base-4.20.0.1: Core data structures and operations
Copyright(c) The University of Glasgow 2001
LicenseBSD-style (see the file libraries/base/LICENSE)
Maintainerlibraries@haskell.org
Stabilitystable
Portabilitynon-portable (requires universal quantification for runST)
Safe HaskellSafe
LanguageHaskell2010

Control.Monad.ST

Description

This module provides support for strict state threads, as described in the PLDI '94 paper by John Launchbury and Simon Peyton Jones Lazy Functional State Threads.

References (variables) that can be used within the ST monad are provided by Data.STRef, and arrays are provided by Data.Array.ST.

Synopsis

The ST Monad

data 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) = _|_

Instances

Instances details
Applicative (ST s)

Since: base-4.4.0.0

Instance details

Defined in GHC.Internal.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 #

Functor (ST s)

Since: base-2.1

Instance details

Defined in GHC.Internal.ST

Methods

fmap :: (a -> b) -> ST s a -> ST s b Source #

(<$) :: a -> ST s b -> ST s a Source #

Monad (ST s)

Since: base-2.1

Instance details

Defined in GHC.Internal.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 #

MonadFix (ST s)

Since: base-2.1

Instance details

Defined in GHC.Internal.Control.Monad.Fix

Methods

mfix :: (a -> ST s a) -> ST s a Source #

Monoid a => Monoid (ST s a)

Since: base-4.11.0.0

Instance details

Defined in GHC.Internal.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 #

Semigroup a => Semigroup (ST s a)

Since: base-4.11.0.0

Instance details

Defined in GHC.Internal.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 #

Show (ST s a)

Since: base-2.1

Instance details

Defined in GHC.Internal.ST

Methods

showsPrec :: Int -> ST s a -> ShowS Source #

show :: ST s a -> String Source #

showList :: [ST s a] -> ShowS 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.

fixST :: (a -> ST s a) -> ST s a Source #

Allow the result of an ST 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#.

stToIO :: ST RealWorld a -> IO a Source #

Embed a strict state thread in an IO action. 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.