* Richard Eisenberg
Moreover, I think this solves the other failures in http://www.haskell.org/pipermail/ghc-devs/2013-October/002961.html. Here is one example, in that email. smallcheck has this: newtype Series m a = Series (ReaderT Depth (LogicT m) a) deriving ( …, MonadLogic)
where logict defines MonadLogic thus:
class (MonadPlus m) => MonadLogic m where msplit :: m a -> m (Maybe (a, m a))
So the “bottom line” check above will attempt to find a cocercion betwem msplit’s type with m=Series m, and with m=ReaderT Depth (LogitT m). Right?
Yes.
So on the left of msplit’s arrow, we’ll ask can we prove
Series m a ~R ReaderT Depth (LogicT m) a
Can we show that? Yes, of course… that is the very newtype coercion for Series.
Well, it's the right-hand side of the arrow that's more troublesome, but that works out in this case, too.
I just tried compiling smallcheck with GHC HEAD, and it didn't work out: Test/SmallCheck/SeriesMonad.hs:41:7: Can't make a derived instance of ‛MonadLogic (Series m)’ (even with cunning newtype deriving): it is not type-safe to use GeneralizedNewtypeDeriving on this class; ‛msplit’, at type ‛forall a. m a -> m (Maybe (a, m a))’, cannot be converted safely In the newtype declaration for ‛Series’ So GHC now looks at the methods, but the problem is still there. What do you guys think? I would agree that msplit's type is problematic (due to the nested m's), but Simon and Richard previously said that it should work, so I'm confused. Roman