#5595: Type error: expected type = actual type ???
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Reporter: basvandijk | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.2.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: GHC rejects valid program
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As [http://www.haskell.org/pipermail/glasgow-haskell-
users/2011-October/021109.html mentioned] on the glasgow-haskell-users
mailinglist, I'm working on a new design of
[http://hackage.haskell.org/package/monad-control monad-control]. However
I get a type error I don't understand. Here's an isolated example:
{{{
{-# LANGUAGE UnicodeSyntax, RankNTypes, TypeFamilies #-}
class MonadTransControl t where
type St t ∷ * → *
liftControl ∷ Monad m ⇒ (Run t → m α) → t m α
restore ∷ Monad o ⇒ St t γ → t o γ
type Run t = ∀ n β. Monad n ⇒ t n β → n (St t β)
foo :: (Monad m, MonadTransControl t) => (Run t -> m α) -> t m α
foo f = liftControl f
}}}
Type checking `foo` this gives the following error:
{{{
Couldn't match expected type `Run t' with actual type `Run t'
Expected type: Run t -> m α
Actual type: Run t -> m α
In the first argument of `liftControl', namely `f'
In the expression: liftControl f
}}}
When I remove the type annotation of `foo` the program type checks.
But when I ask ghci the type of `foo` it tells me it's the same type:
{{{
> :t foo
foo :: (Monad m, MonadTransControl t) => (Run t -> m α) -> t m α
}}}
Note that when I change the associated type synonym to a associated data
type the error disappears.
Is this a bug in GHC?
I'm using ghc-7.2.1.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5595>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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