First encounter with Erlang

dates.erl

-module(dates).
-author("unai").

%% API
-export([classify_day/1]).

classify_day({saturday}) -> classify_day({weekend});
classify_day({sunday}) -> classify_day({weekend});
classify_day({weekend}) -> weekend;
classify_day({_}) -> weekday.

geometry.erl

-module(geometry).
-author("unai").

%% API
-export([area/1]).

area({rectangle, Width, Height}) -> Width * Height;
area({circle, Radius}) -> 3.14159 * Radius * Radius;
area({square, Side}) -> Side * Side.

main.erl

-module(main).
-author("unai").
-export([main/0, server/0]).

-import(geometry, [area/1]).
-import(dates, [classify_day/1]).
-import(myfactorial, [factorial/1]).
-import(io, [fwrite/2, fwrite/1]).

main() ->
  fwrite("Hello, world!~n"),
  %Area = area({rectangle, 10, 4}),
  %fwrite("The area of a rectangle of 10x4 is ~w~n", [area({rectangle, 10, 4})]),
  fwrite("Sunday is the ~w and thursday is ~w~n", [classify_day({sunday}), classify_day({thursday})]),
  fwrite("The factorial of 4 is ~w and of 8 is ~w~n", [factorial(4), factorial(8)]),
  Pid = spawn(fun() -> server() end),
  Pid ! {self(), {rectangle, 6, 10}},
  receive N -> fwrite("Rectangle's area is ~w~n", [N]) end,
  fwrite("Circle's area is ~w~n", [rpc(Pid, {circle, 5})]).


server() ->
  receive
    {From, {rectangle, Width, Height}} ->
      Area = Width * Height,
      %fwrite("The area of the rectangle is ~w~n", [Area]),
      From ! Area,
      server();
    {From, {circle, Radius}} ->
      Area = 3.14159 * Radius * Radius,
      %fwrite("The area of the circle is ~w~n", [Area]),
      From ! Area,
      server();
    {From, Request} ->
      %fwrite("I don't know how to calculate that :(~n"),
      From ! {error, Request},
      server()
  end.

rpc(Pid, Request) ->
  Pid ! {self(), Request},
  receive
    {Pid, Response} ->
      Response
  end.

myfactorial.erl

-module(myfactorial).
-author("unai").

%% API
-export([factorial/1]).

factorial(N) when N > 1 -> N * factorial(N - 1);
factorial(_) -> 1.