Re: [Haskell-cafe] Mapping Haskell Concepts To C# 3

17 May 2008

      On Sat, 2008-05-17 at 20:51 +0100, Sebastian Sylvan wrote:
...
2008/5/17 Kaveh Shahbazian :
        I have question on mapping some Haskell concepts to C# 3 ones.
        Maybe there are not any strict equivalents; yet it helps:
1 - What is the equivalent of "Type Constructor" in C#?
Class declaration. Generic ones. E.g. List<int>, is a type where the
type constructor List has been applied to the type int.
2 - What is the equivalent of "Data Constructor" in C#?
Constructors, I guess.
3 - What is the logical implementation of pattern matching in
        C#? (For example using structures with indicator fields or
        using interfaces and inheritance and dynamically dispatch in
        calling overloaded methods. Also this question contain a
        hidden one...GADTs!)
You can use an abstract base class, and then inherit one class for
each constructor (e.g. base class Expression, and concrete subclasses
Mul, Add, etc.). Then you can use runtime type reflection to figure
out which kind of Expression you have and branch on it.
Ideally you would use dynamic dispatch, not "type reflection" for this,
e.g.
abstract class List<A> {
    public abstract int Length();
    public abstract List<A> Append(List<A> xs);
    public abstract B Foldr<B>(Func cons, B nil);
}

class Nil<A> : List<A> {
    public Nil() {}
    public override int Length() { return 0; }
    public override List<A> Append(List<A> xs) { return xs; }
    public override B Foldr<B>(Func cons, B nil) { return nil; }
}

class Cons<A> : List<A> {
    public Cons(A a, List<A> as) {
        Head = a; Tail = as;
    }
    public override int Length() { return 1 + Tail.Length(); }
    public override List<A> Append(List<A> xs) {
        return new Cons<A>(Head, Tail.Append(xs));
    }
    public override B Foldr<B>(Func cons, B nil) {
        return cons(Head,Tail.Foldr(cons,nil));
    }

    public readonly A Head;
    public readonly List<A> Tail;
}

The Foldr method does make some constructors a bit special, but not too
much.

class Append<A> : List<A> {
    public Append(List <A> xs, List<A> ys) {
        Front = xs; Back = ys;
    }
    public override int Length() {
        return Front.Length() + Back.Length();
    }
    public override List<A> Append(List<A> ys) {
        return new Append<A>(this, ys);
    }
    public override B Foldr<B>(Func cons, B nil) {
        Front.Foldr(cons, Back.Foldr(cons, nil));
    }

    public readonly List<A> Front, Back;
}

We could, of course, define Cons.Append to use the Append class instead.

Re: [Haskell-cafe] Mapping Haskell Concepts To C# 3

Derek Elkins