Property based testing example

kata.erl

-module(kata).
-compile([export_all]).
-include_lib("proper/include/proper.hrl").

%% Test that calendar:is_leap_year/1 is fully compliant with the rules for computing leap years.
%% https://en.wikipedia.org/wiki/Leap_year#Algorithm

-define(ELSE, true).

gen_year() -> ?SUCHTHAT(Y, integer(),  Y > 0 ).

my_is_leap_year(Year) ->
    if
        Year rem 400 == 0 -> true;
        Year rem 100 == 0 -> false;
        Year rem 4 == 0 -> true;
        ?ELSE -> false
    end.

prop_leap_year() ->
    ?FORALL(Year, gen_year(), my_is_leap_year(Year) == calendar:is_leap_year(Year)).