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Control.Monad.ST

Portability | non-portable (requires universal quantification for runST)
Stability | experimental
Maintainer | libraries@haskell.org

Contents

The `ST` Monad
Converting `ST` to `IO`
Unsafe operations

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.
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
unsafeInterleaveST :: ST s a -> ST s a
unsafeIOToST :: IO a -> ST s a
unsafeSTToIO :: ST s a -> IO a

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

Typeable2 ST
Functor (ST s)
Monad (ST s)
MonadFix (ST s)
(Typeable s, Typeable a) => Data (ST s a)
Show (ST s a)

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 | 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#`.
Instances

Typeable RealWorld

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

A monad transformer embedding strict 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`.
Unsafe operations

unsafeInterleaveST :: ST s a -> ST s a | Source

unsafeIOToST :: IO a -> ST s a | Source

unsafeSTToIO :: ST s a -> IO a | Source

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