| Copyright | (c) The University of Glasgow 2001 |
|---|---|
| License | BSD-style (see the file libraries/base/LICENSE) |
| Maintainer | libraries@haskell.org |
| Stability | provisional |
| Portability | non-portable (requires universal quantification for runST) |
| Safe Haskell | Trustworthy |
| Language | Haskell2010 |
Control.Monad.ST.Lazy (signature[?])
Description
This module presents an identical interface to Control.Monad.ST, except that the monad delays evaluation of state operations until a value depending on them is required.
The ST monad
The lazy state-transformer monad.
A computation of type ST s as, and returns a value of type a.
The s parameter is either
- an unstantiated 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 not strict in the state. For example,
runST (writeSTRef _|_ v >>= readSTRef _|_ >> return 2) = 2runST :: (forall s. ST s a) -> a Source #
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 Source #
Allow the result of a state transformer computation to be used (lazily)
inside the computation.
Note that if f is strict, fixST f = _|_
Converting between strict and lazy ST
strictToLazyST :: ST s a -> ST s a Source #
Convert a strict ST computation into a lazy one. The strict state
thread passed to strictToLazyST is not performed until the result of
the lazy state thread it returns is demanded.
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#.