| 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 state-transformer monad.
 A computation of type ST s as, 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) # | Since: base-2.1 | 
| Functor (ST s) # | Since: base-2.1 | 
| MonadFix (ST s) # | Since: base-2.1 | 
| Defined in Control.Monad.Fix | |
| MonadFail (ST s) # | Since: base-4.11.0.0 | 
| Applicative (ST s) # | Since: base-4.4.0.0 | 
| Show (ST s a) # | Since: base-2.1 | 
| Semigroup a => Semigroup (ST s a) # | Since: base-4.11.0.0 | 
| Monoid a => Monoid (ST s a) # | Since: base-4.11.0.0 | 
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 = _|_