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 |
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 ST
monad
The lazy state-transformer monad.
A computation of type
transforms an internal state indexed
by 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) = 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 = _|_
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.
lazyToStrictST :: ST s a -> ST s a Source
Convert a lazy ST
computation into a strict one.
Converting ST
To IO
data RealWorld :: *
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#
.