#6118: Kind variable falls out of scope in instance declaration
------------------------------+---------------------------------------------
Reporter: goldfire | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.5 | Keywords: PolyKinds
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Consider the following code:
{{{
{-# LANGUAGE PolyKinds, DataKinds, KindSignatures, RankNTypes,
TypeFamilies, FlexibleInstances, ScopedTypeVariables #-}
import GHC.Exts
data Nat = Zero | Succ Nat
data List a = Nil | Cons a (List a)
class SingE (a :: k) where
type Demote a :: *
instance SingE (a :: Bool) where
type Demote a = Bool
instance SingE (a :: Nat) where
type Demote a = Nat
instance SingE (a :: Maybe k) where
type Demote a = Maybe (Demote (Any :: k))
instance SingE (a :: List k) where
type Demote (a :: List k) = List (Demote (Any :: k))
}}}
The instance for {{{Maybe}}} fails to compile because {{{k}}} is out of
scope: {{{Not in scope: type variable `k'}}}
The instance for {{{List}}} fails to compile because the {{{k}}} in the
type family instance is not recognized as the same {{{k}}} in the instance
head:
{{{
Kind mis-match
An enclosing kind signature specified kind `List k1',
but `a' has kind `List k'
In the type `(a :: List k)'
In the type instance declaration for `Demote'
In the instance declaration for `SingE (a :: List k)'
}}}
Note that {{{ScopedTypeVariables}}} is enabled.
This was tested on GHC 7.5.20120519.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6118>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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