hof.erl

-module(hof).
-export([doubleAll/1,evens/1,product/1,zip/2,zip_with/3]).
-export([test/0]).

%% Using higher-order functions

doubleAll(Xs) -> lists:map(fun(X) -> 2*X end, Xs).

doubleAll_test() ->
  [] = doubleAll([]),
  [2] = doubleAll([1]),
  [2,4,6,8] = doubleAll([1,2,3,4]),
  passed.

evens(Xs) -> lists:filter(fun(X) -> X rem 2 == 0 end, Xs).

evens_test() ->
  [] = evens([]),
  [] = evens([1,3,5]),
  [2] = evens([2]),
  [2] = evens([2,3,5]),
  [2,4] = evens([1,2,3,4,5]),
  passed.

product(Xs) -> lists:foldl(fun (X, P) -> X * P end, 1, Xs).

product_test() ->
  1 = product([]),
  2 = product([2]),
  6 = product([2,3]),
  passed.

%% Zipping

zip(X, Y) ->
  zip(X, Y, []).
zip([], _, Ac) -> lists:reverse(Ac);
zip(_, [], Ac) -> lists:reverse(Ac);
zip([X|Xs], [Y|Ys], Ac) ->
  zip(Xs, Ys, [{X,Y}|Ac]).

zip_test() ->
  [] = zip([], []),
  [] = zip([1,2], []),
  [] = zip([], [1,2]),
  [{1,2}] = zip([1], [2]),
  [{1,2},{3,4}] = zip([1,3], [2,4]),
  [{1,2},{3,4}] = zip([1,3,5], [2,4]),
  [{1,2},{3,4}] = zip([1,3], [2,4,6]),
  passed.

zip_with(F, X, Y) ->
  zip_with(F, X, Y, []).
zip_with(_, [], _, Ac) -> lists:reverse(Ac);
zip_with(_, _, [], Ac) -> lists:reverse(Ac);
zip_with(F, [X|Xs], [Y|Ys], Ac) ->
  zip_with(F, Xs, Ys, [F(X,Y)|Ac]).

zip_with_test() ->
    [] = zip_with(fun(X,Y) -> X*Y end, [], []),
    [] = zip_with(fun(X,Y) -> X*Y end, [1], []),
    [] = zip_with(fun(X,Y) -> X*Y end, [], [1]),
    [2] = zip_with(fun(X,Y) -> X*Y end, [2], [1]),
    [2,9] = zip_with(fun(X,Y) -> X*Y end, [2,3], [1,3]),
    passed.

zip_with_map(F, X, Y) ->
  lists:map(fun ({A, B}) -> F(A,B) end, zip(X, Y)).

zip_with_map_test() ->
  [] = zip_with_map(fun(X,Y) -> X*Y end, [], []),
  [] = zip_with_map(fun(X,Y) -> X*Y end, [1], []),
  [] = zip_with_map(fun(X,Y) -> X*Y end, [], [1]),
  [2] = zip_with_map(fun(X,Y) -> X*Y end, [2], [1]),
  [2,9] = zip_with_map(fun(X,Y) -> X*Y end, [2,3], [1,3]),
  passed.

zip2(X, Y) ->
  zip_with(fun(A,B) -> {A,B} end, X, Y).

zip2_test() ->
  [] = zip2([], []),
  [] = zip2([1,2], []),
  [] = zip2([], [1,2]),
  [{1,2}] = zip2([1], [2]),
  [{1,2},{3,4}] = zip2([1,3], [2,4]),
  [{1,2},{3,4}] = zip2([1,3,5], [2,4]),
  [{1,2},{3,4}] = zip2([1,3], [2,4,6]),
  passed.

%% Tests

test() ->
  doubleAll_test(),
  evens_test(),
  product_test(),
  zip_test(),
  zip_with_test(),
  zip_with_map_test(),
  zip2_test(),
  passed.