#2157: Equality Constraints with Type Families
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Reporter: hpacheco | Owner: chak
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.9
Severity: normal | Resolution:
Keywords: | Testcase:
Architecture: Multiple | Os: Multiple
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Comment (by hpacheco):
I understood your points and they do make sense, but do you think that
this is a feasible feature request or it should never be supported?
I have just remembered that, still for the family
{{{
type family F a x :: *
type instance F [a] x = Either () (a,x)
type instance F Int x = Either () x
}}}
we can write "partial-evaluated classes for F" whenever not all type
parameters are in use.
For the Functor class this would mean:
{{{
class FunctorF x where
fmapF :: (a -> b) -> F x a -> F x b
instance FunctorF [a] where
fmapF _ (Left _) = Left ()
fmapF f (Right (a,x)) = Right (a,f x)
instance FunctorF Int where
fmapF _ (Left ()) = Left ()
fmapF f (Right n) = Right (f n)
}}}
At first glance it is less generic but seems to work.
However, a previous function (for the family F a :: * -> *)
{{{
hylo :: (Functor (F d)) => d -> (F d c -> c) -> (a -> F d a) -> a -> c
hylo d g h = g . fmap (hylo d g h) . h
}}}
if translated to
{{{
hylo :: (FunctorF d) => d -> (F d c -> c) -> (a -> F d a) -> a -> c
hylo d g h = g . fmapF (hylo d g h) . h
}}}
does not compile with the explicit type signature.
Without a type signature it compiles and infers the signature (does not
make sense to me)
{{{
hylo :: forall t d c a. (FunctorF d) => t -> (F d c -> c) -> (a -> F d a)
-> a -> c
}}}
However, if this signature is passed explicitly, it does not compile
again.
There must be a bug somewhere in this scheme, or am I missing something
huge?
Regards,
hugo
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2157#comment:4>
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