Note that my opposition is against making 'Monad (Either a)' less defined and less tunable than it is at the moment.
It's a common trade-off: the Error constraint limits the instances that are available, but gives you a bit more when you have an instance. One must weigh the relative value of fail vs the unconstrained instance.
If there is no full definition of 'Monad (Either a)' for all 'a', then qualifying 'a' by an additional type class seems to be the standard Haskell solution. As I said, even if you just want to drop 'Error', you could define 'fail s = Left (error s)'. That would still be less defined than the current instance, but more defined than the proposed instance. So you've deliberately chosen not to use 'Left' and not to represent 'fail' in the data type.
If you are just concerned about the Error constraint, simply provide a default instance that maps strMsg to error (see example of default instances with user override in base Data.Typable). That way, you'd get 'Monad (Either a)' without Error constraint, but with strictly more defined fail ('Left _|_' instead of '_|_'), and others can add even more defined fail when needed.
Do you mean an overlapping instance? There are problems with that too.
Yes to both. But it would avoid the problems with the unconstrained Monad instance for Either a, wouldn't it? And most of the time, neither the OverlappingInstances nor the Error constraint would be visible to client code, while the 'fail = error' default would be quite visible (with your instance version, none of the Either lines in client.hs below would return any information before throwing the error). Claus ----------------------- Lib.hs {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE TypeSynonymInstances #-} {-# LANGUAGE FlexibleInstances #-} module Lib where import Control.Monad class Error a where strMsg :: String -> a instance Error String where strMsg = id instance Error a where strMsg = error instance Error a => Monad (Either a) where Left l >>= _ = Left l Right r >>= f = f r return = Right fail = Left . strMsg instance Error a => MonadPlus (Either a) where mzero = Left (strMsg "mzero") Left _ `mplus` x = x Right r `mplus` _ = Right r --------------------client.hs import Lib import Control.Monad x,y,z :: MonadPlus m => m Int x = do 1 <- return 2; return 21 y = do 2 <- return 2; return 42 z = x `mplus` y main = do print (z::Maybe Int) print (x::Maybe Int) print (z::[Int]) print (x::[Int]) print (z::Either String Int) print (z::Either Bool Int) print (x::Either String Int) print (x::Either Bool Int) ----------------- output *Main> main Just 42 Nothing [42] [] Right 42 Right 42 Left "Pattern match failure in do expression at .. client.hs:5:7" Left *** Exception: Pattern match failure in do expression at .. client.hs:5:7