Can't use messages of the form "Your ... is incorrect" in custom list checkers

[dlglin]

Consider the following MWE:

DOCUMENT();

loadMacros('PGstandard.pl', 'PGML.pl', 'PGcourse.pl');

BEGIN_PGML
[_]{List(1,2)->cmp(list_checker => sub {return(0,"Your answer is incorrect");})}
END_PGML

ENDDOCUMENT();

This should display the feedback message "Your answer is incorrect" to the student when they submit, but it doesn't.

The culprit is:

pg/lib/Value/AnswerChecker.pm

Lines 1689 to 1693 in 1603ef5

	if ($score == 0) {
	my $i = 0;
	while ($i <= $#errors) {
	if ($errors[ $i++ ] =~ m/^Your .* is incorrect$/) { splice(@errors, --$i, 1) }
	}

The problem is that this can match a custom error message set by a problem author, which is then silently deleted.

[dpvc]

A couple of possible solutions:

First, you could use Value->Error("Your answer is incorrect") within the list_checker rather than passing the error in the return value. That should produce the desired error, if you only have one message. (If you need a list of messages, that won't work.)

Second, you could use return(0,"Your answer is incorrect.") (Add a period at the end, which will no longer match the pattern for the skipped messages.) It's a hack, but would do the trick.

In terms of a code-based fix, the lines you cite could be moved to the cmp_list_compare function instead, where it would be performed by the default list compare, but not for custom ones. (Though it could be handled better if moved to there. For example, when the message is set at

pg/lib/Value/AnswerChecker.pm

Line 1809 in 1603ef5

push(@errors, "Your$nth $value is incorrect");

you could keep track of the number of answers that receive that message, and then just check at the end if that number is the same as the number of errors and clear the array if so (when the score is zero).