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| Control.Monad.ST.Lazy | | Portability | non-portable (requires universal quantification for runST) | | Stability | provisional | | Maintainer | libraries@haskell.org |
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| 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.
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| Synopsis |
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| The ST monad
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| data ST s a |
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
- an unstantiated type variable (inside invocations of runST), or
- RealWorld (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 |  Instances | |
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| runST :: (forall s . ST s a) -> a |
| 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.
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| fixST :: (a -> ST s a) -> ST s a |
| Allow the result of a state transformer computation to be used (lazily)
inside the computation.
Note that if f is strict, fixST f = _|_.
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| Converting between strict and lazy ST
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| strictToLazyST :: ST s a -> ST s a |
| 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.
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| lazyToStrictST :: ST s a -> ST s a |
| Convert a lazy ST computation into a strict one.
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| Converting ST To IO
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| 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#.
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| stToIO :: ST RealWorld a -> IO a |
| 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.
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| Unsafe operations
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| unsafeInterleaveST :: ST s a -> ST s a |
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| unsafeIOToST :: IO a -> ST s a |
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| Produced by Haddock version 0.8 |
