FutureLearn - Functional Programming in Erlang 1.9

first.erl

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

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

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

% Area of a triangle (Heron's formula)
area(A,B,C) ->
    S = (A+B+C)/2,
    math:sqrt(S*(S-A)*(S-B)*(S-C)).

square(X) ->
    mult(X, X).

treble(X) ->
    mult(3, X).

second.erl

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

% C = sqrt(A*A+B*B)
hypotenuse(A, B) ->
    math:sqrt(first:square(A)+first:square(B)).

% P = A+B+C (C=hypotenuse)
perimeter(A, B) ->
    A+B+hypotenuse(A,B).

% First option:
% Right triangle => you can use fromula A = 1/2BH
% Where A = area, B = base and H = height
area(A, B) ->
    first:mult(1/2,first:mult(A,B)).

% Second option:
% Use heron's formula from module first
area_heron(A, B) ->
    C = hypotenuse(A, B),
    first:area(A, B, C).

test.erl

-module(test).
-include_lib("eunit/include/eunit.hrl").

% To run tests in Erlang shell:
% c(test).
% eunit:test(test).

second_test_() ->
    [?_assert(second:area(3,4) =:= second:area_heron(3,4)),
     ?_assert(second:perimeter(3,4) =:= 12.0),
     ?_assert(second:hypotenuse(3,4) =:= 5.0)].

first_test_() ->
    [?_assert(first:double(4) =:= 8),
     ?_assert(first:treble(4) =:= 12),
     ?_assert(first:square(4) =:= 16),
     ?_assert(first:mult(3,4) =:= 12),
     ?_assert(first:area(3,4,5) =:= 6.0)].