Archive for the ‘Functional Programming’ Category

The Archetype Language (Part 6)

Posted by Dan Vanderboom on June 14, 2010

Overview

This is part of a continuing series of articles about a new .NET language under development called Archetype. Archetype is a C-style (curly brace) functional, object-oriented (class-based), metaprogramming-capable language with features and syntax borrowed from many languages, as well as some new constructs. A major design goal is to succinctly and elegantly implement common patterns that normally require a lot of boilerplate code which can be difficult, error-prone, or just plain onerous to write.

You can follow the news and progress on the Archetype compiler on twitter @archetypelang.

Links to the individual articles:

Part 1 – Properties and fields, function syntax, the me keyword

Part 2 – Start function, named and anonymous delegates, delegate duck typing, bindable properties, composite bindings, binding expressions, namespace imports, string concatenation

Part 3 – Exception handling, local variable definition, namespace imports, aliases, iteration (loop, fork-join, while, unless), calling functions and delegates asynchronously, messages

Part 4 – Conditional selection (if), pattern matching, regular expression literals, agents, classes and traits

Part 5 – Type extensions, custom control structures

Part 6 – If expressions, enumerations, nullable types, tuples, streams, list comprehensions, subrange types, type constraint expressions

Part 7 – Semantic density, operator overloading, custom operators

Part 8 – Constructors, declarative Archetype: the initializer body

Part 9 – Params & fluent syntax, safe navigation operator, null coalescing operators

Conceptual articles about language design and development tools:

Language Design: Complexity, Extensibility, and Intention

Reimagining the IDE

Better Tool Support for .NET

If Expressions

In Archetype, an if expression can be provided for any value. The expression variant of the if statement, instead of taking embedded statement clauses, takes value expressions for its "consequence" clauses.

The if expression serves the same purpose as the ternary conditional operator in C#:

TextField.PasswordChar = DisplayStar ? ‘*’ else ‘ ‘;

Enumerations

Enumerations are represented syntactically like lists in Archetype, using the square brackets to enclose values. The idea is that an enumeration type is simply a list of possible values.

enum RainbowColor [ Red, Orange, Yellow, Green, Blue, Indigo, Violet ];

Enumerations are often formatted to display one value on each line. The following example demonstrates this, and defines a variable of the enumeration’s type.

enum RainbowColor

[

Red,

Orange,

Yellow,

Green,

Blue,

Indigo,

Violet

];

Anonymous Enumeration Types

It normally makes sense for an enumeration type to be named so it can be referenced and used elsewhere. But in cases where an enumeration is only needed privately within a single class or method, an anonymous enumeration type can be defined this way:

ForegroundColor enum [ Black, Gray, DarkBlue ];

Enumeration Assignment

Regardless of whether you’re working with named or anonymous enumerations, assignment is the same. The enumeration type is not used in the assignment, which works well with anonymous enumerations since they don’t have a discoverable name.

// no need for an enumeration name

// so it also works great with anonymous enumeration types

BackgroundColor = Green;

Language services (Intellisense) can still inform the user of the possible values after the equals sign and space are entered.

Nullable Types

The nullable type operator converts a type T to Nullable<T>, the same as in C#. Consider the following examples of normal and bindable nullable properties, and a local variable with an initializer.

Age int?;

HighScore bindable int?;

var Age int? = null;

Additionally, we can define a local variable and infer its type from an assignment, using the nullable type operator to force type inference to use a nullable type.

// give me a nullable type, even though I’m not setting it to null no?

var Age ? = 4;

From this point on, we can assign values (including null) to the Age variable without using the null type operator. In fact, including the ? operator would be invalid.

// update the value of Age; notice we don’t use the nullable ? symbol after the definition

Age = 9;

Tuples

A tuple is an anonymous type consisting of an ordered set of heterogeneous fields. In Archetype, their fields can be named for Intellisense hinting when used as return types, or left unnamed. In local variable definitions, their individual members must either be named or use the anonymous member symbol, the underscore.

The following example shows the syntax for defining a tuple as a return type for a function. In this case, a pair of int values will be returned.

GetMouseLocation (int, int) ()

{

return (100, 50);

}

The members of a tuple in a return type can be named as a hint to the caller of the function.

GetMouseLocation (x int, y int) ()

{

return (100, 50);

}

The function is called and its return tuple value stored like this:

var (x, y) = GetMouseLocation();

Here we’re defining a new tuple type, Tuple<int, int>, which is not named as a whole. We might call it an anonymous tuple. Instead of naming the whole, we’re naming the individual members.

Using the .NET Tuple type, we could also write this:

// we don’t care about the y value here

var (x, _) = GetMouseLocation();

We would then have to reference loc.Item1 and loc.Item2 to access the individual members. Naming the members instead of the whole, however, makes more sense because it provides greater code readability.

This next example demonstrates how tuples can be defined using type inference.

var (a, b) = (1, 2);

var (c, d) = (a, b);

var x = b;

On the first line, a and b are defined as accessors into a new tuple: a is assigned to a value of 1, and b to a value of 2. On the second line, another tuple is defined and its member c is assigned to a while d is assigned to b. The third line demonstrates how you can use the tuple members independently of each other. In this case, the value of b is assigned to x.

If we don’t care about all of the members of a tuple, we can use the underscore character to ignore that member. The next example shows how to extract the x value from our GetMouseLocation function while ignoring the y value.

// we don’t care about the y value here

var (x, _) = GetMouseLocation();

Finally, we have a handy way of swapping values without the need to introduce a third variable.

(a, b) = (b, a);

Archetype is not limited to two-member tuples. The .NET Framework defines tuples up to seven members, so Archetype will handle at least that many. If that proves inadequate, it should be relatively easy to extend this to any number of members.

Streams

I first read about streams (or lazy lists, as in Haskell) in a C Omega document on a Microsoft Research site. They’re analogous to sequences in XQuery and XPath, and are implemented using the IEnumerable<T> type in an iterator. I liked C Omega’s * operator to define a stream because of the way it sets that type apart from a normal type. In C#, it’s not obvious that a function with a return type of IEnumerable<T> should behave any differently from another function until you notice the yield keyword.

If I want a stream defined as a property in C#, I’d have to write something like this:

IEnumerable<int> Numbers

{

get

{

yield return 1;

yield return 3;

yield return 5;

}

In Archetype, the syntax is more succinct and direct:

Numbers int*

{

yield 1;

yield 2;

yield 3;

}

We can be even more terse in such cases by using a comma in the yield list.

Numbers int*

{

yield 1, 2, 3;

}

Using list comprehensions, which we’ll explore in more detail later in this article, we can do this as well:

Numbers int*

{

yield 0..100 skip 5;

}

One note about the list comprehension here: we don’t use square brackets around the numeric range because they are implied in the yield statement. Including them here would cause the yield statement to return the list as a single yielded value.

These examples produce streams with a fixed number of elements, but streams can be infinite as well. This example returns all positive odd numbers starting with one.

OddNumbers int*

{

def i = 1;

loop

{

yield i;

i += 2;

}

Streams are lazy, so while it looks at first glance like an infinite loop from which you’ll never escape, in reality control is driven by the loop that accesses the stream.

loop (var n in OddNumbers)

{

Console.WriteLine(n);

if (n > 100)

break;

}

When other type operators are used, such as the nullable type operator, the stream operator must appear last.

Ages int?*

{

yield 35;

yield null;

}

List Comprehensions

Archetype provides some special syntax for constructing lists called list comprehensions. This is syntactic sugar that provide shortcuts for building lists.

Consider the following syntax in C# and Archetype for constructing a list from 1 to 100.

// C#

var FirstHundred = from x in Enumerable.Range(1, 100) select x;

// Archetype

var FirstHundred = [ 1..100 ];

The square brackets in Archetype specify the construction of a list. Now consider a more complicated list construction. In this case, Linq is employed in both langauges:

// C#

var FirstHundred = from x in Enumerable.Range(1, 100) where x*x > 3 select x*2;

// Archetype

var FirstHundred = from x in [ 1..100 ] where x*x > 3 select x*2;

Here you can see how a list can be used as the source of a query.

Here are some more list comprehension examples:

Subrange Types

One of the gems in Pascal is subrange types. This allows a developer to define a new type that is structurally the same as another type, but whose values are constrained in some way. I’m often bothered by the disparity between database and .NET types. In a database, a string type (such as varchar) has a definite and usually small limit. In .NET, strings can be up to 2 MB, but there hasn’t been a good way in languages like C# and Visual Basic to constrain the length. In various object-relational mappers, a Size attribute is often employed, but this is only metadata and does nothing to prevent the string from becoming too large, so additional work must be carefully performed to constrain the input using control properties and validation logic.

Archetype answers this with subrange types and type constraints. Consider the following:

// an int that can only have a value from 0 to 105

type ValidAge int in [0..105];

We can now use this ValidAge type to define our class properties:

Age ValidAge;

If a type is unlikely to be reused, we can also define subrange types anonymously.

Age int in [0..105];

In fact, any list comprehension can be used in a subrange type expression, including multiple ranges, as long as a single base type is used. This example shows an age property that is valid for underage and retiring age people, but is invalid for any ages in between.

Age int in [0..17, 65..105];

We can limit the length of a string simply:

LastName string#30;

Although in actual practice, it might make more sense to create several named types for various string lengths represented in a database:

type Code string#10;

type Name string#20;

type Summary string#100;

type Description string;

LastName Name;

By using a limited number of named string types, both in your code as well as in the database, it’s much easier to update the lengths as needed with a lot less effort. Archetype adds attributes to the members using these types as well, so this data can be queried and used to inform user interface controls and validation logic, enabling a stronger model-driven approach.

The length of strings doesn’t need to be a single number representing the maximum, however. We can also specify a range of lengths.

Name string#2..3 = ”ZZZ”;

Notice in this example how an initializer must be used. This is because a value of null is actually invalid for the Name property. The minimum allowed length is 2. Not providing an initializer with a valid value produces a compiler error. If we wanted to also allow a null value, we would do so like this:

Name string?#2..3 = “ZZZ”;

As with type constraint expressions—discussed in the next section—Archetype injects the appropriate runtime checks in the property setter before any explicitly specified setter code, and throws an OutOfRangeException if the value doesn’t match the specified type criteria.

Type Constraint Expressions

Related to subrange types, type constraints can be applied equally to named or anonymous types. They allow you to specify a Linq-like where clause that will be used to check values being assigned to properties at runtime. Because they rely on property setter methods, type constraints cannot be used on fields. Fortunately, local variables within methods are also implemented like properties by default, so type constraints are also valid on local variables.

I’ve noticed that for some brands, or some stores carrying those brands, only even-numbered sizes of pants are stocked. This example shows a subrange type representing pants size, using both a subrange type as well as a type constraint expression.

// an int that can only be even

type PantSize int in [0..60] where value % 2 == 0;

Using the modulus operator to obtain the remainder of division, we can be certain now that values of this type will only be even numbers. Type constraint expressions are allowed to call static or global functions, properties, and fields, but they cannot reference instance members.

Summary

This article covered a lot of type fundamentals. It should be obvious at this point how a common thread is being woven into the Archetype language. You’ve probably noticed how almost every construct has named and anonymous counterparts. Another important theme is the ability to extend types with syntax designed to shape them, and the use of Linq-like expressions throughout the language.

There is still some type content to cover, such as variant or tagged union types and duck typing, which I’ll save for a future article. Also coming soon is my work on defining custom query comprehensions for a Linq-like query language which can be easily extended with a simple language feature, as well as operators for higher-order functions like fold, map, and others.

Work on my goal of getting a basic Archetype compiler into everyone’s hands is going slowly but steadily. I have a simple Silverlight IDE running in the cloud that parses Archetype code and will return a .NET assembly. I got the Oslo tools to work on the server, and I’m partially building the AST I need to perform the semantic analysis and use to report compile errors to the user and to generate the C# code which I’ll then compile with csc.exe. I’m using a WCF publish-subscribe pattern to initiate a build from the client and report progress as messages going back to the client. In the next few weeks for sure, and possibly sooner, I’ll post a link to that so you can give Archetype a test drive yourself.

[Part 7 of this series can be found here.]

Posted in Archetype Language, Functional Programming, Language Innovation | 14 Comments »

The Archetype Language (Part 5)

Posted by Dan Vanderboom on May 24, 2010

Overview

You can follow the news and progress on the Archetype compiler on twitter @archetypelang.

Links to the individual articles:

Part 1 – Properties and fields, function syntax, the me keyword

Part 2 – Start function, named and anonymous delegates, delegate duck typing, bindable properties, composite bindings, binding expressions, namespace imports, string concatenation

Part 3 – Exception handling, local variable definition, namespace imports, aliases, iteration (loop, fork-join, while, unless), calling functions and delegates asynchronously, messages

Part 4 – Conditional selection (if), pattern matching, regular expression literals, agents, classes and traits

Part 5 – Type extensions, custom control structures

Part 6 – If expressions, enumerations, nullable types, tuples, streams, list comprehensions, subrange types, type constraint expressions

Part 7 – Semantic density, operator overloading, custom operators

Part 8 – Constructors, declarative Archetype: the initializer body

Part 9 – Params & fluent syntax, safe navigation operator, null coalescing operators

Conceptual articles about language design and development tools:

Language Design: Complexity, Extensibility, and Intention

Reimagining the IDE

Better Tool Support for .NET

Type Extensions

If you’re unfamiliar with extension methods in C# or other languages, this section might blow your mind a little bit. If you love and use extension methods all the time, and don’t know what you’d do without it, my hope is that you enjoy the power that Archetype unleashes with robust type extensibility.

If you’re in the unfamiliar group, type extensions are a way of adding new members to existing types, regardless of whether those types were defined in the same assembly as the extensions or in a different assembly. Contrary to what the name may suggest, no modification of the original type actually occurs; instead, the extensions are fed into Visual Studio’s language services, and Intellisense is updated to make it appear that those additional members are available for an instance of that type. Are there methods you’d like to call on any string object? With extension methods, you can add methods and use them as if they belonged to that class. Here’s how it looks in C#:

public static class MyExtensions

{

public static void ShuffleLetters(this string Text)

{

// …

}

Here we can say var result = “Hello there”.ShuffleLetters(); and the dot triggers Intellisense to pop up and show our ShuffleLetters method.

Extension methods are great. But if you really embrace them and start thinking in terms of opportunities for extensions, you’ll run into a few brick walls. You see, extension methods are just a tease; they’re merely the tip of the iceberg, one isolated fragment of a larger (and seemingly happier) family.

First you begin to wish you could add a property instead of a method so you don’t have to put up with parentheses or a Get- prefix to force a property to look and behave like a method. You might see distasteful expressions like cust.HasChanges(), and there’s nothing you can do about it.

Then you’ll be working with a static class, and you’ll wish you could add a static method, but you can’t add static members. Eventually, you’ll run into that scenario where an additional operator or constructor would be the perfectly elegant way to solve the current problem. But you’ll resign yourself to something kludgy instead.

So having gone down a similar road, I was more than a little frustrated when C# 4.0 was released with no new type extensibility at all. This is one of the stimulants to my starting the Archetype language project: the crystallization of knowledge that no other language team was likely to evolve in the direction and with the priorities that I’ve been developing in my head over the years. I’m beyond the point where I’m willing to just wait and see what happens.

C# has a clever approach to designating a method as an extension. However, it’s somewhat indirect. Instead of saying something like “extend” or “extension” near a class definition, they impose several syntactically-unrelated requirements:

The method must be both public and static.
The first method parameter’s type is the type to be extended.
The first method parameter must be prefixed by the “this” keyword. This hints that we’re adding an instance member.
The class the method is defined in must be both public and static.

You would never guess these requirements, and it’s easy to get one of them wrong and skip a beat fixing it. Now try to add extensions for properties, operators, constructors and finalizers, indexers, and possibly fields, and then throw in static members. How will you designate all of these different things? Lots more cleverness, I’d say, but it’s not likely to be syntactically scalable.

When we design syntax, it’s helpful to design a whole family of related capabilities together. You can see Archetype’s approach in the following example:

Customer object

{

FirstName bindable string;

LastName bindable string;

this (FirstName, LastName) set all;

}

Customer extension

{

BirthDate bindable DateTime;

FullName bindable string

get composite FirstName " " LastName;

this (FirstName, LastName, BirthDate) set all;

static BuildCustomer Customer (FullName string)

{

var i = FullName.IndexOf(" ");

assert i >= 0;

return new Customer(FullName.Substring(0, i), FullName.Substring(i + 1));

}

There’s a lot happening here, so let’s go over what we see here one step at a time.

The original class we’re going to extend, Customer, is defined first. Two bindable properties and a constructor, nothing more.
Creating a wrapper for a class extension doesn’t require that you remember several clever tricks. Instead, you simply write “ClassTarget extension”, and everything within the structure is considered an extension of that type.
BirthDate is an extension property.
FullName is also an extension property, however the composite keyword, combined with references to FirstName and LastName, requires that the Archetype compiler look at the target class as well as the extension class to resolve identifiers. The compiler must also wire the binding infrastructure so the target object stimulates the FullName property to update when either composite part does. Referenced members against the target class must be visible to an external class: private and protected members can’t be referenced from an extension, for example.
The this method is an instance constructor. set all sets FirstName and LastName on the target object and BirthDate on the extension object. Constructor methods don’t require a return type, as they are assumed to be the same as the type they’re defined in.
BuildCustomer is a static factory method. The assert keyword is a way to define checkpoints to ensure that conditions are what they’re expected to be, which are especially valuable at the beginning and end of methods. The basic idea is that you’d be able to define their behavior to throw an exception, log a message, or whatever you like when they’re violated: in debug mode, or in production code. More about this construct in a future article.
Operators are also supported, but are not shown in this example. To see how custom operators are created, see Part 7 of this series for a detailed explanation.

While extension methods are simple to implement in a compiler, extension fields are a little bit more complicated. In addition to this transformation, it is necessary to remove Dictionary entries that refer to objects that have been garbage collected, to prevent these dictionaries from growing uncontrollably. When one or more extension properties are used in an assembly, a background worker thread might occassionally check the keys in these dictionaries to see if any refer to garbage collected objects, and remove them.

The BirthDate property (with its internal storage field) in the above extension class is converted into something like this:

_BirthDates field Dictionary<Customer, DateTime>;

GetBirthDate DateTime (Customer Customer)

{

if (_BirthDates.ContainsKey(cust))

return _BirthDates[cust];

return default(DateTime);

}

SetBirthDate void (Customer Customer, BirthDate DateTime)

{

if (BirthDate == default(DateTime))

{

if (_BirthDates.ContainsKey(Customer))

_BirthDates.Remove(Customer);

}

else

{

if (!_BirthDates.ContainsKey(cust))

_BirthDates.Add(cust, BirthDate);

else

_BirthDates[cust] = BirthDate;

}

The BirthDate property would handle data binding and then call one of these two methods (or simply include their logic within the property get and set methods). Another possibility is to instantiate the internally-named extension class and use that as the value in a dictionary.

A runtime mechanism tracks instances of extension objects and remove them from the dictionary periodically. System.WeakReference provides the mechanism to do this. It involves two WeakReferences per object: a short weak reference that becomes null and signals the need to cleanup, and a long weak reference to use as a key to the dictionary to clean up. This mechanism would be loaded only when necessary, and some configuration on its cleanup behavior will be made available.

Static field and property storage would be easier to implement and wouldn’t require any cleanup.

There are a few members that may not make sense to provide as extensions. Static constructors may provide some value to the extension class, but it wouldn’t be able to reach into the target object at static constructor runtime.

Once the wrinkles of implementation detail are worked out, this rich ecosystem of type extensions will open up clean and elegant solutions for adding missing parts from types that you know belong there.

Custom Control Structures

Every so often, I end up writing a function that behaves like a control structure with the block of statements passed in as a lambda function. I’ve done this to spin up a thread to run code in (described in this article), and discussed what Parallel.For would look like as a custom control structure in my article discussing the future of programming languages. My idea there was to define an extension method in such a way, with the delegate at the end, that the compiler would treat the delegate as a separate closure:

public static class Parallel

{

public static void For(long Start, long Count, Action Action)

{

// …

}

This is how you’d use it currently in C#:

Parallel.For(0, 10, () =>

{

// add code here for the Action delegate parameter

});

My proposal was to use it like this instead:

Parallel.For(0, 10)

{

// add code here for the Action delegate parameter

}

The point I made in that article is worth repeating. First, a word from Anders at PDC08:

“Another interesting pattern that I’m very fond of right now in terms of language evolution is this notion that our static languages, and our programming languages in general, are getting to be powerful enough, that with all of these things we’re picking up from functional programming languages and metaprogramming, that you can–in the language itself–build these little internal DSLs, where you use fluent interface style, and you dot together operators, and you have deferred execution… where you can, in a sense, create little mini languages, except for the syntax.

If you look at parallel extensions for .NET, they have a Parallel.For, where you give the start and how many times you want to go around, and a lambda which is the body you want to execute. And boy, if you squint, that looks like a Parallel For statement.

But it allows API designers to experiment with different styles of programming. And then, as they become popular, we can pick them up and put syntactic veneers on top of them, or we can work to make languages maybe even richer and have extensible syntax like we talked about, but I’m encouraged by the fact that our languages have gotten rich enough that you do a lot of these things without even having to have syntax.” – Anders Hejlsberg

On one hand, I agree with him: the introduction of lambda expressions and extension methods can create some startling new syntax-like patterns of coding that simply weren’t feasible before. I’ve written articles demonstrating some of this, such as New Spin on Spawning Threads and especially The Visitor Design Pattern in C# 3.0. And he’s right: if you squint, it almost looks like new syntax. The problem is that programmers don’t want to squint at their code. As Chris Anderson has noted at the PDC and elsewhere, developers are very particular about how they want their code to look. This is one of the big reasons behind Oslo’s support for authoring textual DSLs with the new MGrammar language [now called M].

Ruby and Groovy also support closures that are supplied external to the method arguments.

Where I originally suggested that a first or final delegate parameter should be automatically supported as an external closure, there are a couple reasons to be more explicit. Consider the following Archetype syntax:

[Keyword]

fork<T>(items IEnumerable<T>, action Action<T> closure)

{

// create Task for each item

}

With this global function, I can write parallelized forking code as though it were a part of the Archetype language. With the Keyword attribute, I can even colorize the function name as if it were a built-in keyword.

fork(customers)

{

// work with each customer

it.DoSomething();

}

But the main reason to explicitly mark it as a closure is so it can be treated as an embedded statement, such that keywords like return and break behave within the context of the containing scope. In other words, if I use return within one of these closures, my intention is to return from the method that closure lives in, not to return out of the closure itself: that’s what the the break keyword is for.

The it keyword above assumes the role of the single object in the collection specified (customers).

This is starting to look pretty good, but the it keyword is a crutch if you think about it. We only need it because we don’t have something nicer like the expression I introduced in Part 3 of this series when I talked about iterating with loop. You may recall there were basically two formats for specifying the bounds and behavior of the loop.

// loop through and reference each object in an IEnumerable

loop (var cust in Customers)

{

}

// i starts at 11, decrements by 2, until it reaches (or passes) 1

loop (var i in 11..1 skip 2)

{

}

To make a custom control structure look and behave like one of the built-in variety, there must be a way to indicate in the parameter list that such an argument is required. So let’s say that we introduce an iterator keyword to indicate we want to support either of the syntaxes shown above.

[Keyword]

fork<T>(items IEnumerable<T> iterator, action Action<T>)

{

// create Task for each item

}

This allows the function’s invocation to define an item identifier which is exposed to the following closure. We could then very naturally write:

fork (var cust in customers)

{

// work with each customer

cust.DoSomething();

}

Now we can reference an identifier that makes sense to us, cust, and the whole thing looks like it’s baked into the language. Viola!

There’s another kind of iterator that pertains to coroutines that I haven’t discussed yet, but in Archetype I call them streams, so there shouldn’t be too much confusion between them.

Named Closures

Let’s take this to the next level. What if we wanted to extend our control structure with a second closure that would execute when all of the tasks that were forked had been completed or canceled? This would complete the fork-join concurrency pattern. Consider the following syntax:

[Keyword]

fork<T> void (items IEnumerable<T> iterator, ForkAction Action<T> closure,

JoinAction Action<TaskList<T>> closure as join)

{

// 1. schedule Task for each item

// 2. when all Tasks have been completed or canceled,

// 3. invoke JoinAction

}

Here is how we use it, taken from Part 3 in the series:

// fork out a bunch of parallel tasks and join when all are done

fork (var cust in Customers)

{

// this code is encapsulated in a task in the TPL

// and scheduled for execution

}

join (tasks)

{

// this code block is executed when all of the tasks

// are either completed or canceled

}

One thing I haven’t mentioned yet is Archetype’s support for optional parameters. They work the same as in C# or VB.NET, and come in handy here. By making the second closure parameter optional (by adding “ = null ” after "closure as join”), we can now use fork alone, or fork-join together, in a single function definition. If we don’t enclose it in a specific namespace or class, it will look exactly like a language keyword.

Another example was brought to my attention (in the comments for article 2). The idea is that a predicate is defined in a closure, syntactically separate from the argument list, inspired from the language Groovy.

print("trying some new syntax") { it.Length > 5 };

But let’s convert this one step at a time. If we use Archetype’s concept of a named closure, we could insert if to make the intent more obvious:

print void (Text string, Predicate bool(string) closure as if)

{

if (Predicate(Text))

Console.WriteLine(Text);

}

print("trying some new syntax") { return it.Length > 5 };

The curly braces—though fantastic for multiple-statement blocks of logic, is overkill for simple expressions (and so is the return keyword). Unless I uncover a good reason not to, I’m inclined to allow Archetype to trade the curly braces for parentheses for simple expressions. We could then write this, which is what I was ultimately looking for.

print("trying some new syntax") if (ShouldPrint && it.Length > 5);

I introduced the ShouldPrint boolean variable here to illustrate that the conditions provided here don’t have to relate to it (and in most cases, probably would not).

You may be wondering if using if as the closure’s name would cause a problem with the if keyword in the Archetype language. The reason it’s not a problem is that it appears between the method name on the left and the semicolon on the right. After the semi-colon, the appearance of the if token would be mapped to the language keyword.

The first closure parameter of a method can be named or nameless, but all subsequent closures must be named. I can’t think of a reason to limit the number of closure parameters other than too many is ridiculous, but I’ll leave that to the discretion of the Archetype developer. Closure parameters can be defined with params for chaining together arbitrary numbers of named closures. When calling a method with no parameters except closures, the parentheses after the function name can be omitted. These methods will be exposed to other languages as having delegate types as well as Iterator and Closure attributes to identify those parameters.

Closure Extension Clauses

Another possibility arises when you consider that the if clause added to the print method could be defined in such a way that it could be applied to any method invocation. I’m not advocating this particular pattern of placing the if condition after the statement to execute, but it will do for the sake of illustration. This feature is purely speculative and I’d be curious to hear some feedback.

I could imagine writing a closure extension clause to solve the print problem more generally, something like this:

any (Predicate bool() closure as if)

{

if (Predicate())

any();

}

Then as long as it’s in scope, I could write code like this:

Start void ()

{

import System.Console;

var debug = true;

WriteLine("Debugging started") if (debug);

var name = ReadLine() if (!debug);

WriteLine("Hello " name "!") if (!debug);

}

One line deserves explanation: name is defined as a string variable regardless of the if clause; it is ReadLine which is executed or not based on the condition specified.

More interesting examples are possible if we leave the world of expression closures and consider multiple-statement closures. One possibility involves adding an async closure extension clause, with the callback handled by the supplied closure.

Start void ()

{

import System.Console;

async FetchData()

{

// respond to callback

}

There are many possibilities here, as this opens up the doors to syntax experimentation without actually having to modify the language grammar itself. This is very similar to macros in languages like Lisp and Nemerle. On one hand, it’s more constrained to ensure that each extensibility point always conforms to a common set of structural principles, but the variety of structural extensibility points make it extremely versatile.

I have a few more ideas for language extensions (or “syntactic sugar shaping”) for other types of clauses (we only covered method invocation here), but I’m going to save that for another article.

Next Steps

This article took some long strides toward defining how Archetype handles type and language extensibility, positioning Archetype as an incredibly flexible and malleable tool with which to define syntactic patterns for solving entire classes of problems more intuitively and elegantly.

I created a CodePlex project for Archetype to give it a home. Over the past few weeks, I’ve created a C# 4.0 parser using the M language to prepare me for the construction of Archetype’s parser and compiler. The C# parser definition is available for download on the CodePlex site. If you’re curious about how languages are parsed and projected into Abstract Syntax Trees (ASTs), download this and open it in Intellipad. You can download Intellipad for free at this Microsoft site.

Once you’ve opened it in Intellipad, find where it says “M Mode” in the upper-right corner. Click that, and select DSL Grammar Mode. Then open the DSL menu and select “Split New Input and Output Views”. You’ll see three window panes. On the left, you can enter or paste in some C# code. The middle contains the C# grammar. On the right, you’ll see a graph of objects that represents how the parser views your code. It’s pretty interesting to see what it comes up with!

My next steps involve cleaning up the Unicode character class issues in the grammar, and then getting it to build an object-graph in memory based on the M Graph (the contents of the right window pane). After that, I can work on generating C# code from that AST. I’ll end up with a C# to C# converter, which seems silly, but I’ll eventually fork the M grammar into its own Archetype grammar and start to change the parser to accomodate the new language. The output will remain C#, since that’s easy to compile to assemblies with the csc.exe compiler, but the input will be the new goodness of Archetype.

Future articles will detail this work, as well as defining new corners of Archetype language syntax.

[Part 6 of this series can be found here.]

Posted in Archetype Language, Functional Programming, Language Innovation | 14 Comments »

The Archetype Language (Part 4)

Posted by Dan Vanderboom on May 8, 2010

Overview

You can follow the news and progress on the Archetype compiler on twitter @archetypelang.

Links to the individual articles:

Part 1 – Properties and fields, function syntax, the me keyword

Part 2 – Start function, named and anonymous delegates, delegate duck typing, bindable properties, composite bindings, binding expressions, namespace imports, string concatenation

Part 3 – Exception handling, local variable definition, namespace imports, aliases, iteration (loop, fork-join, while, unless), calling functions and delegates asynchronously, messages

Part 4 – Conditional selection (if), pattern matching, regular expression literals, agents, classes and traits

Part 5 – Type extensions, custom control structures

Part 6 – If expressions, enumerations, nullable types, tuples, streams, list comprehensions, subrange types, type constraint expressions

Part 7 – Semantic density, operator overloading, custom operators

Part 8 – Constructors, declarative Archetype: the initializer body

Part 9 – Params & fluent syntax, safe navigation operator, null coalescing operators

Conceptual articles about language design and development tools:

Language Design: Complexity, Extensibility, and Intention

Reimagining the IDE

Better Tool Support for .NET

Conditional Selection

The if statement has been a classic across so many languages. In Archetype it is almost identical to C# syntax.

if (expression)

statement;

if (expression)

{

// expression is true

}

else

{

// expression is false

}

When conditions become complicated, reversing all of the boolean logic can be tricky. A common way of reversing it is to surround the expression in parentheses and placing a unary not operator before it. With the required parentheses around the if statement’s condition, it looks like this:

if (!(expression))

statement;

In Archetype, the exclamation point can be placed before the parentheses. It is the only part of the condition that can appear outside the parentheses.

if !(expression)

statement;

Pattern Matching

C-style languages have supported a language construct, switch-case, for providing access to simple jump tables combined with a syntax that is better suited than if for matching many conditions. This has been unfortunately limited to matching against value types (and string in C#) and against constant values at that. It’s unfortunate because the more concise syntax for multiple matching values is good in itself, not only when the matching values are constant values. This constraint is due to the way those compilers build jump tables; it’s a performance optimization technique designed during a time when 8 MHz processors were considered fast.

Pattern matching is one area where functional languages have been very strong. Archetype has a match keyword that serves the purpose.

match (text) "BEGIN" -> HandleBegin();

This first example matches a simple string to a constant value, and calls HandleBegin if there is a match. You could write this using an if statement as well. Here is the equivalent code:

if (text == "BEGIN") HandleBegin();

This next example illustrates several ideas. There are multiple conditions and some of the conditions are grouped together (with the or operator, |) to share the same reaction code. The condition or conditions are listed first, followed by the –> operator, and a statement or code block of statements on the right specifies the reaction code. Also note the numeric range 6..10 and the use of non-constant values (such as x). Any valid expression is allowed here as long as it’s type matches (or can implicitly cast from) the type of the term being evaluated (number). It’s also worth mentioning that the | operator isn’t necessary before each set of conditions as it is in other functional languages. (I’d rather align the left edge of code to the beginning of each condition.)

var x = 1;

var number = 4;

match (number)

{

x -> number += 3;

3 | 5 -> number++;

2 | 4 | 6..10 ->

{

number–;

Log("Numbers are getting too big");

}

Unlike switch, Archetype’s match doesn’t automatically fall through from one match to the next. This feature is rarely used with switch and is a significant source of programming defects. For the few scenarios where you’d like to execute every branch that matches, I’m considering a match all construct which would look like this:

match all (text)

{

"BEGIN" -> LetsBegin();

(Letter | Digit)* -> AddIdentifier(text);

}

In this example, both LetsBegin and AddIdentifier would be called.

Regular Expression Literals

Archetype also supports regular expression literals based on the syntax from Microsoft’s M language as a result of list syntax and operator overloads defined as a library. In many scenarios, Archetype can determine the difference between string and regular expression literals. However, in simple cases such as matching against a single character or simple string of characters, the variable defined will require the specification of the regex type.

var BeginToken regex = "BEGIN";

Without this qualifier, BeginToken would look like a string to the compiler. To ease this problem, Archetype will convert a string to a regex object if the string participates in a regex-typed expression. Here’s an example:

var BeginToken = "BEGIN";

var MyRegEx = BeginToken | ("A".."Z")*;

The range of letters and the * Kleene operator (which means repeat 0 or more times) identifies MyRegEx as a regex identifier.

Let’s take a look at how regular expressions and regular expression literals work with the match construct. First we see the literal embedded directly in the match statement, as the only value to match against.

match (text) ("A".."Z" | "a".."z")* -> DoSomethingUseful();

Next, we’ll look at how we can define the regex objects and use their identifiers. In this way, we can build up libraries of interdependent regular expressions and go even so far as to write sophisticated parsers. This is an important tool for fulfilling the goal of language-oriented development.

var Letter = "A".."Z" | "a".."z";

var Digit = "0".."9";

match (text)

{

"BEGIN" ->

{

MarkBeginning();

NewTransaction();

}

(Letter | Digit)* -> AddIdentifier(text);

}

Finally, you can apply a when clause as a pattern matching guard similar to F#.

var Letter = "A".."Z" | "a".."z";

var Digit = "0".."9";

match (text)

{

"BEGIN" ->

{

MarkBeginning();

NewTransaction();

}

(Letter | Digit)* when (text.Length < 15) -> AddIdentifier(text);

}

This isn’t the last word on pattern matching or regular expressions in Archetype. This is one area I expect to evolve and grow, and to appear in future articles.

Agents, Classes, and Traits

Archetype is a multi-paradigm language, as most commonly-used languages are today. While it has many features which are functional, it’s heavily influenced by object-oriented design ideas. Most object-oriented languages are largely imperative rather than functional. That is to say, it is “programming by side-effects” rather than the goal in functional programming of “no side-effects” (or as few as possible).

Functional programming has grown in popularity relatively recently, considering it’s been around from the beginning of high-level language design. However, it suffers in some areas such as representing stateful behavior in user interfaces. Some clever solutions have been devised (such as the use of monads to trick or fake the logic into representing state in a purely functional way), but the theory and application of these patterns are far from intuitive. I believe this is largely the reason why functional programming languages have been the niche speciality of scientists and mathematicians and not your every day developer.

Because of these contentious forces, Archetype is aimed at being a transitional language: urging us forward in the use of functional patterns, but without abandoning the imperative style of “programming by side-effects”, and striving to look familiar to programmers of imperative languages such as C#, Visual Basic, etc.

Software Agents and the Actor Model

There are some built-in Archetype constructs that will help to make object-oriented programming safer. While it doesn’t propose, like Axum (previously code-named Maestro), to prevent any logic that is unsafe in a concurrent execution environment, it does provide some simple but powerful tools that can be used to reduce the risk considerably.

I’m referring primarily to agents which support the Software Agent or Actor Model of parallel program design. Agents are special classes that run independently (in parallel) of each other, and can only communicate with other agents through messages (introduced in part 3). Specifically, agents are not allowed to call the methods or subscribe to the delegate members of other agents. Since each agent runs without the ability to receive from or give execution control to other agents, there is much smaller chance of coordination problems while executing concurrently.

In every other way, however, agents are defined and composed just like classes. First, we’ll take a look at a simple Customer class.

Customer object, IDisposable

{

FirstName string;

LastName string;

this(FirstName, LastName)

set all;

Dispose()

{

// clean up

}

The class name appears first, followed by a required base type (object here), and a list of interfaces separated by commas.

Archetype supports single-class inheritance with the ability to implement any number of interfaces (or traits, more about that later in this article), as well as any generic type parameters and generic type constraints that you’re used to.

We see some new things here, however. The instance constructor is called this, and the set keyword is used to set the values of class members where they match constructor parameters of the same name. This is the same as writing these lines:

this.FirstName = FirstName;

this.LastName = LastName;

With many parameters in a constructor (or another method), this can save many lines of typing. If you only want to store some of the parameters in properties, you can use a comma separated list: set FirstName, LastName. If your intent is to set all parameters, you can use the abbreviated set all instead, as shown in the example. When set all is used, specifying parameter types is optional.

The following code provides an example of two agents that cooperate with each other.

WebDataAgent agent

{

Subscriptions Dictionary<string, List<guid>>;

this()

{

// initialize the agent

Subscriptions = Dictionary<string, List<guid>>();

}

Subscribe in message(Topic string);

{

if !(Topic in Subscriptions.Keys)

Subscriptions.Add(Topic, new List<guid>);

Subscriptions[Topic].Add(me.Client);

// confirm the subscription

SubscriptionConfirmed(me.Message);

}

SubscriptionConfirmed out message(RequestID guid);

PublishMessage in message(Topic string, Message string)

{

loop (var sub in Subscriptions[Topic])

{

MessagePublished(Topic, Message);

}

MessagePublished out message(Topic string, Message string);

}

UserInterfaceAgent agent

{

CurrentView IView;

this(StartView IView)

CurrentView = StartView;

RequestData out message(RequestID guid, Method string);

DataReceived in message(RequestID guid, Result List<double>)

{

// handle incoming message…

// unhook this message handler

DataReceived -= me;

}

I have more ideas for actor-based programming (such as a built-in RequestID: me.id), but I want to start simple and force myself to work hard to justify any overhead.

Traits

Composing classes together to obtain maximum reuse of code has been a goal of object-oriented programming for a long time, but it usually falls short of the ideal. Languages like C++ that support multiple inheritance are unwieldy due to the additional complexity (see the Diamond Problem), and single inheritance—though sufficient in most scenarios—suffers from limitations that have bothered OOP programmers from the beginning of programming time. Other languages have introduced constructs like Flavors and Mixins, and each has had to deal with its own peculiarities and workarounds. While an in-depth discussion of traits and their advantages over other approaches is beyond the scope of this article, a smart group at the OGI School of Science & Engineering published a paper that illustrates the issues clearly. In it, they explain how traits solve many of the problems while avoiding the pitfalls of other approaches.

I found this characterization to be particularly lucid (the bold emphasis is mine):

Although multiple inheritance makes it possible to reuse any desired set of classes, a class is frequently not the most appropriate element to reuse. This is because classes play two competing roles. A class has a primary role as a generator of instances: it must therefore be complete. But as a unit of reuse, a class should be small. These properties often conflict. Furthermore, the role of classes as instance generators requires that each class have a unique place in the class hierarchy, whereas units of reuse should be applicable at arbitrary places.

– Nathaneal Scharli et al, in their 2003 paper entitled “Traits: Composable Units of Behavior”

The basic idea is that a trait defines a set of functions but no state. Multiple traits are pulled into a class, where they are “flattened”. This means that each trait’s functions are added to the class as if those functions were defined directly in the class. That is, you don’t need to use a member access operator (.) to navigate from the class to the trait and then to the trait’s function. In doing this, it’s possible for function names and signatures to overlap among traits. If the name is the same but the signature is different, they’re applied as overloads. When there’s an actual clash, a conflict-resolution expression is defined to specify the function to use (or ignore).

Although the design of traits involve the lack of any state, Archetype may attempt to include trait-local state. That is, variables that are visible to the functions of that trait, but which can’t be seen from the hosting class or any other trait. (This corresponds to the idea of extension properties, which I’ll discuss in the next article.)

This is an experimental area of the Archetype language, one that will likely change several times before getting right. Here’s an example of what it will probably look like to compose classes out of traits.

Serializable<T> trait

{

provide Serialize string (obj T) { … }

provide Deserialize T (input string) { … }

}

Persistent<T> trait

where T : ref

{

// by function

require Serialize string ();

require Deserialize T (input:string);

// or by trait

require Serializable<T>;

provide HasChanges bool

get, private set;

provide static Load T (id: uid) { … }

provide Save void () { … }

}

Customer object, Serializable, Persistent

{

FirstName string;

LastName string;

FullName string

get FirstName " " LastName;

}

Start void ()

{

var cust = Customer.Load(123);

cust.FirstName = "Dan";

cust.LastName = "Vanderboom";

cust.Save();

}

A few notes about the code:

The type parameter on the trait allows you to constrain the types of object to which the trait can be applied.
The where T : ref is the same as where T : class in C#.
The require and provide keywords specify the methods that trait requires to be present, or provides to the host class. Archetype may also support a require of an entire trait, which would act analogously to subtyping (or rather, more like an #include).
Conflict resolution expressions aren’t shown because their syntax hasn’t yet been decided.
Code formatting applies an italic font to traits, but maintains the color of a user-defined type. This should help to avoid confusing traits with classes. This is only one possible solution, but it suggests the use of multiple ways of categorizing identifiers to apply a mixture of formatting and colorizing behaviors.
This is not a great example of the strength of traits.

I’ve also had some design ideas for runtime mixing of traits into classes, while still accessing everything through strongly-typed variables (think of traits as interfaces), but this will require much more exploration.

Another idea to support some Aspect Oriented feature. Imagine if you could define a trait called Bindable, that when added to a class, would add the bindable type extension modifier to all of its properties.

For better examples of traits and class composition using traits, I recommend reading the above-mentioned paper.

Next Steps

In this article, I covered simple conditional statements as well as functional-style pattern matching. We also looked at agent-based programming based on loosely-coupled messages, which provides greater safety in parallel programming scenarios, traits as a way to compose features on a more granular level and to solve the composition problems that plague single-inheritance languages.

My next article will cover extension of types (extension methods, properties, events, indexers, constructors, and operators), as well as the first of language extensibility options (defining new control structures). I will probably dip out of sight for a few weeks as I get further along in building the parser and compiler, and learn about Visual Studio’s Managed Language Services.

If you don’t already follow me on twitter (@danvanderboom), I do a lot of tweeting about what I’m reading, researching, or considering during the language design process, so this is a good way to get an inside look at that process.

[Part 5 of this series can be found here.]

Posted in Archetype Language, Functional Programming, Language Innovation | 7 Comments »

The Archetype Language (Part 3)

Posted by Dan Vanderboom on April 27, 2010

Overview

You can follow the news and progress on the Archetype compiler on twitter @archetypelang.

Links to the individual articles:

Part 1 – Properties and fields, function syntax, the me keyword

Part 2 – Start function, named and anonymous delegates, delegate duck typing, bindable properties, composite bindings, binding expressions, namespace imports, string concatenation

Part 3 – Exception handling, local variable definition, namespace imports, aliases, iteration (loop, fork-join, while, unless), calling functions and delegates asynchronously, messages

Part 4 – Conditional selection (if), pattern matching, regular expression literals, agents, classes and traits

Part 5 – Type extensions, custom control structures

Part 6 – If expressions, enumerations, nullable types, tuples, streams, list comprehensions, subrange types, type constraint expressions

Part 7 – Semantic density, operator overloading, custom operators

Part 8 – Constructors, declarative Archetype: the initializer body

Part 9 – Params & fluent syntax, safe navigation operator, null coalescing operators

Conceptual articles about language design and development tools:

Language Design: Complexity, Extensibility, and Intention

Reimagining the IDE

Better Tool Support for .NET

Exception Handling

The try keyword can be used within any code block.

ProcessItem void (Item Item)

{

try

{

// throw a runtime exception

}

catch (ex Exception)

{

// handle exception

}

finally

{

// cleanup

}

Additionally, every function can specify its own inline catch and/or finally blocks like this:

ProcessItem void (Item Item)

{

// throw a runtime exception

}

catch (ex Exception)

{

// handle exception

}

finally

{

// cleanup

}

You’ll see this pattern appear in other constructs in Archetype (such as async blocks).

A try block can exist with one or more catch clauses only, a finally clause only, or both. If both are included, the catch clauses must come first. This is true whether they’re defined as part of the function (example 0) or have an explicit try clause (example 1). Since curly braces are optional for single statement code blocks, we can write this:

try Work();

catch (x Exception) Log(x);

finally Finish();

The exception type can be specified by itself (without a name), and the default variable "ex" will be used. If a catch block doesn’t specify an exception type, Exception is presumed.

try Work();

catch (ArgumentException) Log(ex);

catch (NullReferenceException) Log(ex);

catch Log(ex);

finally Finish();

The order of catch blocks must be from most derived to most base (Exception itself must always be last, if present). Incorrect ordering will result in a compiler error.

Catch and finally blocks are scoped as nested within the try block. This enables catch and finally blocks to reference identifiers defined in the try block.

try

{

var answer = 42;

}

catch

{

// valid reference to answer

var a = answer;

}

As with C#, the throw keyword can be used with an Exception variable to wrap and rethrow a caught exception, or throw can be used to in a statement by itself to rethrow the original exception.

Namespace Imports

In Part 2, we saw the first of the import keyword to import namespaces.

import System;

Start int ()

Console.WriteLine("Hello world!");

Like Nemerle, we can also apply the import keyword with classes to access static members without specifying the class name.

import System.Console;

Start void ()

WriteLine("Hello world!");

Another option is to import a namespace or class into a nested function or class scope.

Start void ()

{

import System;

Console.WriteLine("Starting up…");

}

Employee object

{

import System;

Work void ()

{

Console.WriteLine("Working hard!");

}

Similar to the with keyword in Pascal, import can be used to import a namespace or class for a specific code block. This limits a namespace or class import to a limited section of a function.

Start void ()

{

System.Console.WriteLine("The following import doesn’t apply here.");

import System.Console

{

WriteLine("hello");

WriteLine("goodbye");

}

One final thing you can do with import is to specify a namespace alias.

Start void ()

{

import sys = System;

sys.Console.WriteLine("Example of a namespace alias");

}

Aliases

The section on namespace imports above introduced namespace aliasing. In this section, we’ll see how to use the alias keyword to provide additional identifiers to classes, functions, properties, and fields.

The class alias is similar to the import class pattern, except that a new identifier is introduced and must be used to reference its members. (With import, that class’s static members are implicitly accessible.)

DemonstrateAlias void ()

{

alias kid = Geneology.Child;

var Josa = new kid;

Josa.FirstName = "Josa";

Josa.Age = 4;

}

In addition to the alias statement, I’m introducing object instantiation syntax in Nemerle. The constructor of the Geneology.Child class is being called (via its alias, kid) with the new keyword but no parentheses, which are optional when calling a parameterless constructor.

The var keyword is also new here. It is similar to the var keyword in C#, except that it’s required for local variable definitions.

This example demonstrates alias used for a local variable. The syntax is identical for class fields and properties, except that those aliases can be specified at the class level as well as within functions.

DemonstrateAlias void ()

{

var SocialSecurityNumber = "123-456-7890";

alias SSN = SocialSecurityNumber;

System.Console.WriteLine(SSN);

}

This last alias example shows how it can be applied to functions.

DoWork void () { … }

Test void ()

{

alias work = DoWork;

work();

}

Control Flow

We’ve already discussed exception handling, which is a very fundamental kind of control flow structure. In this section, we’ll explore several other constructs that are familiar to every programmer.

In Programming Language Pragmatics, the author (Michael L. Scott) enumerates six essential types of control flow: sequencing, iteration, selection, exception handling, recursion, and concurrency. We’ll cover most of them in this article.

Sequencing is merely the scheduling of one statement to be executed after another. This is the standard model of interpreting source code statements in a code block, so there’s not much more to say about it.

Iteration

Iteration is much more interesting. In Archetype, there are four iteration constructs.

Loop

The first is the incredibly versatile loop. It can take a simple integer expression to loop a specific number of times. It can alternatively take an expression that introduces a variable, a range of values, and an optional skip value (similar to the for keyword in Visual Basic). Finally, it can act like the foreach keyword in C# and iterate over IEnumerable and IEnumerable<T> collections such as streams, lists, etc.

// loop 10 times

loop (10)

{

}

// i starts at 3, increments by 1, until it reaches 9

loop (var i in 3..9)

{

}

// i starts at 11, decrements by 2, until it reaches (or passes) 1

loop (var i in 11..1 skip 2)

{

}

// define cust, then loop through and reference each object in an IEnumerable

loop (var cust in Customers)

{

}

This replaces the archaic syntax of the for loop in C# and older C-style languages, and provides a construct which is much easier to read and write. Each of the integer constants in the examples above can be replaced with expressions (variables, function calls returning integers, etc).

When writing a loop, it’s often necessary to skip the remainder of the current iteration and continue with the next one. In C#, the ambiguous-sounding continue keyword is used. I remember seeing this for the first time and thinking that it meant to continue executing after the loop, which wasn’t the case. So in Archetype, I’m ressurrecting the venerable old keyword next, as in “go to the next iteration in this loop”. To break out of a loop altogether and continue executing after the loop, the break is used.

// define the int i and loop from 0 to 9

loop (var i in 0..9)

{

// do some work

if (DoneWithThisIteration)

next;

if (DoneWithLoop)

break;

// continue with more work

}

Fork-Join

A potent addition to the loop construct is the fork-join pattern. The fork and join words are actually defined in a library, not in the language itself. In a later article, you will see how to create patterns like this.

// fork out a bunch of parallel tasks and join when all are done

fork (var cust in Customers)

{

// this code is encapsulated in a task in the TPL

// and scheduled for execution

}

join (tasks)

{

// this code block is executed when all of the tasks

// are either completed or canceled

}

The join clause’s parameter, named tasks in the example, is a reference to a list of Task Parallel Library (TPL) tasks. This is a handy way to execute code in parallel without having to restructure your code (similar to the Parallel.ForEach method in the TPL). Most of the difficulties of concurrent programming are matters of coordination, however, and so they are often best handled by parallel libraries such as the TPL or the Concurrency & Coordination Runtime (CCR).

While and Until

The next example is the familiar while loop. It matches a condition at the beginning of each iteration and only executes the following code block if the expression provided is true.

// repeat while condition is true

while (a == 10)

{

}

The until loop works similarly, but tests its condition after the following code.

// repeat until the condition is true

until (str.Length == 0)

{

}

Although it makes sense in C# that the until clause should appear at the end, where it’s placed, the reality is that in C# the syntax is awkward: I don’t know whether to keep my curly braces aligned and put the do and until on their own lines so they don’t crowd the embedded code block, or what. With the naming difference in Archetype, and with the debugger’s help in stepping through in the correct way, I’m betting this feature will not only be easy to grasp, but hopefully will be seen to clean up certain coding situations and help make looping syntax more structurally consistent.

The break and next keywords apply to while and until loops as it does for loop constructs (see the Loop section above).

Asynchronous Programming

It’s becoming more common now to make asynchronous calls to web services and other long-running processes that we don’t want our code to sit around waiting for. In Silverlight, for example, the only network communication options we have are asynchronous. But the Asynchronous Programming Model (APM) has a way of confusing and tripping up developers as they try to wrap their heads around it.

Archetype introduces a few language constructs to make asynchronous programming easy.

Calling Methods and Delegates Asynchronously

The async keyword allows you to call any method or delegate asynchronously with the same syntax.

GetData string (Index int)

{

return (Index + 1).ToString();

}

async GetData(42)

{

var result = value;

}

All of the details of dealing with AsyncCallback and IAsyncResult are abstracted away. The value keyword represents the return value of the asynchronously-called method (if applicable). The first access of value may result in an exception in the event that the target method failed when it ran. This can be caught with a standard try-catch block, or the following syntax can be used.

async DoWork()

{

// success

}

catch (ex ArgumentException)

{

// failure

}

finally

{

// final logic

}

All of the standard rules apply regarding the syntax of catch and finally blocks (see the Exception Handling section above in this article).

As another reminder of the optional curly braces for single statements, here is a short and simple async call example.

async DoWork()

NextStep()

catch HandleError(ex)

finally Cleanup();

The ability to specify your intent to call methods and delegates asynchronously without mucking around in the implementation details should go a long way to making developers more productive in high-latency scenarios. In a final example of async, I’ll demonstrate how we can still obtain access to the IAsyncResult variable that is returned by the APM, which is useful if you need to occasionally check if it’s completed.

var ar = async DoWork()

NextStep()

catch HandleError(ex)

finally Cleanup();

Messages

Messages offer an alternative to delegates. As useful and simple as delegates are, the problem with them is that they pass along not only data, but also execution control. Often what we need, in particular for applications that must take advantage of parallel execution, is a way to pass along data without giving up control over execution. This is usually done by pushing a message onto a queue which can be picked up at the receiver’s convenience without holding up the sender.

In Archetype, messages fulfill this need. Like delegates, they can be named or anonymous. Here is an example of each.

message EmptySignal();

SomeAgent agent

{

// using a named message

Started out EmptySignal;

// using an anonymous message

Completed out message(string);

}

Unlike their delegate counterparts, messages don’t have return values. Return values only make sense when you make a synchronous call and give up execution control. Though we can get around this with the async construct and the Asynchronous Programming Model, this is something of a hack.

In the example above, the anonymous message provides the keyword message in place of a return type. In both cases, the out keyword is used to indicate that it is an outgoing message. As you might expect, there is an in keyword to indicate incoming messages.

Messages are one way communications. If you need a response, you need to specify a corresponding incoming message. This applies to the communication of error information as well. If you’re interested in learning more about asynchronous message-based communication, you can refer to my article on the subject.

The out messages act like delegates and can be called like functions, and you can think of in messages as event handlers, although they can also be invoked like methods within the agent.

I am in the process of evaluating Axum and other agent-based languages. I’m particularly interested in the way Axum defines protocol contracts, which are compile-time checks that message A is responded to by message B, and so on. It’s likely that this type of constraint will be implemented using the language extensibility capabilities of Archetype, and that it will be deferred until I understand the issues and challenges better. Until that time, I believe that having the option to define messages as first-class members will provide developers with a greatly-needed tool for safer concurrency programming. It will make the execution control model explicit and obvious at member definition instead of being bolted-on later as a set of imperative instructions in an often-misunderstood corner of the .NET Framework.

Another avenue I’m exploring is a syntax for exposing messages as WCF endpoints. Stay tuned for more information on this subject.

Next Steps

We covered a lot of ground in this article. In these first three articles, many of Archetype’s most fundamental and important constructs have been explained and demonstrated. In the next article, I’ll introduce Archetype’s capabilities and syntax for conditional selection (if), pattern matching (including a much-needed replacement for the switch statement), and the relationship between traits and classes.

There’s a lot more to see, but since this article turned out to be so large, I’ll stop here with promises about the next one.

[Part 4 of this series can be found here.]

Posted in Archetype Language, Functional Programming, Language Innovation | 4 Comments »

The Archetype Language (Part 2)

Posted by Dan Vanderboom on April 27, 2010

Overview

You can follow the news and progress on the Archetype compiler on twitter @archetypelang.

Links to the individual articles:

Part 1 – Properties and fields, function syntax, the me keyword

Part 2 – Start function, named and anonymous delegates, delegate duck typing, bindable properties, composite bindings, binding expressions, namespace imports, string concatenation

Part 3 – Exception handling, local variable definition, namespace imports, aliases, iteration (loop, fork-join, while, unless), calling functions and delegates asynchronously, messages

Part 4 – Conditional selection (if), pattern matching, regular expression literals, agents, classes and traits

Part 5 – Type extensions, custom control structures

Part 6 – If expressions, enumerations, nullable types, tuples, streams, list comprehensions, subrange types, type constraint expressions

Part 7 – Semantic density, operator overloading, custom operators

Part 8 – Constructors, declarative Archetype: the initializer body

Part 9 – Params & fluent syntax, safe navigation operator, null coalescing operators

Conceptual articles about language design and development tools:

Language Design: Complexity, Extensibility, and Intention

Reimagining the IDE

Better Tool Support for .NET

The Purpose of Archetype

You may wonder why I’m designing a new language. As I explained to Vlad in the comments of my introductory article:

I don’t need it. There are plenty of perfectly usable languages out there.

That being said, I want it. I want to spend a lot less time with ceremony and more with substance. I want greater expressive power without sacrificing readability. I want to extend the language syntax and hook into the compiler at certain key points to experiment with new ideas without having to version the base compiler. I want to define traits as composable types and reserve classes for engines of instantiation when it makes sense to do so. I want common concurrency patterns to feel like first-class citizens so that behavior guaranties can be made at compile time. I also want to see if all the language ideas I’ve come up with over the years will really be as valuable as I think they will be.

I have a lot more reasons, too. They’ll be the subject of continued articles in the series.

Hello World!

I would be remiss if I didn’t include a Hello World example.

Start void ()

{

System.Console.WriteLine("Hello world!");

}

This is a complete program. It defines a Start function as a top-level construct (not inside of a class), with a single call to Console.WriteLine, a normal .NET method.

Because any code block can be either a single statement or a pair of curly braces containing zero or more statement, we can shorten our example to look like this:

Start void ()

System.Console.WriteLine("Hello world!");

C# already allows this with constructs like if, while, and for. Archetype takes it to the next level by making it universal.

Requiring the start method of a program to be hosted in a class seems like a kludge to me; and in the spirit of enabling the language to be used for more functional programming, I thought it appropriate to allow this type of functional composition without the ceremony of an enclosing class.

The startup function name will be Start by default, and a modifiable setting on the project options page will let you use a different name for the entry point function. It will also support using a static function in a class.

Delegates

Delegate definitions in Archetype are very close to function definition syntax. Consider this example:

import System;

Start void ()

{

ShowInfo void (); // define a delegate

ShowInfo(); // invoke the delegate

ShowInfo = () => { Console.WriteLine("Name: " me.Name) }; // Name: Start

ShowInfo += { Console.WriteLine("Type: " me.Type) }; // Type: void

ShowInfo();

}

First, we’re introducing the import statement. Our use of it here is identical to the using keyword in C#. The import statement does some other interesting things, which we’ll see in a future article.

The first statement in our Start method defines a delegate called ShowInfo. Note that the only difference between this and a function definition is its lack of a trailing code block. Instead, a semicolon appears after the (empty) parameter list.

The next line invokes the delegate. In C# this would throw a NullReferenceException, which I’ve always found annoying. In Archetype, as with Visual Basic, this gets converted by the compiler into a check for null followed by an invocation if it’s not null. I’ve gone this route because of how rare it is that I actually want to throw an exception in these cases; in C#, I’m constantly writing the null check for delegates to avoid it, wrapping that check and the invocation in OnDoWhatever methods, and that seems wasteful. In Archetype, if you want to throw an exception when a delegate is null, then write the code to throw one explicitly.

The following two lines point the delegate to a specific function (expressed as a lambda, similar to C#) and add a lambda function to the first. The =, +=, and –= operators work as expected with delegates.

Notice that the first lamda function supplies an empty parameter list, but the second one omits it. The parameter list can be omitted when there are no parameters, or when you don’t need to reference the arguments that are passed in.

It’s possible to be even more terse if we have a single non-assignment statement we’re assigning to our delegate variable:

ShowInfo = Console.WriteLine("Name: " me.Name);

Assignment statements cause a problem with parsing because of the right-to-left interpretation of assignments. Consider these statements:

ShowInfo = Console.WriteLine("Name: " me.Name);

ShowInfo = Age = 1;

The first line is a valid delegate assignment. The intention of the second is :“each time ShowInfo is invoked, set Age to 1.” However, the parser reads this as “Set Age to 1, then set ShowInfo to Age,” which is not what we want. As a result, single assignment statement delegates in Archetype require being surrounded in curly braces.

Finally in our Start method above, the last line invokes the delegate again, which in turn calls both lambda functions.

Additional Notes:

The me keyword refers to the current function, Start. As the comments suggest, me.Name returns “Start” and me.Type returns a System.Void Type object. Calling me() as a function would call Start recursively.
String concatenation doesn’t use the + operator. Multiple strings separated by spaces are concatenated automatically. In the case of Console.WriteLine above, where a string literal (“Type: ") is followed by a non-string value (me.Type), the non-string value is converted to a string with ToString. This can occur because the non-string value is listed where a string is expected.

Delegate Parameters

Defining parameters for delegates is easy. If an anonymous function won’t use any of the parameters passed in, it can omit the parentheses entirely. Individual parameters names can be omitted with an underscore character. Otherwise, argument names are supplied as usual. All three variations can be seen in the following example:

Start void ()

{

ShowInfo void (Info string, Priority int);

ShowInfo = (info, priority) { Console.WriteLine("Name: " me.Name) };

ShowInfo += (info, _) { Console.WriteLine("Type: " me.Type.Name) };

ShowInfo += { Console.WriteLine("Info: " info) };

ShowInfo("Fake Info", 10);

}

Named (Non-Anonymous) Delegate Types

So far we’ve only seen anonymous delegate types. The ShowInfo delegate above has a type, but we can’t refer to it by name, and so we can’t share that type with other code. This is fine in many cases. In fact, many times I’m annoyed by the need to go to another file to add a delegate that will never be used elsewhere. But there’s also occassionally a need to expose that type, especially for use by a library or framework consumer.

The following code defines a delegate type, a function that uses that delegate type for its parameter, and a call to the function. That call contains a lambda expression that creates an anonymous function and a delegate object pointing to it, and passes that to the ShowInfo function.

type ShowPersonDelegate void (Name string, Age int);

ShowInfo void (ShowPerson ShowPersonDelegate)

{

ShowPerson("Josa", 4);

ShowPerson("Ava", 1);

}

Start void ()

{

ShowInfo((name, age) { Console.WriteLine(name " is " age) });

}

Another way to think about this is that the delegate keyword simply defines a name (ShowPersonDelegate), which then points to an otherwise-anonymous delegate type: void(Name string, Age int).

The final statement calls ShowInfo, passing in a lambda function, which has the same syntax as C#.

For the sake of comparison, here is the same program using an anonymous delegate. Note that the delegate’s parameter names are optional.

ShowInfo void (ShowPerson void(string,int))

{

ShowPerson("Josa", 4);

ShowPerson("Ava", 1);

}

Start void ()

{

ShowInfo((name, age) { Console.WriteLine(name " is " age) });

}

As in C#, a delegate object will be created automatically if a method name is provided where a matching delegate is requested:

SendText void (Name string, Age int)

{

Console.WriteLine(name " is " age);

}

ShowInfo void (ShowPerson void(string,int))

{

ShowPerson("Josa", 4);

ShowPerson("Ava", 1);

}

Start void ()

{

ShowInfo(SendText);

}

Delegate Duck Typing

To simplify interoperating between named and anonymous delegates, a form of compile-time duck typing is used. Consider the following code:

Predicate1 Func<int, bool>;

Predicate2 bool(int);

Predicate1 = p => p % 2 == 0;

Predicate2 = Predicate1;

Predicate1 and Predicate2 are technically two different delegate types. The anonymous bool(int) delegate will be named something like __anon_bool_int by the compiler. However, the last two lines are valid because bool(int) and Func<int, bool> are structurally equivalent. It is effectively transformed by the compiler into:

Predicate2 = p => Predicate1(p);

Bindable Properties

The bindable keyword can be used to enable data binding support for user interface controls. It’s always bothered me how much boiler plate code must be written in .NET languages for bindable properties. In C#, this is typical:

private int _Age;

public int Age

{

get { return _Age; }

set

{

_Age = value;

PropertyChanged("Age", value);

}

That’s ten lines of a code for a simple integer property! And this is a simple scenario. Compare that to Archetype’s binding property:

Age bindable int;

Much better! With this, we can define many bindable properties is a small space. This is expanded by the compiler into something like this:

_Age field int;

Age int

{

get me.Value;

set PropertyChanged(me.Name, me.Value = value);

}

After being warned about the potential dangers of INotifyPropertyChanged by Michael in the comments of the previous article, I am exploring alternative implementations. Regardless of how it’s implemented (see Part 7 for more details), bindable will be a powerful addition to Archetype developers.

Composite Bindings

Occasionally I need a property which is composed of two or more other properties, and I want to ensure that the proper data binding machinery is notified whenever each constituent property is updated. In C#, I would need to make multiple PropertyChanged calls in each of the individual properties to signal that the composite binding is changing as well. In Archetype, we can use the composite keyword within the composite property itself. Syntactically this is a pull model whereas otherwise we’d be forced to implement a push model. The Archetype syntax looks like this:

FirstName bindable string;

LastName bindable string;

FullName bindable string

get composite FirstName " " LastName;

When the compiler sees the composite keyword after get, it scans the following expression tree. When it finds property references and those properties are marked as bindable, it makes the appropriate transformations to notify of changes. In the underlying implementation, it is a push model, but the developer of Archetype is spared those details. Multiple-statement get functions are supported, and set functions are also supported when using composite.

Bindable Collection Properties

Binding to collections is a little different from binding to single properties. In WPF, Silverlight, and now more broadly in .NET 4.0, types such as ObservableCollection provide several notifications to user interface controls.

Archetype provides special binding expressions to specify common scenarios such as “bind x to the selected item of this collection”.

Here is an example of a collection and its current single selection, bound together in the view model:

Alternatives ObservableCollection<Alternative>;

SelectedAlternative bindable Alternative

bind to Alternatives.SelectedItem;

The following example demonstrates binding a collection to another collection with a discriminating expression (subselection):

Options ObservableCollection<Option>;

SelectedOptions ObservableCollection<Option>

bind to Options.SelectedItems

where item.OptionName.StartsWith("L");

SelectedOptions is an ObservableCollection so that its subselection of contents can itself be bound to a user interface control. The bind to expression sets the binding source, and the where expression specifies a predicate (a function taking “item”, in this case an Option object, as a parameter, and returning a bool) to include only the objects we want. The where expression is optional.

You may notice that SelectedItem and SelectedItems are not valid properties of ObservableCollection. This is because they are extension properties. Archetype supports extension methods just like C#, but it goes further to provide extension properties, indexers, constructors, and operators. I’ll discuss type extensions in a future article.

What this doesn’t address is the possibility of binding a collection to more than one user interface control, and allowing independent selection in each. Because of this, the specifics of binding expressions in Archetype will very likely change before being finalized, but this should give you a taste of the possibilities of language-aware binding.

Next Steps

In the next article, we’ll take a closer look at the import keyword and its special abilities, exception handling, local variable definition, and control flow structures such as if, loop, while, and until. I’ll also introduce one of my favorite Archetype features, the async construct which is used to intuitively call delegates asynchronously.

[Part 3 of this series can be found here.]

Posted in Archetype Language, Functional Programming, Language Innovation | 8 Comments »

The Archetype Language (Part 1)

Posted by Dan Vanderboom on April 26, 2010

Overview

You can follow the news and progress on the Archetype compiler on twitter @archetypelang.

Links to the individual articles:

Part 1 – Properties and fields, function syntax, the me keyword

Part 2 – Start function, named and anonymous delegates, delegate duck typing, bindable properties, composite bindings, binding expressions, namespace imports, string concatenation

Part 3 – Exception handling, local variable definition, namespace imports, aliases, iteration (loop, fork-join, while, unless), calling functions and delegates asynchronously, messages

Part 4 – Conditional selection (if), pattern matching, regular expression literals, agents, classes and traits

Part 5 – Type extensions, custom control structures

Part 6 – If expressions, enumerations, nullable types, tuples, streams, list comprehensions, subrange types, type constraint expressions

Part 7 – Semantic density, operator overloading, custom operators

Part 8 – Constructors, declarative Archetype: the initializer body

Part 9 – Params & fluent syntax, safe navigation operator, null coalescing operators

Conceptual articles about language design and development tools:

Language Design: Complexity, Extensibility, and Intention

Reimagining the IDE

Better Tool Support for .NET

Experiments in Language Design

After 25 years of computer programming in many different languages and a more-than-casual interest in linguistic analysis, I’ve developed a keen appreciation of the best features among them. I also have a relatively steady stream of new language feature ideas. Many years ago I began tinkering with interpreters and compilers. At PDC 2009, I was surprised and delighted to hear the news of the language M (part of Oslo), with which it is possible to write parsers for other languages. Parser generators have been around for a long time, but with such strong support in .NET, it was close enough to home for me to sit up and pay close attention. The desire to create my own language to address the shortcomings I’ve experienced has been perpetually in the back of my mind.

After that PDC, I bought several more books on language and compiler design and began diving in. I’ve been somewhat obsessed with it recently, and the language specification I’m writing is starting to look legitimate, so I think it’s time to start sharing what I’ve come up with and (hopefully) get some good feedback.

The Language

The code name for this language is Archetype. I don’t know if I’ll use this for the final name, but this will do for now. It’s a multi-paradigm language instead of attempting to be pure in any one way, and if you’re familiar with C#, you should be pretty comfortable with the syntax. Yet, if you enjoy the functional programming power of languages like ML, Haskell, OCaml, F#, or Nemerle, or are interested in language constructs to simplify asynchronous and concurrent workflows, you’ll probably like Archetype. While it supports functional programming, one of my goals is to make it appealing and even obvious to developers without a strong functional programming background. It targets the .NET CLR and will therefore run on many platforms and devices, as well as interoperating well with existing .NET assemblies.

To place it in a set of buckets, as languages are classified by paradigm on Wikipedia, Archetype would be considered: imperative, declarative, generic, functional, object-oriented (class-based), language-oriented, reflective, and meta-programming-based.

Current Status

The parser is under development using M (in the Intellipad editor). Though the language design and specification itself is about 70-80% complete, the parser is only about 10% done. Once the parser is a little further along and some interesting samples can be written and parsed, I’ll start building the semantic analyzer and code generation pieces.

The first versions of the compiler will generate C# code instead of IL instructions. It will be a lot faster for me to translate Archetype constructs to C#. The C# compiler, though not concurrent or incremental, is highly optimized and produces great output. This does limit my ability to depart radically from C# semantics, but this is okay: C# is a wonderful language and I plan to keep Archetype pretty closely aligned with it. For example: all of the same operators and precedence rules are borrowed from C#. Archetype does introduce a number of new operators, keywords, and syntactical constructs, but it aims to be close to a superset as far as semantics go.

Disclaimer

Everything is subject to change. Some features are stolen directly from specific languages which I will do my best to identify as I go. Your mileage may vary. Available while quantities last. Batteries not included.

This is a set of experiments. Hopefully it will also be a fun conversation among language enthusiasts.

A Taste of Features

Since this article is already getting long and I have a ton designed already, I’m going to keep the language design part short and present a mere taste of language features.

Properties and Fields

We’ll start with something basic: how to define properties and fields.

Age int;

This first example is a property. The name comes first, which you’ll see everywhere in Archetype. Also notice that the int type is the same as in C#. This is true of all the built-in C# types.

To define a field, the “field” keyword is added before the type. This encourages property definition by default.

Age field int;

The property definition above is short for, but equivalent to, this:

Age int

{

get { return me.Value; }

set { me.Value = value; }

}

When defining properties, it’s so common to require a private “backing field” that I thought it warranted something in the language. C# also does this, but only if you use implicit get and set functions. As soon as you need custom logic for one or the other, you lose this. In Archetype, the “me” keyword refers to the current function or property. In the case of properties, me.Value is the backing field which saves you a line of code for every property that needs one. Reducing code clutter and maximizing information density and conciseness are major design goals in Archetype.

Other “me” properties are available as well, such as Name and Type, which are useful for general-purpose code generation and debugging. In functions, invoking “me” is recursive. C# has the keyword “this” (which Archetype shares), which very usefully refers to the current object. The “me” keyword is roughly analogous to this.GetType().

The curly braces surrounding get and set are optional. If a get or set method is a single statement, the curly braces around it are also optional. We could then write this:

Age int

get me.Value,

set me.Value = value;

Note the lack of a "return" keyword in the get method: “get return” would be redundant. Also, the get and set clauses are separated by a comma. This is a common pattern for multiple clauses in Archetype. The semicolon triggers the end of the statement (in this case, a property declaration statement).

Public class variables are properties by default to promote consistent and forward-looking design techniques (such as compatibility with interfaces), and the field keyword is there to opt out when there is a need (such as performance).

Value types in this language are not nullable by default. The question mark can be used after the value type’s name to indicate it is nullable.

Age int?;

Functions

I’ll have much more to say about functions and delegates in my next article. Here, I’ll just briefly sketch an outline of what they look like and hint at what’s to come.

Save<T> void (Entity T)

{

// …

}

As with properties, the identifier is listed first (along with a generic type parameter), followed by the return type, and finally the parameters in parentheses.

I’ve been comfortable with the type-first definitions in C# for years, but I’ve often begun writing a function whose name came to mind instantly, but whose return type required further thought. However, my fingers would hesitate to type anything until I could determine the return type. After seeing the name come first in Nemerle, it struck me how nice it would be to define functions name-first. The problem that Nemerle has (and Visual Basic, for that matter) is that the parameter list comes next, and the return type is listed last. This has the advantage that it’s easier to write, but suffers from being more difficult to read. When doing a quick scan of code, eyes scanning down through a class, the return types will be all over the place on the screen. In the case of long parameter lists where poorly-formatted code puts the return type off the screen too far to the right, you’d actually have to scroll right to see the return type. This is decidedly worse than type-first.

Then I thought: why not put the two most important parts of a function header first: name and return type, with parameters after them? Then you’d have the best of both worlds: faster to write, and easy to read and understand. Each parameter then follows the “name type” order, consistent with properties and functions.

Next Steps

In my next article on Archetype, I’ll go into much more detail about functions and delegates, where I think Archetype makes some original contributions (at least in terms of syntactical convenience and elegance). I’ll talk about creating basic console applications, the simplest program possible to write, anonymous functions and anonymous delegates, a keyword (actually a custom type extension) to drastically simplify data-bindable property definitions for UI view models, and more.

[Part 2 of this series can be found here.]

Posted in Archetype Language, Functional Programming, Language Innovation | 13 Comments »

Better Tool Support for .NET

Posted by Dan Vanderboom on September 7, 2009

Productivity Enhancing Tools

Visual Studio has come a long way since its debut in 2002. With the imminent release of 2010, we’ll see a desperately-needed overhauling of the archaic COM extensibility mechanisms (to support the Managed Package Framework, as well as MEF, the Managed Extensibility Framework) and a redesign of the user interface in WPF that I’ve been pushing for and predicted as inevitable quite some time ago.

For many alpha geeks, the Visual Studio environment has been extended with excellent third-party, productivity-enhancing tools such as CodeRush and Resharper. I personally feel that the Visual Studio IDE team has been slacking in this area, providing only very weak support for refactorings, code navigation, and better Intellisense. While I understand their desire to avoid stepping on partners’ toes, this is one area I think makes sense for them to be deeply invested in. In fact, I think a new charter for a Developer Productivity Team is warranted (or an expansion of their team if it already exists).

It’s unfortunately a minority of .NET developers who know about and use these third-party tools, and the .NET community as a whole would without a doubt be significantly more productive if these tools were installed in the IDE from day one. It would also help to overcome resistance from development departments in larger organizations that are wary of third-party plug-ins, due perhaps to the unstable nature of many of them. Microsoft should consider purchasing one or both of them, or paying a licensing fee to include them in every copy of Visual Studio. Doing so, in my opinion, would make them heroes in the eyes of the overwhelming majority of .NET developers around the world.

It’s not that I mind paying a few hundred dollars for these tools. Far from it! The tools pay for themselves very quickly in time saved. The point is to make them ubiquitous: to make high-productivity coding a standard of .NET development instead of a nice add-on that is only sometimes accepted.

Consider just from the perspective of watching speakers at conferences coding up samples. How many of them don’t use such a tool in their demonstration simply because they don’t want to confuse their audience with an unfamiliar development interface? How many more demonstrations could they be completing in the limited time they have available if they felt more comfortable using these tools in front of the masses? You know you pay good money to attend these conferences. Wouldn’t you like to cover significantly more ground while you’re there? This is only likely to happen when the tool’s delivery vehicle is Visual Studio itself. Damon Payne makes a similar case for the inclusion of the Managed Extensibility Framework in .NET Framework 4.0: build it into the core and people will accept it.

The Gorillas in the Room

CodeRush and Resharper have both received recent mention in the Hanselminutes podcast (episode 196 with Mark Miller) and in the Deep Fried Bytes podcast (episode 35 with Corey Haines). If you haven’t heard of CodeRush, I recommend watching these videos on their use.

CodeRush Express (which is free)
CodeRush with Refactor!
DXCore (the library you can use to build your own add-ins, and on which CodeRush and Refactor! are built)

For secondary information on CodeRush, DXCore, and the principles with which they were designed, I recommend these episodes of DotNetRocks:

I don’t mean to be so biased toward CodeRush, but this is the tool I’m personally familiar with, has a broader range of functionality, and it seems to get the majority of press coverage. However, those who do talk about Resharper do speak highly of it, so I recommend you check out both of them to see which one works best for you. But above all: go check them out!

Refactor – Rename

Refactoring code is something we should all be doing constantly to avoid the accumulation of technical debt as software projects and the requirements on which they are based evolve. There are many refactorings in Visual Studio for C#, and many more in third-party tools for several languages, but I’m going to focus here on what I consider to be the most important refactoring of them all: Rename.

Why is Rename so important? Because it’s so commonly used, and it has such far-reaching effects. It is frequently the case that we give poor names to identifiers before we clearly understand their role in the “finished” system, and even more frequent that an item’s role changes as the software evolves. Failure to rename items to accurately reflect their current purpose is a recipe for code rot and greater code maintenance costs, developer confusion, and therefore buggy logic (with its associated support costs).

When I rename an identifier with a refactoring tool, all of the references to that identifier are also updated. There might be hundreds of references. In the days before refactoring tools, one would accomplish this with Find-and-Replace, but this is dangerous. Even with options like “match case” and “match whole word”, it’s easy to rename the wrong identifiers, rename pieces of string literals, and so on; and if you forget to set these options, it’s worse. You can go through each change individually, but that can take a very long time with hundreds of potential updates and is a far cry from a truly intelligent update.

Ultimately, the intelligence of the Rename refactoring provides safety and confidence for making far-reaching changes, encouraging more aggressive refactoring practices on a more regular basis.

Abolishing Magic Strings

I am intensely passionate about any tool or coding practice that encourages refactoring and better code hygiene. One example of such a coding practice is the use of lambda expressions to select identifiers instead of using evil “magical strings”. From my article on dynamically sorting Linq queries, the use of “magic strings” would force me to write something like this to dynamically sort a Linq query:

Customers = Customers.Order("LastName").Order("FirstName", SortDirection.Descending);

The problem here is that “LastName” and “FirstName” are oblivious to the Rename refactoring. Using the refactoring tool might give me a false sense of security in thinking that all of my references to those two fields have been renamed, leading me to The Pit of Despair. Instead, I can define a function and use it like the following:

public static IOrderedEnumerable<T> Order<T>(this IEnumerable<T> Source, 
    Expression<Func<T, object>> Selector, SortDirection SortDirection)
{
    return Order(Source, (Selector.Body as MemberExpression).Member.Name, SortDirection);
}

Customers = Customers.Order(c => c.LastName).Order(c => c.FirstName, SortDirection.Descending);

This requires a little understanding of the structure of expressions to implement, but the benefit is huge: I can now use the refactoring tool with much greater confidence that I’m not introducing subtle reference bugs into my code. For such a simple example, the benefit is dubious, but multiply this by hundreds or thousands of magic string references, and the effort involved in refactoring quickly becomes overwhelming.

Coding in this style is most valuable when it’s a solution-wide convention. So long as you have code that strays from this design philosophy, you’ll find yourself grumbling and reaching for the inefficient and inelegant Find-and-Replace tool. The only time it really becomes an issue, then, is when accessing libraries that you have no control over, such as the Linq-to-Entities and the Entity Framework, which makes extensive use of magic strings. In the case of EF, this is mitigated somewhat by your ability to regenerate the code it uses. In other libraries, it may be possible to write extension methods like the Order method shown above.

It’s my earnest hope that library and framework authors such as the .NET Framework team will seriously consider alternatives to, and an abolition of, “magic strings” and other coding practices that frustrate otherwise-powerful refactoring tools.

Refactoring Across Languages

A tool is only as valuable as it is practical. The Rename refactoring is more valuable when coding practices don’t frustrate it, as explained above. Another barrier to the practical use of this tool is the prevalence of multiple languages within and across projects in a Visual Studio solution. The definition of a project as a single-language container is dubious when you consider that a C# or VB.NET project may also contain HTML, ASP.NET, XAML, or configuration XML markup. These are all languages with their own parsers and other language services.

So what happens when identifiers are shared across languages and a Rename refactoring is executed? It depends on the languages involved, unfortunately.

When refactoring a C# class in Visual Studio, the XAML’s x:Class value is also updated. What we’re seeing here is cross-language refactoring, but unfortunately it only works in one direction. There is no refactor command to update the x:Class value from the XAML editor, so manually changing it causes my C# class to become sadly out of sync. Furthermore, this seems to be XAML specific. If I refactor the name of an .aspx.cs class, the Inherits attribute of the Page directive in the .aspx file doesn’t update.

How frequent do you think it is that someone would want to change a code-behind file for an ASP.NET page, and yet would not want to change the Inherits attribute? Probably not very common (okay, probably NEVER). This is a matter of having sensible defaults. When you change an identifier name in this way, the development environment does not respond in a sensible way by default, forcing the developer to do extra work and waste time. This is a failure in UI design for the same reason that Intellisense has been such a resounding success: Intellisense anticipates our needs and works with us; the failure to keep identifiers in sync by default is diametrically opposed to this intelligence. This represents a fragmented and inconsistent design for an IDE to possess, thus my hope that it will be addressed in the near future.

The problem should be recognized as systemic, however, and addressed in a generalized way. Making individual improvements in the relationships between pairs of languages has been almost adequate, but I think it would behoove us to take a step back and take a look at the future family of languages supported by the IDE, and the circumstances that will quickly be upon us with Microsoft’s Oslo platform, which enables developers to more easily build tool-supported languages (especially DSLs, Domain Specific Languages).

Even without Oslo, we have seen a proliferation of languages: IronRuby, IronPython, F#, and the list goes on. A refactoring tool that is hard-coded for specific languages will be unable to keep pace with the growing family of .NET and markup languages, and certainly unable to deal with the demands of every DSL that emerges in the next few years. If instead we had a way to identify our code identifiers to the refactoring tool, and indicate how they should be bound to identifiers in other languages in other files, or even other projects or solutions, the tools would be able to make some intelligent decisions without understanding each language ahead of time. Each language’s language service could supply this information. For more information on Microsoft Oslo and its relationship to a world of many languages, see my article on Why Oslo Is Important.

Without this cross-language identifier binding feature, we’ll remain in refactoring hell. I offered a feature suggestion to the Oslo team regarding this multi-master synchronization of a model across languages that was rejected, much to my dismay. I’m not sure if the Oslo team is the right group to address this, or if it’s more appropriate for the Visual Studio IDE team, so I’m not willing to give up on this yet.

A Default of Refactor-Rename

The next idea I’d like to propose here is that the Rename refactoring is, in fact, a sensible default behavior. In other words, when I edit an identifier in my code, I more often than not want all of the references to that identifier to change as well. This is based on my experience in invoking the refactoring explicitly countless times, compared to the relatively few times I want to “break away” that identifier from all the code that references.

Think about it: if you have 150 references to variable Foo, and you change Foo to FooBar, you’re going to have 150 broken references. Are you going to create a new Foo variable to replace them? That workflow doesn’t make any sense. Why not just start editing the identifier and have the references update themselves implicitly? If you want to be aware of the change, it would be trivial for the IDE to indicate the number of references that were updated behind the scenes. Then, if for some reason you really did want to break the references, you could explicitly launch a refactoring tool to “break references”, allowing you to edit that identifier definition separately.

The challenge that comes to mind with this default behavior concerns code that spans across solutions that aren’t loaded into the IDE at the same time. In principle, this could be dealt with by logging the refactoring somewhere accessible to all solutions involved, in a location they can all access and which gets checked into source control. The next time the other solutions are loaded, the log is loaded and the identifiers are renamed as specified.

Language Property Paths

If you’ve done much development with Silverlight or WPF, you’ve probably run into the PropertyPath class when using data binding or animation. PropertyPath objects represent a traversal path to a property such as “Company.CompanyName.Text”. The travesty is that they’re always “magic strings”.

My argument is that the property path is such an important construct that it deserves to be an core part of language syntax instead of just a type in some UI-platform-specific library. I created a data binding library for Windows Forms for which I created my own property path syntax and type, and there are countless non-UI scenarios in which this construct would also be incredibly useful.

The advantage of having a language like C# understand property path syntax is that you avoid a whole class of problems that developers have used “magic strings” to solve. The compiler can then make intelligent decisions about the correctness of paths, and errors can be identified very early in the cycle.

Imagine being able to pass property paths to methods or return then from functions as first-class citizens. Instead of writing this:

Binding NameTextBinding = new Binding("Name") { Source = customer1; }

… we could write something like this, have access to the Rename refactoring, and even get Intellisense support when hitting the dot (.) operator:

Binding NameTextBinding = new Binding(@Customer.Name) { Source = customer1; }

In this code example, I use the fictitious @ operator to inform the compiler that I’m specifying a property path and not trying to reference a static property called Name on the Customer class.

With property paths in the language, we could solve our dynamic Linq sort problem cleanly, without using lambda expressions to hack around the problem:

Customers = Customers.Order(@Customer.LastName).Order(@Customer.FirstName, SortDirection.Descending);

That looks and feels right to me. How about you?

Summary

There are many factors of developer productivity, and I’ve established refactoring as one of them. In this article I discussed tooling and coding practices that support or frustrate refactoring. We took a deep look into the most important refactoring we have at our disposal, Rename, and examined how to get the greatest value out of it in terms of personal habits, as well as long-term tooling vision and language innovation. I proposed including property paths in language syntax due to its general usefulness and its ability to solve a whole class of problems that have traditionally been solved using problematic “magic strings”.

It gives me hope to see the growing popularity of Fluent Interfaces and the use of lambda expressions to provide coding conventions that can be verified by the compiler, and a growing community of bloggers (such as here and here) writing about the abolition of “magic strings” in their code. We can only hope that Microsoft program managers, architects, and developers on the Visual Studio and .NET Framework teams are listening.

Posted in Data Binding, Data Structures, Design Patterns, Development Environment, Dynamic Programming, Functional Programming, Language Innovation, LINQ, Oslo, Silverlight, Software Architecture, User Interface Design, Visual Studio, Visual Studio Extensibility, Windows Forms | Leave a Comment »

The Future of Programming Languages

Posted by Dan Vanderboom on November 6, 2008

Two of the best sessions at the PDC this year were Anders Hejlsberg’s The Future of C# and a panel on The Future of Programming.

A lot has been said and written about dynamic programming, metaprogramming, and language syntax extensions–not just academically over the past few decades, but also as a recently growing buzz among the designers and users of mainstream object-oriented languages.

Dynamic Programming

After a scene-setting tour through the history and evolution of C#, Anders addressed how C# 4.0 would allow much simpler interoperation between C# and dynamic languages. I’ve been following Charlie Calvert’s Language Futures website, where they’ve been discussing these features early on with the development community. It’s nice to see how seriously they take the feedback they’re getting, and I really think it’s going to have a positive impact on the language as a whole. Initial thoughts revolved around creating a new block of code with code like dynamic { DynamicStuff.SomeUndefinedProperty = “whatever”; }.

But at the PDC we saw that instead dynamic will be a type for our dynamic objects, and so dynamic lookup of members will only be allowed for those variables. Anders’ demo showed off interactions with JavaScript and Python, as well as Office via COM, all without the ugly Type.Missing parameters (optional parameter support also played a part in that). Other ideas revolved around easing Reflection access, and XML document access for Xml nodes dynamically.

Meta-Programming

At the end of his talk, Anders showed a stunning demo of metaprogramming working within C#. It was an early prototype, so all language features were not supported, but it worked similar to Eval where the code was constructed inside a string and then compiled at runtime. But it was flexible and powerful enough that he could create delegates to functions that he Eval’ed up into existence. Someone in the audience asked how this was different from Lisp macros, to which Anders replied: “This is basically Lisp macros.”

Before you get too excited (or worried) about this significant bit of news, Anders made no promises about when metaprogramming would be available, and he subtly suggested that it may very well be a post-4.0 feature. As he said in the Future of Programming Panel, however: “We’re rewriting the compiler in managed code, and I’d say one of the big motivators there is to make it a better metaprogramming system, sort of open up the black box and allow people to actually use the compiler as a service…”

Regardless of when it arrives, I hope they will give serious consideration to providing syntax checking of this macro or meta code, instead of treating it blindly at compile-time as a “magic string”, as has so long plagued the realm of data access. After all, one of the primary advantages of Linq is to enable compile-time checking of queries, to enforce not only strict type checking, but to also more fundamentally ensure that data sources and their members are valid. The irregularity of C#’s syntax, as opposed to Lisp, will make that more difficult (thanks to Paul for pointing this out), but I think most developers will eventually agree it’s a worthwhile cause. Perhaps support for nested grammars in the generic sense will set the stage for enabling this feature.

Language Syntax Extensions

If metaprogramming is about making the compiler available as a service, language extensions are about making the compiler service transparent and extensible.

The majority (but not all) of the language design panel stressed caution in evolving and customizing language syntax and discussed the importance of syntax at length, but they’ve been considering the demands of the development community seriously. At times Anders vacillated between trying to offer alternatives and admitting that, in the end, customization of language syntax by developers would prevail; and that what’s important is how we go about enabling those scenarios without destroying our ability to evolve languages usefully, avoiding their collapse from an excess of ambiguity and inconsistency in the grammar.

“Another interesting pattern that I’m very fond of right now in terms of language evolution is this notion that our static languages, and our programming languages in general, are getting to be powerful enough, that with all of these things we’re picking up from functional programming languages and metaprogramming, that you can–in the language itself–build these little internal DSLs, where you use fluent interface style, and you dot together operators, and you have deferred execution… where you can, in a sense, create little mini languages, except for the syntax.

If you look at parallel extensions for .NET, they have a Parallel.For, where you give the start and how many times you want to go around, and a lambda which is the body you want to execute. And boy, if you squint, that looks like a Parallel For statement.

But it allows API designers to experiment with different styles of programming. And then, as they become popular, we can pick them up and put syntactic veneers on top of them, or we can work to make languages maybe even richer and have extensible syntax like we talked about, but I’m encouraged by the fact that our languages have gotten rich enough that you do a lot of these things without even having to have syntax.” – Anders Hejlsberg

One idea that came up several times (and which I alluded to above) is the idea of allowing nested languages, in a similar way that Linq comprehensions live inside an isolated syntactic context. C++ developers can redefine many operators in flexible ways, and this can lead to code that’s very difficult to read. This can perhaps be blamed on the inability of the C++ language to provide alternative and more comprehensive syntactic extensibility points. Operators are what they have to work with, so operators are what get used for all kinds of things, which change per type. But their meaning gets so overloaded, literally, that they lose any obvious (context-free) meaning.

But operators don’t have to be non-alphabetic tokens, and the addition of new keywords or symbols could be introduced in limited contexts, such as a modifier for a member definition in a type (to appear alongside visibility, overload, override, and shadowing keywords), or within a delimited block of code such as an r-value, or a curly-brace block for new flow control constructs (one of my favorite ideas and an area most in need of extensions). Language extensions might also be limited in scope to specific assemblies, only importing extensions explicitly, giving library authors the ability to customize their own syntax without imposing a mess on consumers of the library.

Another idea would be to allow the final Action delegate parameter of a function to be expressed as a curly-brace-delimited code block following the function call, in lieu of specifying the parameter within parentheses, and removing the need for a semicolon. For example, with a method defined like this:

public static class Parallel
{
    // Action delegate defined last, to take advantage of C# syntactic sugar
    public static void For(long Start, long Count, Action Action)
    {
        // TODO: implement
    }
}

…a future C# compiler might allow you to write code like this:

Parallel.For(0, 10)
{
    // add code here for the Action delegate parameter
}

As Dr. T points out to me, however, the tricky part will consist of supporting local returns: in other words, when you call return inside that delegate’s code block, you really expect it to return from the enclosing method, not the one defined by the delegate parameter. Support for continue or break would also make for a more intuitive fit. If there’s one thing Microsoft does right, it’s language design, and I have a lot of confidence that issues like this will continue to be recognized and ultimately implemented correctly. In reading their blogs and occasionally sharing ideas with them, it’s obvious they’re as passionate about the language and syntax as I am.

The key for language extensions, I believe, will be to provide more structured extensibility points for syntax (such as control flow blocks), instead of opening up the entire language for arbitrary modification. As each language opens up some new aspect of its syntax for extension, a number of challenges will surface that will need to be dealt with, and it will be critical to solve these problems before continuing on with further evolution of the language. Think of all we’ve gained from generics, and the challenges of dealing with a more complex type system we’ve incurred as a result. We’re still getting updates in C# 4.0 to address shortcomings of generics, such as issues regarding covariance and contravariance. Ultimately, though, generics were well worth it, and I believe the same will be said of metaprogramming and language extensions.

Looking Forward

I’ll have much more to say on this topic when I talk about Oslo and MGrammar. The important points to take away from this are that mainstream language designers are taking these ideas to heart now, and there are so many ideas and options out there that we can and will experiment to find the right combination (or combinations) of both techniques and limitations to make metaprogramming and language syntax extensions useful, viable, and sustainable.

Posted in Conferences, Design Patterns, Dynamic Programming, Functional Programming, Language Extensions, LINQ, Metaprogramming, Reflection, Software Architecture | 1 Comment »

Functional Programming as Intensity of Expression

Posted by Dan Vanderboom on September 20, 2008

On my long drive home last night, I was thinking about the .NET Rocks episode with Ted Neward and Amanda Laucher on F# and functional programming. Though they’re writing a book on F# together, it seems even they have a hard time clearly articulating what functional programming is all about, and where it’s all headed in terms of mainstream commercial use… aside from scientific and data transformation algorithms, that is (as with the canonical logging example when people explain AOP).

I think the basic error is in thinking that Functional is a Style of programming. Yet, to say that so-called Imperative-based languages are non-functional is ridiculous. Not in the sense that they “don’t work”, but that they’re based on Objects “instead of” Functions.

This isn’t much different from the chicken-and-egg problem. Though the chicken-and-egg conundrum has a simple (but unobvious) answer, it doesn’t really matter whether the root of program logic is a type or a function. If I write a C# program with a Program class, the Main static function gets called. Some action is the beginning of a program, so one might argue that functions should be the root-most logical construct. However, you’d then have to deal with functions containing types as well as types containing functions, and as types can get very large (especially with deep inheritance relationships), you’d have to account for functions being huge, spanning multiple code files, and so on. There’s also the issue of types being organizational containers for functions (and other members). Just as we use namespaces to organize our types, so we use types to organize functions. This doesn’t prevent us from starting execution with a function or thinking of the program’s purpose functionally; it just means that we organize it inside a logical container that we think of as a “thing”.

Does this limit us from thinking of business processes as functional units? Ted Neward suggests that we’ve been trained to look for the objects in a system, and base our whole design process on that. But this isn’t our only option for how to think about design, even in our so-called imperative languages. If we’re thinking about it wrong, we can and should change the process; we don’t need to blame our design deficiencies on the trivial fact of which programming construct is the root one. In fact, there’s no reason we should use any one design principle to the exclusion of others. Looking for the things in the system is and will remain a valuable approach for discovering and defining database schemas and object models. The very fact that “functional languages” aren’t perceived as especially useful for stateful components isn’t a fault of a style of programming, but is rather a natural consequence of functions being an incomplete aspect of a general purpose programming language. Functional is a subset of expressive capability.

Where “functional languages” have demonstrated real value is not in considering functions as root-level constructs (this may ultimately be a mistake), but rather in increasing the flexibility of a language to be much more expressive when defining functions. Making functions first-class citizens that can be passed as parameters, returned as function values, and stitched together with metaprogramming techniques, is a huge step in the right direction. The use of simple constructs such as operators to match patterns, reverse the evaluation of functions and the flow of values with piping, and perform complex set- and list-based operations, all increase the expressive intensity and density of the functions in a language. This can only add to the richness of our existing object models.

Sticking objects together in extensible and arbitrarily complex structures is routine for us, but now we’re seeing a trend toward the same kind of composability in functions. Of course, even this isn’t new, per se; the environmental forces that demand this power just haven’t become significant enough to require that level of power in mainstream languages, because technology evolution (like evolution in general) tends to work by adapting solutions that are “good enough”.

It’s common to hear how F# is successfully incorporating “both functional and imperative” styles into one language, and this is important because what we need is not so much the transition to a functional style, as I’ve mentioned already, but a growth of greater functional expressiveness and power in existing, successful, object-oriented languages.

So let our best and favorite languages grow, and add greater expressive powers to them, not only for defining functions, but also in declaring data structures, compile-time constraints and guarantees, and anything else that will help to raise the level of abstraction and therefore the productivity with which we can naturally express and fulfill our business needs.

Ultimately, “functional programming” is not a revolutionary idea, but rather an evolutionary step forward. Even though it’s impact is great, there’s no need to start from scratch, to throw out our old models. Incompatibility between functional and imperative is an illusion perpetuated by an unclear understanding of their relationship and each aspect’s purpose.

Posted in Design Patterns, Functional Programming, Object Oriented Design, Problem Modeling, Software Architecture | 4 Comments »

Observations on the Evolution of Software Development

Posted by Dan Vanderboom on September 18, 2008

Neoteny in the Growth of Software Flexibility and Power

Neoteny is a biological phenomenon of an organism’s development observed across multiple generations of a species. According to Wikipedia, neoteny is “the retention, by adults in a species, of traits previously seen only in juveniles”, and accounts for many evolutionary shifts, including the human brain’s ability to remain elastic and malleable later in life than those of our distant ancestors.

So how does this relate to software? Software is a great deal like an organic species. The species emerged (not long ago), incubated in a more or less fragile state for a number of decades, and continues to evolve today. Each software application or system built is a new member of the species, and over the generations they have become more robust, intelligent, and useful. We’ve even formed a symbiotic relationship with software.

Consider the fact that software running on computers was at one time compiled to machine language code for a specific processor. With the invention of platform-independent instruction sets and their associated runtimes performing just-in-time compilation (Java’s JVM and .NET Framework’s CLR), we’ve delayed the actual production of machine language code until it’s actually needed on the target machine. The compiler produces a slightly more abstract representation of the program logic, and an extra translation step at installation or runtime is needed to complete the process to make the software usable.

With the growing popularity of dynamic languages such as Lisp, Python, and the .NET Framework’s upcoming release of its Dynamic Language Runtime (DLR), we’re taking another step of neoteny. Instead of a compiler generating instruction byte codes, a “compiler for any dynamic language implemented on top of the DLR has to generate DLR abstract trees, and hand it over to the DLR libraries” (per Wikipedia). These abstract syntax trees (AST), normally an intermediate artifact created deep within the bowels of a traditional compiler (and eventually discarded), are now persisted as compiler output.

Traits previously seen only in juveniles… now retained by adults. Not too much of a metaphorical stretch! The question is: how far can we go? And I think the answer depends on the ability of hardware to support the additional “just in time” processing that needs to occur, executing more of the compiler’s tail-end tasks within the execution runtime itself, providing programming languages with greater flexibility and power until the compilation stages we currently execute at design-time almost entirely disappear (to be replaced, perhaps, by new pre-processing tasks.)

I remember my Turbo Pascal compiler running on a 33 MHz processor with 1 MB of RAM, and now my cell phone runs at 620 MHz (with a graphics accelerator) and has gigabytes of memory and storage. And yet with the state of things today, the inclusion of language-specific compilers within the runtime is still quite infeasible. In the .NET Framework, there are too many potential languages that people might attempt to include in such a runtime: C#, F#, VB, Boo, IronPython, etc. Trying to cram all of those compilers into a universal runtime that would fit (and perform well) on a cell phone or other mobile device isn’t yet feasible, which is why we have technologies with approaches like System.Reflection.Emit (on the full .NET Framework), and Mono.Cecil (which works on Compact Framework as well). These work at the platform-independent CIL level, and so can interpret and generate programs generically, interact with each others’ components, and so on. One metaprogramming mechanism can therefore be reused across all .NET languages, and this metalinguistic programming trend is being discussed on the C# and other language design teams.

I’ve just started using Mono.Cecil, chosen because it is cross-platform friendly (and open source). The API isn’t very intuitive, but because the source is available, and because extension methods can go a long way to making it more accessible, it’s a great option. The documentation is sparse, and assembly generation has some performance issues, but it’s a work-in-progress with tremendous potential. If you’re doing any kind of static analysis or have any need to dynamically generate and consume types and assemblies (to get around language limitations, for example), I’d encourage you to check it out. A comparison of Mono.Cecil to System.Reflection can be found here. Another library called LinFu, which performs lots of mind-bending magic and actually uses Mono.Cecil, is also worth exploring.

VB10 will supposedly be moving to the DLR to become a truly dynamic language, which considering their history of support for late binding, makes a lot of sense. With a dynamic language person on the C# 4.0 team (Jim Hugunin from IronPython), one wonders if C# won’t eventually go the same route, while keeping its strongly-typed feel and IDE feedback mechanisms. You might laugh at the idea of C# supporting late binding (dynamic lookup), but this is being planned regardless of the language being static or dynamic.

As the DLR evolves, performance optimizations are being discovered and implemented that may close the gap between pre-compiled and dynamically interpreted languages. Combine this with manageable concurrent execution, and the advantages we normally attribute to static languages may soon disappear altogether.

The Precipitous Growth of Software System Complexity

We’re truly on the cusp of a precipitous period of growth for software complexity, as an exploding array of devices and diverse platforms around the world connect in an ever-more immersive Internet. Taking full advantage of parallel and distributed computing environments by solving the challenges of concurrency and coordination, as well as following the trend toward increased integration among software components, is pushing software complexity into new orders of magnitude. The strategies we come up with for organizing these systems will have to take several key factors into consideration, and we will have to raise the level of abstraction to a point that may be hard for us to imagine with our existing tools and languages.

One aspect that’s clear is the rise of declarative or intention-based syntax, whether represented as XML, Domain Specific Langauges (DSL), attribute decoration, or a suite of new visual modeling editors. This is in part a consequence of raising the abstraction level, as lower-level libraries are entrusted to solve common problems and take advantage of common opportunities.

Another is the use of Inversion of Control (IoC) containers and dependency injection in component based architectures, thereby standardizing the lifecycle of the application and its components, and providing a common environment or ecosystem for all of its components, as well as introducing a common protocol for component location, creation, access, and disposal. This level of consistency is valuable for sharing a common understanding of how to troubleshoot software components. The more predictable a component’s interaction with the rest of the system, the easier it is to debug and modify; conversely, the more unique it and its communication system is, the more disparity there is among components, and the more difficult to understand and modify without introducing errors. If software is a species and applications are individuals, then components are the cells of a system.

Even the introduction of functional programming languages into the mainstream over the past couple years is due, in part, to the ability of those languages to provide more declarative support, more syntactic flexibility, and new ways of dealing with concurrency and coordination issues (such as immutable values) and light-weight, ad hoc data structures (tuples).

Balancing the Forces of Coupling, Cohesion, and Modularity

On a fundamental level, the more that components are independent, the less coupled and the more modular and flexible they are. But the more they can communicate with and are allowed to benefit from each other, the more interdependent they become. This adds to cohesiveness and synergy, but also stronger coupling to a community of abstractions.

A composition of services has layers and segments of interdependence, and while there are dependencies, these should be dependencies on abstractions (interfaces and not implementations). Since there will be at least one implementation of each service, and the extensibility exists to build others as needed, dependency is only a liability when the means for fulfilling it are not extensible. Both sides of a contract need to be fulfilled regardless; service-oriented or component-based designs merely provide a mechanism for each side to implement and fulfill its part of the contract, and ideally the system also provides a discovery mechanism for the service provider to publish its availability for other components to discover and consume it.

If you think about software components as a hierarchy or tree of services, with services of one layer depending on more root services, it’s easy to see how this simplifies the perpetual task of adding new and revising existing functionality. You’re essentially editing an outline, and you have opportunities to move services around, reorganize dependencies easily, and have many of the details of the software’s complexity absorbed into this easy-to-use outline structure (and its supporting infrastructure). Systems of arbitrary complexity become feasible, and then relatively routine. There’s a somewhat steep learning curve to get to this point, but once you’ve crossed it, your opportunities extend endlessly for no additional mental cost. At least not in terms of how to compose your system out of individual parts.

Absorbing Complexity into Frameworks

The final thing I want to mention is that a rise in overall complexity doesn’t mean that the job of software developers necessarily has to become more difficult than it is currently. With the proper design of components that abstract away the complexity into reusable frameworks with intuitive interfaces, developers at the business logic level don’t need to be aware of the inner complexity, in the same way that software developers are largely absolved of the responsibility of thinking about the processor’s inner workings. As we build our technology stack higher and higher, like the famed Tower of Babel, we must make sure that it’s organized and structured in a way to support that upward growth and the load imposed upon it… so it doesn’t come crashing down.

The requirements for building components tomorrow will not be the same as they were yesterday. As illustrated in this account of the effort involved in a feature change at Microsoft, in the future, we will also want to consider issues such as tool-assisted refactorability (and patterns that frustrate this, such as “magic strings”), and due to an explosion of component libraries, discoverability of types, members, and their use.

A processor can handle any complexity of instruction and data flow. The trick is in organizing all of this in a way that other developers can understand and work with.

Posted in Compact Framework, Component Based Engineering, Concurrency, Design Patterns, Development Environment, Distributed Architecture, Functional Programming, Mobile Devices, Object Oriented Design, Problem Modeling, Reflection, Service Oriented Architecture, Software Architecture, Visual Studio | 1 Comment »

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Messages

Next Steps

The Purpose of Archetype

Hello World!

Delegates

Delegate Parameters

Named (Non-Anonymous) Delegate Types

Delegate Duck Typing

Bindable Properties

Composite Bindings

Bindable Collection Properties

Next Steps

Experiments in Language Design

The Language

Current Status

Disclaimer

A Taste of Features

Properties and Fields

Functions

Next Steps

Productivity Enhancing Tools

The Gorillas in the Room

Refactor – Rename

Abolishing Magic Strings

Refactoring Across Languages

A Default of Refactor-Rename

Language Property Paths

Summary

Dynamic Programming

Meta-Programming

Language Syntax Extensions

Looking Forward

Neoteny in the Growth of Software Flexibility and Power

The Precipitous Growth of Software System Complexity

Balancing the Forces of Coupling, Cohesion, and Modularity

Absorbing Complexity into Frameworks