Contents
ST MonadST to IODescription
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.
TheST Monad
The strict state-transformer monad. A computation of type ST s as, 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) -> aSource
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 aSource
Allow the result of a state transformer computation to be used (lazily) inside the computation. Note that if f is strict, fixST f = _|_
ST to IO
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 aSource
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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