Warning with formula matrix when some entries are blank.

[somiaj]

Here is a test problem.

DOCUMENT();
loadMacros(qw(PGstandard.pl PGML.pl));

Context('Matrix');
$M = Matrix([[Formula('x')], [Formula('y')]]);

BEGIN_PGML
[``[$M]=``] [_]*{$M}
END_PGML
ENDDOCUMENT();

If you enter in 'x' for the top row and keep the bottom row blank in the problem editor, and click on check answer or show correct answer twice in a row, a warning will appear.

Can't call method "type" on unblessed reference at [PG]/lib/Value/AnswerChecker.pm line 1882 

Unsure why two submits are needed and the warning won't appear after the first. Also if you make both answer boxes blank, you need to click two submits before the warning goes away.

[dpvc]

One thing to note is that $M is not a matrix of formulas, it is a matrix-valued formula internally, so I suspect that the error is coming from the post-filter that tries to check if the previous formula is equivalent to the current one. That is why it took two tries. Because one of the answers is blank, the previous formula has some non-MathObject entries, and probably can't be compared to another formula, leading to the error. I will have to look into it, but that is my initial thought.

[dpvc]

As an aside, $M could be more easily created via

$m = Matrix([['x'], ['y']]);

as the strings will be parsed as formulas automatically.