The ST monad

data ST s a Source

The lazy ST monad. The ST monad allows for destructive updates, but is escapable (unlike IO). A computation of type ST s a returns a value of type a, and executes in "thread" s. 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 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 an ST 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 an ST 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 Source

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 ST 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.