First, the Fraction context doesn't require that factions be reduced.
More important, however, is the fact that the check for the reduced fraction is done in the Fraction answer checker, not in the production of a Fraction object, and when you have an Assignment rather than a plain Fraction, it is the Assignment's answer checker that is being used, not the Fraction's.
Here is one way to get the effect you are looking for:
loadMacros(
"contextFraction.pl",
"parserAssignment.pl",
);
Context("LimitedFraction")->flags->set(reduceFractions => 0);
parser::Assignment->Allow;
$ans = Compute("x=1/2");
Context()->texStrings;
BEGIN_TEXT
\($ans\) is \{$ans->ans_rule\}
END_TEXT
Context()->normalStrings;
ANS($ans->cmp->withPostFilter(sub {
my $ansHash = shift;
if ($ansHash->{score}) {
my ($cvar,$cfrac) = $ansHash->{correct_value}->value; # get the variable and fraction
my ($svar,$sfrac) = $ansHash->{student_value}->value; # get the variable and fraction
my $check = $cfrac->cmp->evaluate($sfrac->string); # do a full answer check
$ansHash->{score} = $check->{score}; # transfer the score
$ansHash->{ans_message} = $check->{ans_message}; # and any message
}
return $ansHash;
}));
Here we use a post-filter on the answer checker to get the correct and student fractions out of the assignments, and perform a new answer check on the fraction parts separately. This will produce the "not reduced" message, if needed, and we copy the score and message into the original answer hash.
Hope that helps.
Davide