#6049: Kind variable generalization error in GADTs
---------------------------------------+------------------------------------
Reporter: goldfire | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type
checker)
Version: 7.5 | Keywords: PolyKinds
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Testcase:
Blockedby: | Blocking:
Related: |
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The following code fails to compile with 7.5.20120426:
{{{
{-# LANGUAGE DataKinds, KindSignatures, PolyKinds, GADTs,
ExistentialQuantification #-}
data SMaybe :: (k -> *) -> Maybe k -> * where
SNothing :: forall (s :: k -> *). SMaybe s Nothing
SJust :: forall (s :: k -> *) (a :: k). s a -> SMaybe s (Just a)
}}}
The reported error is
{{{
Kind mis-match
The first argument of `SMaybe' should have kind `k1 -> *',
but `s' has kind `k -> *'
In the type `SMaybe s (Just a)'
In the definition of data constructor `SJust'
In the data declaration for `SMaybe'
}}}
The code compiles fine without the explicit {{{forall}}} annotations, but
other, more involved code I'm working on (involving {{{Proxy}}}) requires
the annotations, so simply omitting the explicit kind annotations is not
always a viable workaround.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6049>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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