Haskell: Overlapping instances

Consider the following example program:

next :: Int -> Int
next i
  | 0 == m2 = d2
  | otherwise = 3 * i + 1
  where
    (d2, m2) = i `divMod` 2

loopIteration :: MaybeT (StateT Int IO) ()
loopIteration = do
  i <- get
  guard $ i > 1
  liftIO $ print i
  modify next

main :: IO ()
main = do
  (`runStateT` 31) . runMaybeT . forever $ loopIteration
  return ()

It can only use get instead of lift get because instance MonadState s m => MonadState s (MaybeT m) is defined in the MaybeT module.

Many such instances are defined in kind of a combinatoric explosion manner.

It would have been nice (although impossible? why?) if we had the following type-class:

{-# LANGUAGE MultiParamTypeClasses #-}

class SuperMonad m s where
  lifts :: m a -> s a

Let's try to define it as such:

{-# LANGUAGE FlexibleInstances, ... #-}

instance SuperMonad a a where
  lifts = id

instance (SuperMonad a b, MonadTrans t, Monad b) => SuperMonad a (t b) where
  lifts = lift . lifts

Using lifts $ print i instead of liftIO $ print i works, which is nice.

But using lifts (get :: StateT Int IO Int) instead of (get :: MaybeT (StateT Int IO) Int) doesn't work.

GHC (6.10.3) gives the following error:

Overlapping instances for SuperMonad
                            (StateT Int IO) (StateT Int IO)
  arising from a use of `lifts'
Matching instances:
  instance SuperMonad a a
  instance (SuperMonad a b, MonadTrans t, Monad b) =>
           SuperMonad a (t b)
In a stmt of a 'do' expression:
    i <- lifts (get :: StateT Int IO Int)

I can see why "instance SuperMonad a a" applies. But why does GHC think that the other one does, too?

Solution

To follow up ephemient's excellent answer: Haskell type classes use an open-world assumption: some idiot can come along later and add an instance declaration that's not a duplicate and yet overlaps with your instance. Think of it as an adversary game: if an adversary can make your program ambiguous, the compiler bleats.

If you're using GHC you can of course say to the compiler "to hell with your paranoia; allow me my ambiguous instance declaration":

{-# LANGUAGE OverlappingInstances #-}

If later evolution of your program leads to overload resolution you didn't expect, the compiler gets 1,000 I-told-you-so points :-)

Deprecation Note

This pragma has been deprecated since GHC 7.10, and per-instance pragmas should be used instead. More detail can be found in the GHC documentation.