| Copyright | (c) The University of Glasgow 2001 |
|---|---|
| License | (c) The University of Glasgow 2001 |
| Maintainer | libraries@haskell.org |
| Stability | experimental |
| Portability | non-portable (requires universal quantification for runST) |
| Safe Haskell | Unsafe |
Control.Monad.ST
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 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) = _|_runST :: (forall s. ST s a) -> aSource
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 aSource
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
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#.