Re: Mapping Haskell Concepts To C# 3
On Sat, 2008-05-17 at 20:51 +0100, Sebastian Sylvan wrote:
>
>
> 2008/5/17 Kaveh Shahbazian <kaveh.shahbazian <at> gmail.com>:
> 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<A,B,B> 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<A,B,B> 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<A,B,B> 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<A,B,B> 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.
.
Luke
RSS Feed