#3409: type variable out of scope in worker/wrapper transformation
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Reporter: judahj | Owner: chak
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.10.3
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Changes (by simonpj):
* difficulty: => Unknown
Comment:
No, it's quite different. Here is a much smaller example:
{{{
data T = forall a. T a (Funny a)
type Funny a = Bool
f :: T -> Bool
f (T x n) = n
}}}
This kills Lint because the type of the RHS is `(Funny a)` where `a` is
existentially quantified.
For future reference, I boiled down the original program to this simpler
form:
{{{
newtype Size s = Size Int
data ArrayS d e = ArrayS d e
data Array1 e = forall s . Array1 (Size s) (ArrayS (Size s) e)
-- Array1 :: forall e s. Size s -> ArrayS (Size s) e -> Array1 e
copy :: Int -> Array1 a -> Array1 a
copy _ (Array1 s a) = Array1 s $ (ArrayS s (bang a))
-- Array1 s :: ArrayS (Size s) a -> Array1 a
-- s :: Size s
-- a :: ArrayS (Size s) a
-- ArrayS :: Size s -> a -> ArrayS (Size s) a
-- i :: AccessIx (ArrayS (Size s) a) = Ix s
-- bang a :: AccessResult (ArrayS (Size s) a) = a
-- ArrayS s (bang a) :: ArrayS (Size s) (AccessResult (ArrayS (Size s)
a))
class Access a where
type AccessResult a
bang :: a -> AccessResult a
instance Access (ArrayS d a) where
type AccessResult (ArrayS d a) = a
}}}
Just recording this for now. There's nothing actually wrong, but Lint
should not bleat. I'll have to think about what the best fix is.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3409#comment:3>
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