instances are the pride and joy of Haskell's module system.. 1) instance documentation Once introduced, instances propagate more or less everywhere, but I'm having trouble identifying the source modules for surprising instances with ghc, where I don't have the sources of the libraries, only .hi files and haddock-generated documentation.. Here's an example error message from trying to link together two pieces of Haskell software, both working fine on their own (using ghc-5.04 on cygwin): Duplicate instance declarations: ../tools/base/lib/Monads/ExceptM.hs:26: Functor (Either x) <No locn>: Functor (Either e) Duplicate instance declarations: ../tools/base/lib/Monads/ExceptM.hs:29: Monad (Either x) <No locn>: Monad (Either e) Now, how am I supposed to find out where those other instances come from? Obviously, the error message could be better, but let's just assume I'd be interested in the question independent of any error messages: where do instances come from? The way haddock groups that information in the ghc docs is not very helpful in this case: looking at the Prelude doc, I see that, yes, Either is an instance of Functor and Monad (huh, in the Haskell Prelude? I don't think so..), but not where those unexpected instances are defined.. Fortunately, ghc-5.02.3 on sunos is more informative in its error message (is that a sunos/windows difference, or did the info get lost between ghc-5.02.3 and ghc-5.04?) and suggests that the other instance might come from MonadError (btw, why?). Even armed with that suggestion, I don't know an easy way to verify this, though: the docs don't mention a Functor instance for Either in Control.Monad.Error, they do in Control.Monad.. The .hi files don't seem to give me a hint, either.. Am I missing a trick? Or is the documentation of instances incomplete? 2) ghci instance handling One way (the good old primitive way) is to test. ghci seems to confirm that the Functor instance comes from MonadError, not from Monad, not from Prelude, but.. ghci doesn't seem to clean up instances when switching between modules? $ ghci -package lang ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 5.04, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Loading package haskell98 ... linking ... done. Loading package lang ... linking ... done. Prelude> Prelude> fmap not (Right True::Either Bool Bool) <interactive>:1: No instance for (Functor (Either Bool)) arising from use of `fmap' at <interactive>:1 In the definition of `it': fmap not (Right True :: Either Bool Bool) Prelude> :m Control.Monad Prelude Control.Monad> fmap not (Right True::Either Bool Bool) <interactive>:1: No instance for (Functor (Either Bool)) arising from use of `fmap' at <interactive>:1 In the definition of `it': fmap not (Right True :: Either Bool Bool) Prelude> :m MonadError Prelude MonadError> fmap not (Right True::Either Bool Bool) Right False Prelude MonadError> :m Prelude Prelude> fmap not (Right True::Either Bool Bool) Right False Prelude> fmap not (Left True::Either Bool Bool) Left True Prelude> Cheers, Claus PS. user's guide, ghci section, has %lt; instead of < (<)