FutureLearn Functional Programming In Erlang 1.9: First Erlang Program

first.erl

-module(first).
-export([double/1, mult/2, area/3, square/1, treble/1]).

mult(X,Y) ->
    X*Y.

double(X) ->
    mult(2,X).

area(A,B,C) ->
    S = (A+B+C)/2,
    math:sqrt(S*(S-A)*(S-B)*(S-C)).

square(X) ->
    X*X.

treble(X) ->
    X*3.

first_tests.erl

-module(first_tests). 
-include_lib("eunit/include/eunit.hrl").
-export([square_test/0, square_test2/0]).

square_test() -> 
    % problem with following is that it returns the
    % result of the last equation.
    1 = first:square(1), 
    4 = first:square(2), 
    9 = first:square(3).

square_test2() -> 
    % apparently these are using macros.
    % returns ok if all succeeds, otherwise
    % fails with expected and actual values - nice
    ?assertEqual(1, first:square(1)), 
    ?assertEqual(4, first:square(2)), 
    ?assertEqual(9, first:square(3)).

second.erl

-module(second).
-export([hypotenuse/2, perim/2, area/2]).

hypotenuse(Side1, Side2) ->
    math:sqrt(first:square(Side1) + first:square(Side2)).

perim(Side1, Side2) ->
    % Could also have done this in one line (but liked breaking it over two):
    % Side1 + Side2 + hypotenuse(Side1, Side2).
    Side3 = hypotenuse(Side1, Side2),
    Side1 + Side2 + Side3.

area(Side1, Side2) ->
    % Should have used first:mult...
    %(Side1 * Side2) / 2.
    first:mult(Side1, Side2) / 2.