| 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
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.
The ST Monad
The strict ST monad.
The ST monad allows for destructive updates, but is escapable (unlike IO).
A computation of type returns a value of type ST s aa, and
execute in "thread" s. The s parameter is either
- an uninstantiated type variable (inside invocations of
runST), or RealWorld(inside invocations ofstToIO).
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
| MonadFail (ST s) # | Since: base-4.11.0.0 |
| MonadFix (ST s) # | Since: base-2.1 |
Defined in Control.Monad.Fix | |
| Applicative (ST s) # | Since: base-4.4.0.0 |
| Functor (ST s) # | Since: base-2.1 |
| Monad (ST s) # | Since: base-2.1 |
| Monoid a => Monoid (ST s a) # | Since: base-4.11.0.0 |
| Semigroup a => Semigroup (ST s a) # | Since: base-4.11.0.0 |
| Show (ST s a) # | Since: base-2.1 |
runST :: (forall s. ST s a) -> a #
Return the value computed by a state thread.
The forall ensures that the internal state used by the ST
computation is inaccessible to the rest of the program.
Converting ST to IO
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#.