Y combinator: Difference between revisions

====[http://rosettacode.org/mw/index.php?oldid=287744#Swift Swift]====

<code>T -> TResult</code> in Swift corresponds to <code>Func<T, TResult></code> in C#.

'''Verbatim'''

The more idiomatic version doesn't look much different.

<lang csharp>using System;

static class Program {

struct RecursiveFunc<F> {

public Func<RecursiveFunc<F>, F> o;

}

static Func<A, B> Y<A, B>(Func<Func<A, B>, Func<A, B>> f) {

var r = new RecursiveFunc<Func<A, B>> { o = w => f(_0 => w.o(w)(_0)) };

return r.o(r);

}

static void Main() {

// C# can't infer the type arguments to Y either; either it or f must be explicitly typed.

var fac = Y((Func<int, int> f) => _0 => _0 <= 1 ? 1 : _0 * f(_0 - 1));

var fib = Y((Func<int, int> f) => _0 => _0 <= 2 ? 1 : f(_0 - 1) + f(_0 - 2));

Console.WriteLine($"fac(5) = {fac(5)}");

Console.WriteLine($"fib(9) = {fib(9)}");

}

}</lang>

Without recursive type:

<lang csharp> public static Func<A, B> Y<A, B>(Func<Func<A, B>, Func<A, B>> f) {

Func<dynamic, Func<A, B>> r = z => { var w = (Func<dynamic, Func<A, B>>)z; return f(_0 => w(w)(_0)); };

return r(r);

}</lang>

Recursive:

<lang csharp> public static Func<In, Out> Y<In, Out>(Func<Func<In, Out>, Func<In, Out>> f) {

return x => f(Y(f))(x);

}</lang>