Dear libraries@..., Shouldn't Control.Monad.ST export ioToST :: IO a -> ST RealWorld a, as defined in GHC.IO? Hugs for some reason does not have it, but it would seem trivial to write, given their stToIO: stToIO :: ST RealWorld a -> IO a stToIO (ST f) = IO f It doesn't look like anyone else even *has* ST...
Hello Samuel, Tuesday, August 25, 2009, 4:36:31 AM, you wrote:
Shouldn't Control.Monad.ST export ioToST :: IO a -> ST RealWorld a, as defined in GHC.IO?
no. ST is a subset of IO monad with a limited set of operators guaranteeing referential transparence, as a result runST is a pure operation. if you provide ability to run in ST monad arbitrary operations, this guarantee will break. there are pother way to do the same - use IO operations and unsafePerformIO -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com
On Monday 24 August 2009 11:36:18 pm Bulat Ziganshin wrote:
Hello Samuel,
Tuesday, August 25, 2009, 4:36:31 AM, you wrote:
Shouldn't Control.Monad.ST export ioToST :: IO a -> ST RealWorld a, as defined in GHC.IO?
no. ST is a subset of IO monad with a limited set of operators guaranteeing referential transparence, as a result runST is a pure operation. if you provide ability to run in ST monad arbitrary operations, this guarantee will break. there are pother way to do the same - use IO operations and unsafePerformIO
The proposed ioToST yields a value of type 'ST RealWorld a', which is not usable with runST, as it is not sufficiently polymorphic, and thus cannot be used to write unsafePerformIO. The only way you can eventually do anything with a value of the above type (short of unsafeCoerce and such) is to use stToIO to put it back into IO, and run it from main. -- Dan
On Mon, Aug 24, 2009 at 11:59:18PM -0400, Dan Doel wrote:
On Monday 24 August 2009 11:36:18 pm Bulat Ziganshin wrote:
Hello Samuel,
Tuesday, August 25, 2009, 4:36:31 AM, you wrote:
Shouldn't Control.Monad.ST export ioToST :: IO a -> ST RealWorld a, as defined in GHC.IO?
no. ST is a subset of IO monad with a limited set of operators guaranteeing referential transparence, as a result runST is a pure operation. if you provide ability to run in ST monad arbitrary operations, this guarantee will break. there are pother way to do the same - use IO operations and unsafePerformIO
The proposed ioToST yields a value of type 'ST RealWorld a', which is not usable with runST, as it is not sufficiently polymorphic, and thus cannot be used to write unsafePerformIO.
The only way you can eventually do anything with a value of the above type (short of unsafeCoerce and such) is to use stToIO to put it back into IO, and run it from main.
A useful thing you can do with this is make 'pre-fusioned' monad transformers, like if you want to combine a writer monad with the IO O monad, you could do something like newtype IOW w a = IOW (RealWorld -> (# RealWorld, w, a #) instance Monoid w => Monad (IOW w) where return x = IOW $ \w -> (# w, mempty, x #) ... then use ioToST to write your MonadIO instance. John -- John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/
participants (4)
-
Bulat Ziganshin -
Dan Doel -
John Meacham -
Samuel Bronson