More GND + role inference woes
eir at cis.upenn.edu
Mon Nov 4 18:52:45 UTC 2013
This is subtler than I thought.
See the ticket commentary here: http://ghc.haskell.org/trac/ghc/ticket/8503
On Nov 4, 2013, at 11:02 AM, Richard Eisenberg <eir at cis.upenn.edu> wrote:
> Bah. This is a bug. I will fix.
> Thanks for pointing it out!
> On Nov 4, 2013, at 10:37 AM, Roman Cheplyaka <roma at ro-che.info> wrote:
>> * Richard Eisenberg <eir at cis.upenn.edu> [2013-10-16 13:28:54-0400]
>>>> 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)
>>>> ( …, 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?
>>>> 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:
>> 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
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