#2146: Decomposition rule for equalities is too weak in case of higher-kinded
type
families
-------------------------------------+--------------------------------------
Reporter: chak | Owner: chak
Type: bug | Status: new
Priority: normal | Milestone: 6.10 branch
Component: Compiler (Type checker) | Version: 6.9
Severity: minor | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Multiple
Os: Multiple |
-------------------------------------+--------------------------------------
Comment (by claus):
I just noticed that `GHCi, version 6.9.20080514` accepts partially applied
type families, which would permit `foo2`'s type equality to succeed:
{{{
{-# LANGUAGE TypeFamilies #-}
type family F a :: * -> *
type instance F Int = []
type instance F Bool = Id
type family Id a :: *
type instance Id a = a
foo2 :: (F Int a ~ F Bool [a]) => a -> [a]
foo2 = undefined
}}}
(note the partially applied `Id`)
I thought this was covered in #2157, but both that ticket and its summary
at TypeFunctionsStatus only talk about partially applied type synonyms,
assuming that partially applied type families are already impossible.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2146#comment:4>
GHC <http://www.haskell.org/ghc/>
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