#7024: Problems with polymorphic kinds imported from module
------------------------------+---------------------------------------------
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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There seems to be a problem when importing polymorphic kinds from a module
in a cabalized package. One way to spot the problem is to {{{cabal install
singletons}}}, import {{{Singletons.Lib}}} and run {{{:t SNil}}} in ghci.
ghci reports a stack overflow. Another way to access (what I think is) the
same error is to compile the following:
{{{
{-# LANGUAGE TypeFamilies, DataKinds, PolyKinds, TypeOperators,
GADTs, RankNTypes #-}
import Singletons.Lib
type family Map (f :: k1 -> k2) (a :: [k1]) :: [k2]
type instance Map f '[] = '[]
type instance Map f (h ': t) = (f h) ': (Map f t)
sMap :: forall (f :: k1 -> k2) (b :: [k1]).
(forall a. Sing a -> Sing (f a)) -> Sing b -> Sing (Map f b)
sMap _ SNil = SNil
sMap f (SCons h t) = SCons (f h) (sMap f t)
}}}
The error message is
{{{
Couldn't match kind `BOX' against `[k1]'
Kind incompatibility when matching types:
k1 :: BOX
b :: [k1]
In the pattern: SNil
In an equation for `sMap': sMap _ SNil = SNil
}}}
Neither of these problems surface when checking the type of {{{SNil}}} or
compiling the above code when the singletons sources are at hand and are
interpreted.
I tried to make a small test case for this but was unable to reproduce the
error after a number of attempts, including using Template Haskell to
generate the {{{Sing}}} instance for type-level lists, as is done in the
singletons library.
This was all tested on 7.5.20120620. There's a good chance this bug is
related to #7022.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7024>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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