Control.Monad.ST (original) (raw)

Contents

• The ST Monad
• Converting ST to IO

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 byData.Array.ST.

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

The ST Monad

data ST s a Source #

The strict [ST](Control-Monad-ST.html#t:ST "Control.Monad.ST") monad. The [ST](Control-Monad-ST.html#t:ST "Control.Monad.ST") monad allows for destructive updates, but is escapable (unlike IO). A computation of type `[ST](Control-Monad-ST.html#t:ST "Control.Monad.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](Control-Monad-ST.html#v:runST "Control.Monad.ST")), or
• [RealWorld](Control-Monad-ST.html#t:RealWorld "Control.Monad.ST") (inside invocations of [stToIO](/package/ghc-internal-9.1201.0/docs/GHC-Internal-Control-Monad-ST.html#v:stToIO "GHC.Internal.Control.Monad.ST")).

It serves to keep the internal states of different invocations of [runST](Control-Monad-ST.html#v:runST "Control.Monad.ST") separate from each other and from invocations of[stToIO](/package/ghc-internal-9.1201.0/docs/GHC-Internal-Control-Monad-ST.html#v:stToIO "GHC.Internal.Control.Monad.ST").

The [>>=](Control-Monad.html#v:-62--62--61- "Control.Monad") and [>>](Control-Monad.html#v:-62--62- "Control.Monad") operations are strict in the state (though not in values stored in the state). For example,

[runST](Control-Monad-ST.html#v:runST "Control.Monad.ST") (writeSTRef | v >>= f) = |

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](Control-Monad-ST.html#t:ST "Control.Monad.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](Control-Monad-ST.html#t:ST "Control.Monad.ST") computation to be used (lazily) inside the computation.

Note that if f is strict, `[fixST](Control-Monad-ST.html#v:fixST "Control.Monad.ST")` f = _|_.

Converting ST to IO

data RealWorld Source #

[RealWorld](Control-Monad-ST.html#t:RealWorld "Control.Monad.ST") is deeply magical. It is primitive, but it is not_unlifted_ (hence ptrArg). We never manipulate values of type[RealWorld](Control-Monad-ST.html#t:RealWorld "Control.Monad.ST"); it's only used in the type system, to parameterise [State#](GHC-Exts.html#t:State-35- "GHC.Exts").

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

Embed a strict state thread in an [IO](Prelude.html#t:IO "Prelude") action. The [RealWorld](Control-Monad-ST.html#t:RealWorld "Control.Monad.ST") parameter indicates that the internal state used by the [ST](Control-Monad-ST.html#t:ST "Control.Monad.ST") computation is a special one supplied by the [IO](Prelude.html#t:IO "Prelude") monad, and thus distinct from those used by invocations of [runST](Control-Monad-ST.html#v:runST "Control.Monad.ST").