mylists.erl

-module(mylists).

% interleave X with Y.
interleave(X, Y) -> interleave(X, Y, []).
interleave([], [], A) -> A;
interleave([], T, A) -> lists:reverse(A) ++ T;
interleave(T, [], A) -> lists:reverse(A) ++ T;
interleave([X | XT], [Y | YT], A) -> interleave(XT, YT, [X, Y | A]);

% intersperse X among Ys
intersperse(X, Ys) -> intersperse(X, Ys, []).
intersperse(X, [], A) -> lists:reverse(A);
intersperse(X, [Y], A) -> lists:reverse([ Y, X | A ]);
intersperse(X, [Y | YT], A) -> intersperse(X, YT, [ Y, X | A ]).


prop_interleave_length() ->
    ?FORALL({X, Y}, {list(any()), list(any())},
            length(X) + length(Y) == length(interleave(X, Y))).

prop_intersperse_length() ->
    ?FORALL({B, Ts}, {any(), list(any())},
            length(intersperse(B, Ts)) == (length(Ts) * 2) - 1).
prop_intersperse_start() ->
    ?FORALL({B, Ts}, {any(), list(any())},
            Ts == [] orelse
                hd(Ts) == hd(intersperse(B, Ts)).