Functor type class ported to C#

FuctorTest.cs

// Functor type class ported to C#
// Only tested with: Mono C# compiler version 2.4.2.3
//
// Because .NET doesn't support higher kinded generics, we can't exactly port
// the Functor type class.
// However, if we're willing to give up a bit of type safety, we can get the
// IFuctor interface to work.
//
// We only have to add a single type cast, directly after using fmap. E.g:
//     var xs = new List<int>();
//     var ys = (List<int>) myList.fmap(x => x + 1);

using System;
using GC = System.Collections.Generic;

// class Functor f where
//   fmap :: (a -> b) -> f a -> f b

interface IFunctor<A>
{
	IFunctor<B> fmap<B>(Func<A, B> f);
}

class FunctorTest
{
	public static void Main()
	{
		// List test
		var xs = new List<int>{3, 5, 1, 9};
		var ys = (List<int>) xs.fmap(x => x * x);
		ys.Reverse(); // The cast on previous line is really needed here

		Console.WriteLine(ys);

		// Maybe test 1
		var m1 = new Maybe<string>();
		var m2 = (Maybe<string>) m1.fmap(s => s.ToUpper());

		Console.WriteLine(m2);

		// Maybe test 2
		var m3 = new Maybe<string>("4815162342");
		var m4 = (Maybe<long>) m3.fmap(s => Int64.Parse(s));

		Console.WriteLine(m4);
	}
}

class List<A> : GC.List<A>, IFunctor<A>
{
	public IFunctor<B> fmap<B>(Func<A, B> f)
	{
		List<B> ys = new List<B>();

		foreach (var x in this)
			ys.Add(f(x));

		return ys;
	}

	// "Pretty" printer for lists
	public override string ToString()
	{
		string s = base.ToString() + ": ";

		foreach (var x in this)
			s += x.ToString() + ", ";

		return s;
	}
}

class Maybe<A> : IFunctor<A>
{
	public IFunctor<B> fmap<B>(Func<A, B> f)
	{
		if (value == null)
			return new Maybe<B>();
		else
			return new Maybe<B>(f(value));
	}

	public Maybe() {}
	public Maybe(A x)
	{
		value = x;
	}

	public override string ToString()
	{
		return value == null ? "Nothing" : "Just " + value.ToString();
	}

	A value;
}