Behaviour Preservation across Code Versions in Erlang

Listing 1

happy0.erl.

(1) -spec main(pos_integer(),pos_integer()) ->

(2)  [pos_integer()].

(3) main(N, M) ->

(4)  happy_list(N, M, []).

(5)

(6) happy_list(_, N, L) when length(L) =:= N ->

(7)  lists:reverse(L);

(8) happy_list(X, N, L) ->

(9)  Happy = is_happy(X),

(10)  if Happy ->

(11)   happy_list(X + 1, N, [X∣L]);

(12)  true ->

(13)   happy_list(X + 1, N, L) end.

(14)

(15) is_happy(1) -> true;

(16) is_happy(4) -> false;

(17) is_happy(N) when N > 0 ->

(18)  N_As_Digits =

(19)   [Y - 48 ∣∣

(20)   Y <- integer_to_list(N)],

(21)  is_happy(

(22)   lists:foldl(

(23)    fun(X, Sum) ->

(24)     (X X) + Sum

(25)    end,

(26)    0,

(27)    N_As_Digits));

(28) is_happy(_) -> false.