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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 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

The `ST` Monad
data ST s a

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 unstantiated 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`.

Instances

MonadFix (ST s)
(Typeable a, Typeable b) => Typeable (ST a b)
MArray (STArray s) e (ST s)
MArray (STUArray s) Bool (ST s)
MArray (STUArray s) Char (ST s)
MArray (STUArray s) Int (ST s)
MArray (STUArray s) Word (ST s)
MArray (STUArray s) (Ptr a) (ST s)
MArray (STUArray s) (FunPtr a) (ST s)
MArray (STUArray s) Float (ST s)
MArray (STUArray s) Double (ST s)
MArray (STUArray s) (StablePtr a) (ST s)
MArray (STUArray s) Int8 (ST s)
MArray (STUArray s) Int16 (ST s)
MArray (STUArray s) Int32 (ST s)
MArray (STUArray s) Int64 (ST s)
MArray (STUArray s) Word8 (ST s)
MArray (STUArray s) Word16 (ST s)
MArray (STUArray s) Word32 (ST s)
MArray (STUArray s) Word64 (ST s)
Functor (ST s)
Monad (ST s)
Show (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 `ST` to `IO`
data RealWorld

A special argument for the `ST` type constructor, indexing a state embedded in the `IO` monad by `stToIO`.

stToIO :: ST RealWorld a -> IO a
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
unsafeIOToST :: IO a -> ST s a
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