Control.Monad.ST.Safe (original) (raw)

Contents

• The [ST](Control-Monad-ST-Safe.html#t:ST) Monad
• Converting [ST](Control-Monad-ST-Safe.html#t:ST) to [IO](System-IO.html#t:IO)

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.

Safe API Only.

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](Control-Monad-ST-Safe.html#t:ST) Monad

data ST s a Source

The strict state-transformer monad. A computation of type `[ST](Control-Monad-ST-Safe.html#t: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](Control-Monad-ST-Safe.html#v:runST)), or
• [RealWorld](Control-Monad-ST-Safe.html#t:RealWorld) (inside invocations of [stToIO](Control-Monad-ST.html#t:stToIO)).

It serves to keep the internal states of different invocations of [runST](Control-Monad-ST-Safe.html#v:runST) separate from each other and from invocations of[stToIO](Control-Monad-ST.html#t:stToIO).

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

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

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

Return the value computed by a state transformer computation. The forall ensures that the internal state used by the [ST](Control-Monad-ST-Safe.html#t:ST) computation is inaccessible to the rest of the program.

fixST :: (a -> ST s a) -> ST s aSource

Allow the result of a state transformer computation to be used (lazily) inside the computation. Note that if f is strict, `[fixST](Control-Monad-ST-Safe.html#v:fixST)` f = _|_.

Converting [ST](Control-Monad-ST-Safe.html#t:ST) to [IO](System-IO.html#t:IO)

data RealWorld [Source](/packages/archive/
//doc/html/src/GHC-Prim.html#RealWorld)

RealWorld is deeply magical. It is primitive, but it is not_unlifted_ (hence ptrArg). We never manipulate values of typeRealWorld; it's only used in the type system, to parameterise State#.

stToIO :: ST RealWorld a -> IO aSource

A monad transformer embedding strict state transformers in the [IO](System-IO.html#t:IO) monad. The [RealWorld](Control-Monad-ST-Safe.html#t:RealWorld) parameter indicates that the internal state used by the [ST](Control-Monad-ST-Safe.html#t:ST) computation is a special one supplied by the [IO](System-IO.html#t:IO) monad, and thus distinct from those used by invocations of [runST](Control-Monad-ST-Safe.html#v:runST).