Control.Monad.ST

data ST :: Type -> Effect

The ST effect represents local mutation, i.e. mutation which does not "escape" into the surrounding computation.

An ST computation is parameterized by a phantom type which is used to restrict the set of reference cells it is allowed to access.

The runST function can be used to handle the ST effect.

data STRef :: Type -> Type -> Type

The type STRef h a represents a mutable reference holding a value of type a, which can be used with the ST h effect.

newSTRef :: forall r h a. a -> Eff (st :: ST h | r) (STRef h a)

Create a new mutable reference.

readSTRef :: forall r h a. STRef h a -> Eff (st :: ST h | r) a

Read the current value of a mutable reference.

modifySTRef :: forall r h a. STRef h a -> (a -> a) -> Eff (st :: ST h | r) a

Modify the value of a mutable reference by applying a function to the current value.

writeSTRef :: forall r h a. STRef h a -> a -> Eff (st :: ST h | r) a

Set the value of a mutable reference.

runST :: forall r a. (forall h. Eff (st :: ST h | r) a) -> Eff r a

Run an ST computation.

Note: the type of runST uses a rank-2 type to constrain the phantom type h, such that the computation must not leak any mutable references to the surrounding computation.

It may cause problems to apply this function using the $ operator. The recommended approach is to use parentheses instead.

pureST :: forall a. (forall h. Eff (st :: ST h) a) -> a

A convenience function which combines runST with runPure, which can be used when the only required effect is ST.

Note: since this function has a rank-2 type, it may cause problems to apply this function using the $ operator. The recommended approach is to use parentheses instead.