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