#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 | Resolution:
Keywords: | Difficulty: Unknown
Os: Unknown | Testcase:
Architecture: Unknown |
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Old description:
> 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
> )
>
> ---------------------------------------------------------------------------
New description:
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>
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