eqc.erl

%% Original property, based on docs

prop_seq() ->
  ?FORALL({From, To, Incr}
    , {int(), int(), int()}
    , begin
        try
          %% According to docs
          length(lists:seq(From, To, Incr)) == (To - From + Incr) div Incr
        catch
          %% if we fail, these are supposed to be the only conditions why we fail
          _:_ -> (To < From - Incr andalso Incr > 0) orelse
            (To > From - Incr andalso Incr < 0) orelse
            (Incr == 0 andalso From /= To)
        end
      end).



> eqc:quickcheck(eqc:prop_seq()).
Failed! After 1 tests.
{0,0,0}
false

%% Nope :) Works for lists:seq(0, 0, 0) and it shouldn't. Let's update our conditions

prop_seq() ->
  ?FORALL({From, To, Incr}
    , {int(), int(), int()}
    , begin
        try
          %% According to docs + update
          case {From, To, Incr} of
            {0, 0, 0} -> equals([0], lists:seq(From, To, Incr));
            _         ->length(lists:seq(From, To, Incr)) == (To - From + Incr) div Incr
          end
        catch
          %% if we fail, these are supposed to be the only conditions why we fail
          _:_ -> 
            (To < From - Incr andalso Incr > 0) orelse
            (To > From - Incr andalso Incr < 0) orelse
            (Incr == 0 andalso From /= To)
        end
      end).
      
47> eqc:quickcheck(jesse_eqc:prop_seq()).
....................................................................................................
OK, passed 100 tests
true
48> eqc:quickcheck(jesse_eqc:prop_seq()).
....................................................................................................
OK, passed 100 tests
true
49> eqc:quickcheck(jesse_eqc:prop_seq()).
...Failed! After 4 tests.
{-1,-1,0}
Shrinking xxxx(0 times)
{-1,-1,0}
false



%%% oooops :) Wrong again :)