Re: [Haskell-beginners] What is wrong with my types signatures.

Thanks a lot.

Here's my guess. I can't be sure without class instances and some function types that are not in code.

sh' <- lift $ runMaybeT $ shapeND d :: Pipe a (XrdMeshFrame sh) IO (Maybe sh?) ...

f <- lift $ runMaybeT $ rowND d (fromIndex sh' i') -- <- sh' is (Maybe sh), guessing that fromIndex does not take a Maybe.

This was the problem. Thanks. PS: The error message was right (as usual :), but I do not know why I was not able to found the cuprite.