Control.Monad.ST.Lazy

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.

The ST monad

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

Monad (ST s)

Since: 2.1

Methods

(>>=) :: ST s a -> (a -> ST s b) -> ST s b

(>>) :: ST s a -> ST s b -> ST s b

return :: a -> ST s a

fail :: String -> ST s a

Functor (ST s)

Since: 2.1

Methods

fmap :: (a -> b) -> ST s a -> ST s b

(<$) :: a -> ST s b -> ST s a

MonadFix (ST s)

Since: 2.1

Methods

mfix :: (a -> ST s a) -> ST s a

MonadFail (ST s)

Since: 4.10

Methods

fail :: String -> ST s a

Applicative (ST s)

Since: 2.1

Methods

pure :: a -> ST s a

(<*>) :: ST s (a -> b) -> ST s a -> ST s b

liftA2 :: (a -> b -> c) -> ST s a -> ST s b -> ST s c

(*>) :: ST s a -> ST s b -> ST s b

(<*) :: ST s a -> ST s b -> ST s a

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.

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 = _|_.

Converting between strict and lazy ST

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.

lazyToStrictST :: ST s a -> ST s a

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#.

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.