Monday, March 18, 2013

Disassembled Expression

For this post, I took the sample function body from Fig. 1.3 and compiled it using a C# 2010 compiler (version 4.0.30319.1), then disassembled the executable into IL.

The result was pretty interesting, and not too different from what I imagine happens in a modern compilable functional language.

The most interesting thing, to me, is that the lambda expression was turned into a class (with the automatically generated name <>c__DisplayClass1).  Translating from CIL into C#-like pseudo-code, the class looks something like this:
class <>c__DisplayClass1 {
    public int32 z;
    public int32 k;

    public DateTime <Main>b__0(DateTime x, int32 y) {
        return x.AddYears(int.Parse(y.ToString()) + z * k);

Fig. 3.1: Pseudo-code representation of an internally generated class for a C# lambda expression
This class basically represents the lambda expression.  The environment part is represented by the public members z and k, and the method <Main>b__0 represents the control part.  To invoke this lambda expression, first the environment part needs to be filled out by creating an instance of the class, then setting the values in the environment part (creating a closure).  The closure could then be passed around and eventually evaluated by calling the method <Main>b__0 with the parameters x and y.

So, for me, the next question is going to be this:  If we generate classes like this to represent lambda expressions, how will we curry functions, and how will we create higher order functions?  Those are likely to be the topics of the next post.

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