GHC.ST

The ST Monad.

Documentation

newtype 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) = _|_

Constructors

ST (STRep s a)

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

data STret s a

Constructors

STret (State# s) a

type STRep s a = State# s -> (# State# s, a #)

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.

Unsafe functions

liftST :: ST s a -> State# s -> STret s a

unsafeInterleaveST :: ST s a -> ST s a

unsafeInterleaveST allows an ST computation to be deferred lazily. When passed a value of type ST a, the ST computation will only be performed when the value of the a is demanded.

unsafeDupableInterleaveST :: ST s a -> ST s a

unsafeDupableInterleaveST allows an ST computation to be deferred lazily. When passed a value of type ST a, the ST computation will only be performed when the value of the a is demanded.

The computation may be performed multiple times by different threads, possibly at the same time. To prevent this, use unsafeInterleaveST instead.

Since: base-4.11