Delegate returning itself

[Ali Çehreli via Digitalmars-d-learn]

On 12/06/2014 07:28 AM, Jonathan Marler wrote:
> Is there a way to create a delegate that returns itself?

Y combinator helps exactly with that:

   http://rosettacode.org/wiki/Y_combinator#D

Copying the code from there:

import std.stdio, std.traits, std.algorithm, std.range;

auto Y(S, T...)(S delegate(T) delegate(S delegate(T)) f) {
     static struct F {
         S delegate(T) delegate(F) f;
         alias f this;
     }
     return (x => x(x))(F(x => f((T v) => x(x)(v))));
}

void main() { // Demo code:
     auto factorial = Y((int delegate(int) self) =>
         (int n) => 0 == n ? 1 : n * self(n - 1)
     );

     auto ackermann = Y((ulong delegate(ulong, ulong) self) =>
         (ulong m, ulong n) {
             if (m == 0) return n + 1;
             if (n == 0) return self(m - 1, 1);
             return self(m - 1, self(m, n - 1));
     });

     writeln("factorial: ", 10.iota.map!factorial);
     writeln("ackermann(3, 5): ", ackermann(3, 5));
}

Ali