Is there any particular reason to not move the instance to the prelude? A module was failing when imported from one place and ok when imported from another, so I spent a frustrating 10 minutes searching for the instance. I couldn't find a way to search haddock for instances (not even grep on the html), nor hoogle, nor anything else. Finally I turned to google and found a couple of haskell-cafe threads with people wondering why other people's code failed for them. This was especially frustrating because I knew I had written my own instance when I originally couldn't find it, then removed my own when I did find it, then forgot where I found it *two times* so I knew it existed, just not where. So this is the third time I've done this search. Now I have a few commented 'import Control.Monad.Error ()' scattered about in a few "low level" modules to try to make sure it's visible everywhere and next time I go hunting I can look in my own modules. If orphan instances are discouraged, shouldn't the instance for Monad Either be declared where Either is? This should probably go for Functor Either too. I notice that Monad Maybe is in Prelude, and it would be a pain if it weren't. If there's no particular reason to not move it, I'll do a libraries proposal... or since it involves the prelude should this be haskell-prime?