Control.Monad.ST.Safe

Deprecated: Safe is now the default, please use Control.Monad.ST instead

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.

The ST Monad

data ST s a

The strict 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 execute 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 strict in the state (though not in values stored in the state). For example,

runST (writeSTRef _|_ v >>= f) = _|_

Instances

MonadFail (ST s)

Since: base-4.11.0.0

Defined in GHC.ST

Methods

fail :: String -> ST s a

MonadFix (ST s)

Since: base-2.1

Defined in Control.Monad.Fix

Methods

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

Applicative (ST s)

Since: base-4.4.0.0

Defined in GHC.ST

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

Functor (ST s)

Since: base-2.1

Defined in GHC.ST

Methods

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

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

Monad (ST s)

Since: base-2.1

Defined in GHC.ST

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

Monoid a => Monoid (ST s a)

Since: base-4.11.0.0

Defined in GHC.ST

Methods

mempty :: ST s a

mappend :: ST s a -> ST s a -> ST s a

mconcat :: [ST s a] -> ST s a

Semigroup a => Semigroup (ST s a)

Since: base-4.11.0.0

Defined in GHC.ST

Methods

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

sconcat :: NonEmpty (ST s a) -> ST s a

stimes :: Integral b => b -> ST s a -> ST s a

Show (ST s a)

Since: base-2.1

Defined in GHC.ST

Methods

showsPrec :: Int -> ST s a -> ShowS

show :: ST s a -> String

showList :: [ST s a] -> ShowS

runST :: (forall s. ST s a) -> a

Return the value computed by a state thread. 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 an ST computation to be used (lazily) inside the computation.

Note that if f is strict, fixST f = _|_.

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

Embed a strict state thread in an IO action. 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.