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| Control.Monad.ST | | Portability | non-portable (requires universal quantification for runST) | | Stability | experimental | | Maintainer | libraries@haskell.org |
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| Description |
| 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.
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| Synopsis |
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| The ST Monad
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| data ST s a |
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
- an uninstantiated 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 strict in the state (though not in
values stored in the state). For example,
runST (writeSTRef _|_ v >>= f) = _|_ |  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 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#.
| | Instances | |
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| stToIO :: ST RealWorld a -> IO a |
| A monad transformer embedding strict 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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| unsafeSTToIO :: ST s a -> IO a |
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| Produced by Haddock version 0.8 |
