Control-Monad-ST.purs

module Control.Monad.ST where

import Prelude

import Control.Monad.Eff (Eff(), runPure)

-- | 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.
foreign import data ST :: * -> !

-- | The type `STRef s a` represents a mutable reference holding a value of type `a`, which can be used with the `ST s` effect.
foreign import data STRef :: * -> * -> *

-- | Create a new mutable reference.
foreign import newSTRef :: forall a h r. a -> Eff (st :: ST h | r) (STRef h a)

-- | Read the current value of a mutable reference.
foreign import readSTRef :: forall a h r. STRef h a -> Eff (st :: ST h | r) a

-- | Modify the value of a mutable reference by applying a function to the current value.
foreign import modifySTRef :: forall a h r. STRef h a -> (a -> a) -> Eff (st :: ST h | r) a

-- | Set the value of a mutable reference.
foreign import writeSTRef :: forall a h r. STRef h a -> a -> Eff (st :: ST h | r) a

-- | Run an `ST` computation.
-- |
-- | Note: the type of `runST` uses a rank-2 type to constrain the phantom type `s`, 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.
foreign import runST :: forall a r. (forall h. Eff (st :: ST h | r) a) -> Eff r 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.
pureST :: forall a. (forall h r. Eff (st :: ST h | r) a) -> a
pureST st = runPure (runST st)