Data/Array/ST.hs

    {-# LANGUAGE RankNTypes #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Array.ST
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  non-portable (uses Data.Array.MArray)
    --
    -- Mutable boxed and unboxed arrays in the 'Control.Monad.ST.ST' monad.
    --
    -----------------------------------------------------------------------------

    module Data.Array.ST (
       -- * Boxed arrays
       STArray,             -- instance of: Eq, MArray
       runSTArray,

       -- * Unboxed arrays
       STUArray,            -- instance of: Eq, MArray
       runSTUArray,

       -- * Overloaded mutable array interface
       module Data.Array.MArray,
     ) where

    import Data.Array.Base  ( STUArray, UArray, unsafeFreezeSTUArray )
    import Data.Array.MArray
    import Control.Monad.ST ( ST, runST )

    import GHC.Arr          ( STArray, Array, unsafeFreezeSTArray )

    -- | A safe way to create and work with a mutable array before returning an
    -- immutable array for later perusal.  This function avoids copying
    -- the array before returning it - it uses 'unsafeFreeze' internally, but
    -- this wrapper is a safe interface to that function.
    --
    runSTArray :: (forall s . ST s (STArray s i e)) -> Array i e
    runSTArray st = runST (st >>= unsafeFreezeSTArray)

    -- | A safe way to create and work with an unboxed mutable array before
    -- returning an immutable array for later perusal.  This function
    -- avoids copying the array before returning it - it uses
    -- 'unsafeFreeze' internally, but this wrapper is a safe interface to
    -- that function.
    --
    runSTUArray :: (forall s . ST s (STUArray s i e)) -> UArray i e
    runSTUArray st = runST (st >>= unsafeFreezeSTUArray)

    -- INTERESTING... this is the type we'd like to give to runSTUArray:
    --
    -- runSTUArray :: (Ix i, IArray UArray e,
    --              forall s. MArray (STUArray s) e (ST s))
    --         => (forall s . ST s (STUArray s i e))
    --         -> UArray i e
    --
    -- Note the quantified constraint.  We dodged the problem by using
    -- unsafeFreezeSTUArray directly in the defn of runSTUArray above, but
    -- this essentially constrains us to a single unsafeFreeze for all STUArrays
    -- (in theory we might have a different one for certain element types).