-module(fn).
-author("rgibson").

%% API
-export([compose/1, element_as/3, either/2]).

compose([H,T]) ->
    fun(X) -> H(T(X)) end;
compose([H|T]) ->
    TCompose = compose(T),
    fun(X) -> H(TCompose(X)) end.

%% not using function guards to save extra element function call
element_as(integer, Pos, Tuple) ->
    ensure_element_type(fun is_integer/1, integer, Pos, Tuple);
element_as(atom, Pos, Tuple) ->
    ensure_element_type(fun is_atom/1, atom, Pos, Tuple);
element_as(list, Pos, Tuple) ->
    ensure_element_type(fun is_list/1, list, Pos, Tuple);
element_as(T, Pos, Tuple) ->
    error(unmatched_elem_msg(Pos, T, Tuple)).

ensure_element_type(Pred, Type, Pos, Tuple) ->
    Elem = element(Pos, Tuple),
    case Pred(Elem) of
        true -> Elem;
        false -> error(unmatched_elem_msg(Pos, Type, Tuple))
    end.

unmatched_elem_msg(Pos, Type, Tuple) ->
    lists:flatten(io_lib:format(<<"Unable to match element ~p as ~p in tuple ~p">>,
                                [Pos, Type, Tuple])).

either(Fn, Args) ->
    try
        {ok, apply(Fn, Args)}
    catch
        _: Error -> {error, Error}
    end.