** Using WeBWorK **

|Matrices must have at least one entry at line 41 of (eval 14775)
line $ans = Matrix($reply->{'diff'});
apparently I cannot get the sage result using WEBWORK dictionary (hash perl)
here is the code (found in your github)

## Template for calling Sage from within a WebWork pg file
## BEGIN_DESCRIPTION
## Sample problem embedding Sage in WW
## END_DESCRIPTION

DOCUMENT();

loadMacros(
"PGstandard.pl",
"MathObjects.pl",
);

###############################################
##
## pg initializations and regular WeBWorK code


Context("Matrix");
Context()->variables->add(u => 'Real');
Context()->variables->add(v => 'Real');

$SageCode1=<<END;
def ff():
return choice([sin,cos,exp,ln,lambda x: x])
def combination(*args):
return prod([choice(i) for i in args])
u,v=var('u v')
def simple_expression():
return combination(range(1,10),[u,u^2],[v,v^2])
def random_expression():
return ff()(simple_expression())+ff()(simple_expression())
f(u,v)=(random_expression(),random_expression(),simple_expression())
WEBWORK['function']=f(u,v)
WEBWORK['diff']=f.diff()(u,v)
END

# Sage
$reply = AskSage($SageCode1,{seed=>$problemSeed, accepted_tos=>true});

$F = Formula($reply->{function});
$ans = Matrix($reply->{'diff'});

Context()->texStrings;
BEGIN_TEXT
Find the total derivative \(Df\) of \[f(u,v)={$F}\]
\{ $ans->ans_array(50)\}
END_TEXT

Context()->normalStrings;
ANS($ans->cmp());
ENDDOCUMENT();

Any Idea ? Marc