base-3.0.2.0: Basic librariesSource codeContentsIndex
Control.Monad.ST
Portabilitynon-portable (requires universal quantification for runST)
Stabilityexperimental
Maintainerlibraries@haskell.org
Contents
The ST Monad
Converting ST to IO
Unsafe operations
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.
Synopsis
data ST s a
runST :: (forall s . ST s a) -> a
fixST :: (a -> ST s a) -> ST s a
data RealWorld
stToIO :: ST RealWorld a -> IO a
unsafeInterleaveST :: ST s a -> ST s a
unsafeIOToST :: IO a -> ST s a
unsafeSTToIO :: ST s a -> IO a
The ST Monad
data ST s aSource

The strict state-transformer monad. A computation of type ST s a transforms an internal state indexed by s, and returns a value of type a. The s parameter is either

  • an uninstantiated type variable (inside invocations of runST), or
  • RealWorld (inside invocations of stToIO).

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) = _|_
show/hide Instances
runST :: (forall s . ST s a) -> aSource
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 aSource
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
data RealWorld Source
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#.
show/hide Instances
stToIO :: ST RealWorld a -> IO aSource
A monad transformer embedding strict state transformers in the IO monad. The RealWorld parameter indicates that the internal state used by the ST computation is a special one supplied by the IO monad, and thus distinct from those used by invocations of runST.
Unsafe operations
unsafeInterleaveST :: ST s a -> ST s aSource
unsafeIOToST :: IO a -> ST s aSource
unsafeSTToIO :: ST s a -> IO aSource
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