Copyright | (c) The University of Glasgow 2001 |
---|---|
License | BSD-style (see the file libraries/base/LICENSE) |
Maintainer | [email protected] |
Stability | provisional |
Portability | non-portable (requires universal quantification for runST) |
Safe Haskell | Trustworthy |
Language | Haskell2010 |
Deprecated: Safe is now the default, please use Control.Monad.ST.Lazy instead
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.
Safe API only.
The lazy 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 not strict in the state. For example,
runST
(writeSTRef _|_ v >>= readSTRef _|_ >> return 2) = 2
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 = _|_
.
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.
lazyToStrictST :: ST s a -> ST s a Source
Convert a lazy ST
computation into a strict one.
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
A monad transformer embedding lazy state transformers in the IO
monad. 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-Lazy-Safe.html