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Control.Monad.ST

Copyright (c) The University of Glasgow 2001
License BSD-style (see the file libraries/base/LICENSE)
Maintainer [email protected]
Stability experimental
Portability non-portable (requires universal quantification for runST)
Safe Haskell Trustworthy
Language Haskell2010

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.

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

The ST Monad

data ST s a Source

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) = _|_
Instances
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

fail :: String -> 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

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

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 Source

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 :: 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 Source

Embed a strict state transformer 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.

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.6.1/docs/html/libraries/base-4.12.0.0/Control-Monad-ST.html