Hi, In recent versions of transformers, 'ErrorT' has been deprecated in favour of 'ExceptT': https://hackage.haskell.org/package/transformers-0.4.2.0/docs/Control-Monad-... This doesn't seem like a trivial like-for-like replacement since the implementation of 'fail' is different - in ErrorT it produces an exception within the ErrorT itself, and in ExceptT it just calls fail in the underlying monad. Is there any guidance or simple trick to find if a program is relying on the old behaviour? Otherwise migrating could be rather painful/dangerous, particularly as tests often don't cover exceptional cases well. I've spent a while searching for past discussion of the change, but haven't found anything, so please do point me at anything relevant if this has already been discussed. Cheers, Ganesh