Control.Monad.ST

base-4.8.2.0: Basic libraries

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

Control.Monad.ST

Contents

- The `ST` Monad
- Converting `ST` to `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.

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

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) |
Functor (ST s) |
MonadFix (ST s) |
Applicative (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 :: * 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

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`.