Erlang list manipulation for https://www.futurelearn.com/courses/functional-programming-erlang/1/steps/155694#fl-comments

l2.erl

%%%-------------------------------------------------------------------
%%% @author Josh Rehman
%%% @copyright (C) 2017, JavaJosh Enterprises
%%% @doc
%%%
%%% @end
%%% Created : 27. Feb 2017 6:54 PM
%%%-------------------------------------------------------------------
-module(l2).
-author("Josh Rehman").

%% API
-export([double/1, evens/1, take/2, odds/1]).


% Define an Erlang function double/1 to double the elements of a list of numbers.
double(X) -> double(X, []).

double([],Doubled) -> Doubled;
double([X|Xs], Doubled) ->
  double(Xs, [2*X | Doubled]).

% Define a function evens/1 that extracts the even numbers from a list of integers.
evens(X) ->
  evens(X, []).
evens([], Evens) ->
  Evens;
evens([X|Xs], Evens) when X rem 2 == 0 ->
  evens(Xs, [X|Evens]);
evens([_X|Xs], Evens) ->
  evens(Xs, Evens).

% Define a function odds/1 that extracts odd numbers, using the case statement. (This one is a bonus).

%%odd(X) when X rem 2 == 1 -> true;
%%odd(X) -> false.

% Wanted to use a case statement, but failed.
odds(X) -> odds(X,[]).
odds([],X) -> X;
odds([X|Xs], Odds) ->
  case X of
    X when (X rem 2) == 1 -> odds(Xs, [X|Odds]);
    _ -> odds(Xs,Odds)
  end.

% Define a function take that takes the first N elements from a list.
% Problem: it returns them in reverse order.
take(N,X) ->
  reverse(take(N,X,[])).

take(0, _X, Taken) ->
  Taken;
take(N,[X|Xs], Taken) ->
  take(N-1, Xs, [X | Taken]).

% This feels a little wasteful. Would be better to have
% an "append" function somewhere.
reverse(X) -> reverse(X,[]).
reverse([],Result) -> Result;
reverse([X|Xs],Result) ->
  reverse(Xs, [X|Result]).