HORIZON HASKELLDocslts/ghc-9.10.x248f8f02026-10-05Search names, modules, packages, or :: a typeCtrl K

GHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05

Modulerebase-1.21.2Haskell2010

Rebase.GHC.ST

  • 3 types
  • 4 values
  • Packagerebase-1.21.2
  • Exports7
  • LanguageHaskell2010
  • LicenceMIT
  • SourceST.hs
newtypenewtype 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

Instances36Monad, Functor, MonadFix, Applicative, MonadThrow, PrimBase, …
valuerunST :: (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.

valueunsafeInterleaveST :: 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.

valueunsafeDupableInterleaveST :: 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.