Import the attached file from hugs and evaluate the following expression: get ((AttributeLeaf (MyLabel "x") 4)::Env1) (MyLabel "x") The result should be Just 4. But if you replace the operator =?= with the builtin operator == on line 31 and then retry the same expression, the evaluation diverges. The =?= operator is defined as l =?= l' = l == l' Since I have found this workaround (after two days of looking for bugs in my code) the matter is no longer urgent to me, but I'd like to know if and when you fix it. Thank you.