FutureLearn - Functional Programming in Erlang 2.11 - Where do I begin?

take.erl

-module(more_lists).
-export([reverse/1,take/2,take2/2,test_take/0,test_take2/0]).

%% Take with direct recursion
-spec take(integer(), [T]) -> [T].
take(_, []) -> []; % handles case where N > length(Xs)
take(0, _) -> [];
take(N, [X|Xs]) when N > 0 -> [X | take(N-1, Xs)].

test_take() ->
    [] = take(0,"hello"),
    "hell" = take(4,"hello"),
    "hello" = take(5,"hello"),
    "hello" = take(9,"hello"),
    success.

%% Reverse a list (helper for the tail recursive take).
-spec reverse([T]) -> [T].
reverse(Xs) ->  reverseR(Xs, []).

-spec reverseR([T], [T]) -> [T].
reverseR([], Ys) -> Ys;
reverseR([X|Xs], Ys) -> reverseR(Xs, [X|Ys]).

%% Take with tail recursion
-spec take2(integer(), [T]) -> [T].
take2(N, Xs) -> reverse(takeR(N, Xs, [])).

%% Helper for tail recursive take
-spec takeR(integer(), [T], [T]) -> [T].
takeR(_, [], Ys) -> Ys;
takeR(0, _, Ys) -> Ys;
takeR(N, [X|Xs], Ys) when N > 0 ->
    takeR(N-1, Xs, [X | Ys]).

test_take2() ->
    [] = take2(0,"hello"),
    "hell" = take2(4,"hello"),
    "hello" = take2(5,"hello"),
    "hello" = take2(9,"hello"),
    success.