Monday, October 8, 2007

Function Cells in C#

I found this interesting syntactic hack to mimic function cells in C#. Suppose you know the signature and name of a function at compile time, but the value will be computed at runtime and may change over time. First, define a delegate type for the signature of the function:

delegate object FooFunction (string s, int i);

In the class where the function Foo will be defined, we create a static variable to hold the delegate:

static FooFunction currentFooFunction;

and a Property to read it:

public static FooFunction Foo
        return currentFooFunction;

You can now write code like this:
object bar = Foo("test", 4);
which appears to be an ordinary static function call, but in actuality is a call that indirects through the currentFooFunction.

The basic trick here is to have a Property that returns a delegate. The Property code can perform an arbitrary computation to return the delegate, and the compiler knows that a delegate returned from a Property call can be used in a syntactic function position.

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