Insane Fun's

I ran across an Erlang fun that could recursively calculate exponentials and it was very perplexing at first.

fun(X) ->

  G = fun(_, 0) -> 1;

         (F, Y) -> Y * F(F, Y-1)

  end,

  G(G, X)

end.

This is double the fun from what I am used to seeing. The outer fun constructs another fun that is designed to take itself as a parameter. Mind blown!

After looking this over for a bit I realized it wasn’t tail-call optimized so after a couple stabs I took it to the next level.

fun(X) ->

  G = fun(_, A, 0) -> A;

         (F, A, N) -> F(F, A*N, N-1)

  end,

  G(G, 1, X)

end.

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