#2572: scoped type variables: bad error message, and wrong semantics?
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Reporter: igloo | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.10.1
Component: Compiler (Type checker) | Version: 6.9
Severity: normal | Keywords:
Difficulty: Unknown | Testcase: reify
Architecture: Unknown | Os: Unknown
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With this module (a cut-down version of reify):
{{{
{-# OPTIONS -fglasgow-exts #-}
module Foo where
type GTypeFun = forall a . a -> ()
gmapType :: Int -> GTypeFun
gmapType _ (_ :: a) = undefined
}}}
GHC gives the rather perplexing error:
{{{
Foo.hs:8:12:
The scoped type variables `a' and `a'
are bound to the same type (variable)
Distinct scoped type variables must be distinct
In the pattern: _ :: a
In the definition of `gmapType': gmapType _ (_ :: a) = undefined
}}}
Having locations for the two `a`s would be useful.
This module gives the same error:
{{{
{-# OPTIONS -fglasgow-exts #-}
module Foo where
gmapType :: Int -> (forall a . a -> ())
gmapType _ (_ :: a) = undefined
}}}
GHC 6.8.2 accepts both modules.
I don't think that the forall in the type signature should bring `a` into
scope for the body, as it doesn't bring it into scope for the type
signature.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2572>
GHC <http://www.haskell.org/ghc/>
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