But what is the inferred type for f x = do { [v] <- x, return v } I think the proposal is to have a MonadFail constraint. As far as I understand what you are saying, it would make sense for f :: Monad m => m [a] -> m a g x = do { [v] <- x; return v } (with failure giving a runtime error). From: Libraries [mailto:libraries-bounces@haskell.org] On Behalf Of Johan Tibell Sent: 10 June 2015 14:01 To: Mario Blažević Cc: Haskell Libraries Subject: Re: MonadFail proposal (MFP): Moving fail out of Monad An idea. Could the new desugaring be the same as for normal functions with a missing case? Code that uses fail would have to add the constraint still, but code that really didn't want to use fail would 1) get a warning about an incomplete pattern match and 2) get a runtime error for the failed pattern match produced by GHC. Is there anything I'm not thinking of here why this wouldn't work?