#7123: Dictionary method should be applicable even in existential context
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Reporter: heisenbug | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Changes (by igloo):
* difficulty: => Unknown
Old description:
> {{{
> {-# LANGUAGE GADTs, TypeFamilies #-}
>
> class Foo a where
> type Bar a
> baz :: Bar a -> Int
>
> instance Foo Int where
> type Bar Int = (Int, Int)
> baz (i, _) = i
>
> data Quux where
> Quux :: Foo a => a -> Bar a -> Quux
>
> q = Quux (42 :: Int) (1, 2)
>
> i = case q of Quux _ b -> baz b
> }}}
>
> repro.hs:18:31:
> Could not deduce (Bar a0 ~ Bar a)
> from the context (Foo a)
> bound by a pattern with constructor
> Quux :: forall a. Foo a => a -> Bar a -> Quux,
> in a case alternative
> at repro.hs:18:15-22
> NB: `Bar' is a type function, and may not be injective
> The type variable `a0' is ambiguous
> Possible fix: add a type signature that fixes these type variable(s)
> In the first argument of `baz', namely `b'
> In the expression: baz b
> In a case alternative: Quux _ b -> baz b
> Failed, modules loaded: none.
New description:
{{{
{-# LANGUAGE GADTs, TypeFamilies #-}
class Foo a where
type Bar a
baz :: Bar a -> Int
instance Foo Int where
type Bar Int = (Int, Int)
baz (i, _) = i
data Quux where
Quux :: Foo a => a -> Bar a -> Quux
q = Quux (42 :: Int) (1, 2)
i = case q of Quux _ b -> baz b
}}}
{{{
repro.hs:18:31:
Could not deduce (Bar a0 ~ Bar a)
from the context (Foo a)
bound by a pattern with constructor
Quux :: forall a. Foo a => a -> Bar a -> Quux,
in a case alternative
at repro.hs:18:15-22
NB: `Bar' is a type function, and may not be injective
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
In the first argument of `baz', namely `b'
In the expression: baz b
In a case alternative: Quux _ b -> baz b
Failed, modules loaded: none.
}}}
--
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7123#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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