[Haskell-beginners] case in monadic function

Hello! I have a function like this: http://rafb.net/p/8E66FI29.html, starting with evalState. The problem is with checkPredName cause it's return type is SemanticError what's not a monadic value but case waiting for m a (as the error message sais), but if I use return to checkPredName then ofc the pattern match will fail. How can I fix this? Regards, David