Copyright	(c) The University of Glasgow 2001
License	BSD-style (see the file libraries/base/LICENSE)
Maintainer	libraries@haskell.org
Stability	provisional
Portability	non-portable (requires universal quantification for runST)
Safe Haskell	Trustworthy
Language	Haskell2010

Control.Monad.ST.Lazy

Contents

The ST monad
Converting between strict and lazy ST
Converting ST To IO

Description

This module presents an identical interface to Control.Monad.ST, except that the monad delays evaluation of state operations until a value depending on them is required.

Synopsis

data ST s a
runST :: (forall s. ST s a) -> a
fixST :: (a -> ST s a) -> ST s a
strictToLazyST :: ST s a -> ST s a
lazyToStrictST :: ST s a -> ST s a
data RealWorld :: Type
stToIO :: ST RealWorld a -> IO a

The `ST` monad

data ST s a #

The lazy 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 not strict in the state. For example,

runST (writeSTRef _|_ v >>= readSTRef _|_ >> return 2) = 2

Instances

Monad (ST s) #	Since: base-2.1
Instance details Defined in Control.Monad.ST.Lazy.Imp Methods (>>=) :: ST s a -> (a -> ST s b) -> ST s b # (>>) :: ST s a -> ST s b -> ST s b # return :: a -> ST s a # fail :: String -> ST s a #
Functor (ST s) #	Since: base-2.1
Instance details Defined in Control.Monad.ST.Lazy.Imp Methods fmap :: (a -> b) -> ST s a -> ST s b # (<$) :: a -> ST s b -> ST s a #
MonadFix (ST s) #	Since: base-2.1
Instance details Defined in Control.Monad.ST.Lazy.Imp Methods mfix :: (a -> ST s a) -> ST s a #
MonadFail (ST s) #	Since: base-4.10
Instance details Defined in Control.Monad.ST.Lazy.Imp Methods fail :: String -> ST s a #
Applicative (ST s) #	Since: base-2.1
Instance details Defined in Control.Monad.ST.Lazy.Imp Methods pure :: a -> ST s a # (<>) :: ST s (a -> b) -> ST s a -> ST s b # liftA2 :: (a -> b -> c) -> ST s a -> ST s b -> ST s c # (>) :: ST s a -> ST s b -> ST s b # (<*) :: ST s a -> ST s b -> ST s a #

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 between strict and lazy `ST`

strictToLazyST :: ST s a -> ST s a #

Convert a strict ST computation into a lazy one. The strict state thread passed to strictToLazyST is not performed until the result of the lazy state thread it returns is demanded.

lazyToStrictST :: ST s a -> ST s a #

Convert a lazy ST computation into a strict one.

Converting `ST` To `IO`

data RealWorld :: Type 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 #

A monad transformer embedding lazy 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.

The ST monad

Converting between strict and lazy ST

Converting ST To IO

The `ST` monad

Converting between strict and lazy `ST`

Converting `ST` To `IO`