Copyright	(c) The University of Glasgow 1992-2002
License	see libraries/base/LICENSE
Maintainer	cvs-ghc@haskell.org
Stability	internal
Portability	non-portable (GHC Extensions)
Safe Haskell	Unsafe
Language	Haskell2010

GHC.ST

Contents

Unsafe functions

Description

The ST Monad.

Synopsis

newtype ST s a = ST (STRep s a)
data STret s a = STret (State# s) a
type STRep s a = State# s -> (# State# s, a #)
runST :: (forall s. ST s a) -> a
liftST :: ST s a -> State# s -> STret s a
unsafeInterleaveST :: ST s a -> ST s a
unsafeDupableInterleaveST :: ST s a -> ST s a

Documentation

newtype ST s a Source #

The strict ST monad. The ST monad allows for destructive updates, but is escapable (unlike IO). A computation of type ST s a returns a value of type a, and execute in "thread" s. 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) = _|_

Constructors

ST (STRep s a)

Instances

Instances details

Monad (ST s) #	Since: base-2.1
Instance details Defined in GHC.ST Methods (>>=) :: ST s a -> (a -> ST s b) -> ST s b Source # (>>) :: ST s a -> ST s b -> ST s b Source # return :: a -> ST s a Source #
Functor (ST s) #	Since: base-2.1
Instance details Defined in GHC.ST Methods fmap :: (a -> b) -> ST s a -> ST s b Source # (<$) :: a -> ST s b -> ST s a Source #
MonadFix (ST s) #	Since: base-2.1
Instance details Defined in Control.Monad.Fix Methods mfix :: (a -> ST s a) -> ST s a Source #
MonadFail (ST s) #	Since: base-4.11.0.0
Instance details Defined in GHC.ST Methods fail :: String -> ST s a Source #
Applicative (ST s) #	Since: base-4.4.0.0
Instance details Defined in GHC.ST Methods pure :: a -> ST s a Source # (<>) :: ST s (a -> b) -> ST s a -> ST s b Source # liftA2 :: (a -> b -> c) -> ST s a -> ST s b -> ST s c Source # (>) :: ST s a -> ST s b -> ST s b Source # (<*) :: ST s a -> ST s b -> ST s a Source #
Show (ST s a) #	Since: base-2.1
Instance details Defined in GHC.ST Methods showsPrec :: Int -> ST s a -> ShowS Source # show :: ST s a -> String Source # showList :: [ST s a] -> ShowS Source #
Semigroup a => Semigroup (ST s a) #	Since: base-4.11.0.0
Instance details Defined in GHC.ST Methods (<>) :: ST s a -> ST s a -> ST s a Source # sconcat :: NonEmpty (ST s a) -> ST s a Source # stimes :: Integral b => b -> ST s a -> ST s a Source #
Monoid a => Monoid (ST s a) #	Since: base-4.11.0.0
Instance details Defined in GHC.ST Methods mempty :: ST s a Source # mappend :: ST s a -> ST s a -> ST s a Source # mconcat :: [ST s a] -> ST s a Source #

data STret s a Source #

Constructors

STret (State# s) a

type STRep s a = State# s -> (# State# s, a #) Source #

runST :: (forall s. ST s a) -> a Source #

Return the value computed by a state thread. The forall ensures that the internal state used by the ST computation is inaccessible to the rest of the program.

Unsafe functions

liftST :: ST s a -> State# s -> STret s a Source #

unsafeInterleaveST :: ST s a -> ST s a Source #

unsafeInterleaveST allows an ST computation to be deferred lazily. When passed a value of type ST a, the ST computation will only be performed when the value of the a is demanded.

unsafeDupableInterleaveST :: ST s a -> ST s a Source #

unsafeDupableInterleaveST allows an ST computation to be deferred lazily. When passed a value of type ST a, the ST computation will only be performed when the value of the a is demanded.

The computation may be performed multiple times by different threads, possibly at the same time. To prevent this, use unsafeInterleaveST instead.

Since: base-4.11