Prolog Queen Attack error

Programming
1

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?

2

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!)

3

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
4

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)

5

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)

6

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.

7

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