Copyright | (c) The University of Glasgow 2001 |
---|---|
License | BSD-style (see the file libraries/base/LICENSE) |
Maintainer | [email protected] |
Stability | experimental |
Portability | non-portable (requires universal quantification for runST) |
Safe Haskell | Trustworthy |
Language | Haskell2010 |
Deprecated: Safe is now the default, please use Control.Monad.ST instead
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.
Safe API Only.
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
runST
), orRealWorld
(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) = _|_
runST :: (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 = _|_
.
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 Source
Embed a strict state transformer in an IO
action. 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
.
© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.2.1/docs/html/libraries/base-4.10.0.0/Control-Monad-ST-Safe.html