Re: More GND + role inference woes

Bah^2. This is subtler than I thought. See the ticket commentary here: http://ghc.haskell.org/trac/ghc/ticket/8503 Richard On Nov 4, 2013, at 11:02 AM, Richard Eisenberg wrote:

Bah. This is a bug. I will fix.

Thanks for pointing it out! Richard

On Nov 4, 2013, at 10:37 AM, Roman Cheplyaka wrote:

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* Richard Eisenberg [2013-10-16 13:28:54-0400]

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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.

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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

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