Recursion examples - FP in Erlang 2.3

recursion.erl

%% @doc Recursion examples - FP in Erlang 2.3

-module(recursion).
-export([fib/1, pieces/1, test/0]).

%% @doc Give N-th element of Fibonacci sequence

fib(0) -> 0;
fib(1) -> 1;
fib(N) when N > 1 ->
    fib(N-2) + fib(N-1).

test_fib() ->
    1 = recursion:fib(2),
    3 = recursion:fib(4),
    8 = recursion:fib(6),
    987 = recursion:fib(16),
    % 1100087778366101931 = recursion:fib(88), % Ctrl-C Ctrl-C helps
    ok.

%% @doc Step by step evaluation of fib(4)
%%
%% fib(4)
%% = fib(2) + fib(3)
%% = (fib(0) + fib(1)) + (fib(1) + fib(2))
%% = (0 + 1) + (1 + (fib(0) + fib(1)))
%% = 1 + (1 + (0 + 1))
%% = 1 + (1 + 1)
%% = 1 + 2
%% = 3

%% @doc Maximum number of pieces into which one can cut a piece of paper with N
%% straight cuts

pieces(0) -> 1; % no cuts so paper in 1 piece
pieces(1) -> 2;
pieces(N) when N > 1 ->
    N + pieces(N - 1).

test_pieces() ->
    4 = recursion:pieces(2),
    7 = recursion:pieces(3),
    11 = recursion:pieces(4),
    ok.

test() ->
    ok = test_fib(),
    ok = test_pieces(),
    ok.

TDD - as you suggested :-)