Hi, I was surprised to see my code break with ghc-7, due to changed monad instances: With ghc-6.12.3 I have: Prelude> :m Control.Monad.Error Prelude Control.Monad.Error> fail "Bla" :: Either String () Loading package mtl-1.1.0.2 ... linking ... done. Left "Bla" but with ghc-7.0.4 and ghc-7.2.2 I get a fatal exception: *Main> fail "Bla" :: Either String () *** Exception: Bla I always viewed "Either String" as an alternative to Maybe with a failure message. The instance in ghc-7 comes from Control.Monad.Instances: instance Monad (Either e) where return = Right Left l >>= _ = Left l Right r >>= k = k r which indeed uses "error" as default implementation of fail. Is there (supposed to be) an overlapping "instance Monad (Either String)" somewhere else? Any comments (or notes) that I missed? Cheers Christian