Functions as results in practice

hof2.erl

-module(hof2).

-export([composition/1, iteration/1, test/0, twice/2]).

composition(Fs) ->
    fun (X) -> lists:foldr(fun (F, Y) -> F(Y) end, X, Fs)
    end.

composition_test() ->
    F = composition([fun (X) -> X * 2 end,
		     fun (X) -> X + 1 end]),
    6 = F(2),
    ok.

twice(F, X) -> F(F(X)).

twice_test() -> 18 = twice(fun (X) -> 3 * X end, 2), ok.

iteration(0) -> fun (_, X) -> X end;
iteration(N) ->
    fun (F, X) -> F2 = iteration(N - 1), F2(F, F(X)) end.

iteration_test() ->
    F = iteration(3), 8 = F(fun (X) -> 2 * X end, 1).

test() ->
    composition_test(), twice_test(), iteration_test(), ok.