#303: Rebindable syntax doesn't work as advertised
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Reporter: nobody | Owner: nobody
Type: bug | Status: reopened
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.8.3
Severity: normal | Resolution:
Keywords: | Testcase:
Architecture: Unknown | Os: Unknown
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Changes (by ryani):
* status: closed => reopened
* os: => Unknown
* testcase: =>
* version: 6.4 => 6.8.3
* architecture: => Unknown
* resolution: Fixed =>
Comment:
This seems to be broken in 6.8.3, at least for "weird" monad types. Test
case follows:
{{{
{-# LANGUAGE NoImplicitPrelude #-}
module T where
import qualified Prelude as P
class IxMonad m where
return :: a -> m i i a
(>>=) :: m i j a -> (a -> m j k b) -> m i k b
(>>) :: m i j a -> m j k b -> m i k b
m >> n = m >>= \_ -> n
fail :: P.String -> m i j a
fail s = P.error s
data T a b c = T
instance IxMonad T where
return _ = T
m >>= f = T
fail _ = T
testM :: T (a,b) b a
testM = T
test1 = testM >>= \x -> return x
test2 = do
x <- testM
return x
}}}
test1 compiles fine, but test2 (which should be identical up-to-sugaring)
fails.
{{{
$ ghc -c -XNoImplicitPrelude T.hs
T.hs:27:3:
Occurs check: cannot construct the infinite type: b = (a, b)
Expected type: T (a, b) b t
Inferred type: T (a, b) (a, b) a
In the expression: return x
In the expression:
do x <- testM
return x
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/303#comment:7>
GHC <http://www.haskell.org/ghc/>
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