| Portability | non-portable (requires universal quantification for runST) | 
|---|---|
| Stability | experimental | 
| Maintainer | libraries@haskell.org | 
| Safe Haskell | Unsafe | 
Control.Monad.ST
Description
This library provides support for strict state threads, as described in the PLDI '94 paper by John Launchbury and Simon Peyton Jones Lazy Functional State Threads.
The ST Monad
data ST s a
The strict state-transformer monad.
 A computation of type ST s as, and returns a value of type a.
 The s parameter is either
-  an uninstantiated type variable (inside invocations of runST), or
-  RealWorld(inside invocations ofstToIO).
It serves to keep the internal states of different invocations
 of runST separate from each other and from invocations of
 stToIO.
The >>= and >> operations are strict in the state (though not in
 values stored in the state).  For example,
runST (writeSTRef _|_ v >>= f) = _|_runST :: (forall s. ST s a) -> a
Return the value computed by a state transformer computation.
 The forall ensures that the internal state used by the ST
 computation is inaccessible to the rest of the program.
fixST :: (a -> ST s a) -> ST s a
Allow the result of a state transformer computation to be used (lazily)
 inside the computation.
 Note that if f is strict, fixST f = _|_
Converting ST to IO
RealWorld is deeply magical.  It is primitive, but it is not
 	unlifted (hence ptrArg).  We never manipulate values of type
 	RealWorld; it's only used in the type system, to parameterise State#. 
Unsafe Functions
unsafeInterleaveST :: ST s a -> ST s a
Deprecated: Please import from Control.Monad.ST.Unsafe instead; This will be removed in the next release
unsafeIOToST :: IO a -> ST s a
Deprecated: Please import from Control.Monad.ST.Unsafe instead; This will be removed in the next release
unsafeSTToIO :: ST s a -> IO a
Deprecated: Please import from Control.Monad.ST.Unsafe instead; This will be removed in the next release