More GND + role inference woes
Richard Eisenberg
eir at cis.upenn.edu
Mon Nov 4 16:02:25 UTC 2013
Bah. This is a bug. I will fix.
Thanks for pointing it out!
Richard
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)
>>> 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
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