Prolog Queen Attack error

I’m doing the Prolog Queen Attack exercise and getting the following error on my solution

% PL-Unit: create_tests ...... done
% PL-Unit: attack_tests 
% All 6 tests passed
ERROR: -g run_tests,halt: append_args/3: Unknown procedure: '$messages':to_list/2

Is this an issue with my solution?

Probably! If you want a more concrete answer, consider requesting mentoring (after submitting code using the CLI) or sharing your code here. (Please use a ``` codeblock and not an image to share code; use the preview to ensure it’s properly formatted!)

I’m using the web editor, and can’t “submit” because the test doesn’t succeed. This is my code:

%! create(+DimTuple)
% The create/1 predicate succeeds if the DimTuple contains valid chessboard 
% dimensions, e.g. (0,0) or (2,4).
create((DimX, DimY)) :-
	0< DimX, DimX < 8,
    0 < DimY, DimY < 8.

%! attack(+FromTuple, +ToTuple)
% The attack/2 predicate succeeds if a queen positioned on ToTuple is 
% vulnerable to an attack by another queen positioned on FromTuple.
attack((FromX, FromY), (ToX, ToY)):-
	FromX = ToX; % same row
    FromY = ToY; % same col
    subtract(ToY, FromY) is subtract(FromX, ToX); % neg slope
    subtract(ToY, FromY) is subtract(ToX, FromX). % pos slope

OK, i think this is a bug with the tests itself. I get the error even with no solution code at all, with a blank file. (expectedly, i’m not passing the tests either)

I’ve found the Prolog track does sometimes just give awful error messages and local development’s a bit nicer, but the actual failure here is expected. Submitting a blank file is going to fail since it can’t find the attack predicate. It doesn’t have this error if you instead submit it with the always-true definition:
attack((FromX, FromY), (ToX, ToY)).

The actual issue is that a subtract/2 function doesn’t exist - you need to use -/2 (docs)

Thanks everyone. Turns out the root error was that I didn’t know prolog uses a special =:=/2 arithmetic equality. My 3rd and 4th diagonal were passing due to coincidence.

Thanks for getting back to us! Our Prolog introduction video will also cover this (it’s not out yet though).