Is it the Algebra module for Moode? http://docs.moodle.org/24/en/question/type/algebra, https://tracker.moodle.org/browse/CONTRIB/component/10326, https://moodle.org/mod/forum/discuss.php?d=98670
I worked nine years on such a system, called "ActiveMath" http://activemath.org/ then and now "Math-Bridge" http://math-bridge.org/, where I designed and implemented precisely the exercise system that does the answer evaluation: http://matracas.org/escritos/#exercise_system_report
There you can apply different tests to the student's answer, and one common use was to first check for the exact correct form, like "1/2", with a "syntactical" comparison of the expression tree parsed from the textual input. You would get the "Correct!" feedback for that one.
Then you could compare it "semantically" with the expected answer, which sent the expression to a Computer Algebra System for simplification in a specific context (set of simplification rules, depending on the task), so if you answered "2/4" you would get the feedback "That's correct, but not fully simplified. Please give the irreducible form.".
The exercise author can include any number of such classifier expressions to catch different forms of the correct answer, different half-done answers, and wrong answers, giving adequate feedback for each.
Feedback is not just text, but a complete "exercise subgraph" that could be entire sub-exercises intended to correct the misconception corresponding to the wrong answer given by the student.