#1369: Type error when compiling ST-monad-like code
-------------------------------+--------------------------------------------
Reporter: [EMAIL PROTECTED] | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.6.1
Severity: normal | Keywords:
Difficulty: Unknown | Os: Unknown
Testcase: | Architecture: Unknown
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Compiling the module below works fine in GHC 6.4.2.
In GHC 6.6 and 6.6.1, it gives a type error.
/Koen
>>>
{-# OPTIONS -fglasgow-exts #-}
module Bug where
import Control.Monad.ST
import Data.STRef
---------------------------------------------------------------------------
newtype M s a =
MkM (STRef s Int -> ST s a)
runM :: (forall s . M s a) -> a
runM mm =
runST (
do ref <- newSTRef 0
m ref
)
where
MkM m = mm
---------------------------------------------------------------------------
-- the instance declaration and function definition of "inc" are just here
-- for giving context; removing them still makes runM not type check in
GHC 6.6
instance Monad (M s) where
return x =
MkM (\_ -> return x)
MkM m >>= k =
MkM (\ref ->
do x <- m ref
let MkM m' = k x
m' ref
)
inc :: M s Int
inc = MkM (\ref ->
do n <- readSTRef ref
writeSTRef ref (n+1)
return n
)
---------------------------------------------------------------------------
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1369>
GHC <http://www.haskell.org/ghc/>
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