GHC/ST.hs

    {-# LANGUAGE Unsafe #-}
    {-# LANGUAGE NoImplicitPrelude, MagicHash, UnboxedTuples, RankNTypes #-}
    {-# OPTIONS_HADDOCK hide #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.ST
    -- Copyright   :  (c) The University of Glasgow, 1992-2002
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC Extensions)
    --
    -- The 'ST' Monad.
    --
    -----------------------------------------------------------------------------

    module GHC.ST (
            ST(..), STret(..), STRep,
            fixST, runST,

            -- * Unsafe functions
            liftST, unsafeInterleaveST, unsafeDupableInterleaveST
        ) where

    import GHC.Base
    import GHC.Show

    default ()

    -- The state-transformer monad proper.  By default the monad is strict;
    -- too many people got bitten by space leaks when it was lazy.

    -- | The strict state-transformer monad.
    -- A computation of type @'ST' s a@ transforms an internal state indexed
    -- by @s@, and returns a value of type @a@.
    -- The @s@ parameter is either
    --
    -- * an uninstantiated type variable (inside invocations of 'runST'), or
    --
    -- * 'RealWorld' (inside invocations of 'Control.Monad.ST.stToIO').
    --
    -- It serves to keep the internal states of different invocations
    -- of 'runST' separate from each other and from invocations of
    -- 'Control.Monad.ST.stToIO'.
    --
    -- The '>>=' and '>>' operations are strict in the state (though not in
    -- values stored in the state).  For example,
    --
    -- @'runST' (writeSTRef _|_ v >>= f) = _|_@
    newtype ST s a = ST (STRep s a)
    type STRep s a = State# s -> (# State# s, a #)

    -- | @since 2.01
    instance Functor (ST s) where
        fmap f (ST m) = ST $ \ s ->
          case (m s) of { (# new_s, r #) ->
          (# new_s, f r #) }

    -- | @since 4.4.0.0
    instance Applicative (ST s) where
        {-# INLINE pure #-}
        {-# INLINE (*>)   #-}
        pure x = ST (\ s -> (# s, x #))
        m *> k = m >>= \ _ -> k
        (<*>) = ap
        liftA2 = liftM2

    -- | @since 2.01
    instance Monad (ST s) where
        {-# INLINE (>>=)  #-}
        (>>) = (*>)
        (ST m) >>= k
          = ST (\ s ->
            case (m s) of { (# new_s, r #) ->
            case (k r) of { ST k2 ->
            (k2 new_s) }})

    data STret s a = STret (State# s) a

    -- liftST is useful when we want a lifted result from an ST computation.  See
    -- fixST below.
    liftST :: ST s a -> State# s -> STret s a
    liftST (ST m) = \s -> case m s of (# s', r #) -> STret s' r

    noDuplicateST :: ST s ()
    noDuplicateST = ST $ \s -> (# noDuplicate# s, () #)

    -- | '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.
    {-# INLINE unsafeInterleaveST #-}
    unsafeInterleaveST :: ST s a -> ST s a
    unsafeInterleaveST m = unsafeDupableInterleaveST (noDuplicateST >> m)

    -- | '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 4.11
    {-# NOINLINE unsafeDupableInterleaveST #-}
    -- See Note [unsafeDupableInterleaveIO should not be inlined]
    -- in GHC.IO.Unsafe
    unsafeDupableInterleaveST :: ST s a -> ST s a
    unsafeDupableInterleaveST (ST m) = ST ( \ s ->
        let
            r = case m s of (# _, res #) -> res
        in
        (# s, r #)
      )

    -- | Allow the result of a state transformer computation to be used (lazily)
    -- inside the computation.
    -- Note that if @f@ is strict, @'fixST' f = _|_@.
    fixST :: (a -> ST s a) -> ST s a
    fixST k = ST $ \ s ->
        let ans       = liftST (k r) s
            STret _ r = ans
        in
        case ans of STret s' x -> (# s', x #)

    -- | @since 2.01
    instance  Show (ST s a)  where
        showsPrec _ _  = showString "<<ST action>>"
        showList       = showList__ (showsPrec 0)

    {-# INLINE runST #-}
    -- | Return the value computed by a state transformer computation.
    -- The @forall@ ensures that the internal state used by the 'ST'
    -- computation is inaccessible to the rest of the program.
    runST :: (forall s. ST s a) -> a
    runST (ST st_rep) = case runRW# st_rep of (# _, a #) -> a
    -- See Note [Definition of runRW#] in GHC.Magic