list_maker.erl

-module(listmaker).
-export([add_to_head/2, add_to_tail/2, add_to_tail_recursive/2]).
-import(flatten, [flatten/1]).

%% add a non-list item to the beginning of a list
-spec(add_to_head(any(), list()) -> list()).
add_to_head(Item, List) -> [Item | List].

%% add a non-list item to the end of a list
-spec(add_to_tail(any(), list()) -> list()).
add_to_tail(Item, List) -> lists:reverse([ Item | lists:reverse(List) ]).

%% add a non-list item to the end of a list, recursively
-spec(add_to_tail_recursive(any(), list()) -> list()).
add_to_tail_recursive(Item, List) -> add_to_tail_recursive(Item, lists:reverse(List), poop).

%% helper to first revese the string received
add_to_tail_recursive(Item, [], poop) -> lists:reverse(Item);
%% then build the list with the item to add, backwards
add_to_tail_recursive(Item, [H|T], poop) -> add_to_tail_recursive( flatten([Item |  [ H | [] ]]), T, poop).

shell.erl

Eshell V5.9.3.1  (abort with ^G)
1> c(listmaker).
{ok,listmaker}
2> listmaker:add_to_tail(8, [1,2,3]).
[1,2,3,8]
3> 


2> listmaker:add_to_tail_recursive(5, [1,2,3,4]).
[1,2,3,4,5]

to return a list without the constructor you should pass in a list as the Item and glue the two lists together with the ++ operator, such as:

-spec(add_to_tail(list(), list()) -> list()).
add_to_tail(Item, List) -> List ++ Item.

expected output should be:

1> c(listmaker).
{ok,listmaker}
2> MyList = [4,5,6].
[4,5,6]
3> listmaker:add_to_tail([8], MyList).
[4,5,6,8]

now with the recursive version i get this:

10> listmaker:add_to_tail_recursive(5, [1,2,3,4]).
** exception error: bad argument
     in function  hd/1
        called as hd([])
     in call from listmaker:add_to_tail_recursive/2 (listmaker.erl, line 10)

now i just get:

36> listmaker:add_to_tail_recursive(5, [1,2,3,4]).
[4,3,2,1,5]

WHERE TO REVERSE? arrararrr.
i know i need to reverse the string i'm getting but i don't know how/when/where.

ok so that poop atom is not the most elegant solution... but works for now?