#1795: typechecker loops on simple program with fundep
-------------------------+--------------------------------------------------
Reporter: guest | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone: 6.8.1
Component: Compiler | Version: 6.8
Severity: major | Resolution:
Keywords: | Difficulty: Unknown
Os: Multiple | Testcase: yes
Architecture: x86 |
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Old description:
> The program below causes the "Renamer/typechecker" phase to loop.
> Confirmed with GHC 6.8.20070927 on Linux and 6.9.20071018 on Windows (so
> I'm not absolutely certain it's present in latest 6.8, but it seems very
> likely).
>
> The program has an infinite type - the definition of translate causes
> (String, a) to be unified with a. Removing the fundep in MkA causes a
> reasonable error to be reported.
>
> {{{
> {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
> FlexibleInstances #-}
> module X() where
>
> data A a = A
>
> class MkA a b | a -> b where
> mkA :: a -> A b
>
> instance MkA a a where
>
> translate :: (String, a) -> A a
> translate a = mkA a
> }}}
>
> Ganesh
New description:
The program below causes the "Renamer/typechecker" phase to loop.
Confirmed with GHC 6.8.20070927 on Linux and 6.9.20071018 on Windows (so
I'm not absolutely certain it's present in latest 6.8, but it seems very
likely).
The program has an infinite type - the definition of translate causes
(String, a) to be unified with a. Removing the fundep in MkA causes a
reasonable error to be reported.
{{{
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
FlexibleInstances #-}
module X() where
data A a = A
class MkA a b | a -> b where
mkA :: a -> A b
instance MkA a a where
translate :: (String, a) -> A a
translate a = mkA a
}}}
Ganesh
PS: See also #1797 and #1781
Comment (by simonpj):
Definitely a bug. It happens because the improvement gives rise to an
equality `(a ~ (String,a))`, and this equality is simply generated but not
discharged, by `TcSimplify.tcImprove`.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1795#comment:3>
GHC <http://www.haskell.org/ghc/>
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