[project @ 2005-04-21 09:40:41 by simonmar]

[ghc-base.git] / Data / Array / ST.hs

diff --git a/Data/Array/ST.hs b/Data/Array/ST.hs
index 143f792..42829f9 100644 (file)
--- a/Data/Array/ST.hs
+++ b/Data/Array/ST.hs
@@ -1,35 +1,77 @@
 -----------------------------------------------------------------------------
--- 
+-- |
 -- Module      :  Data.Array.ST
 -- Copyright   :  (c) The University of Glasgow 2001
--- License     :  BSD-style (see the file libraries/core/LICENSE)
+-- License     :  BSD-style (see the file libraries/base/LICENSE)
 -- 
 -- Maintainer  :  libraries@haskell.org
 -- Stability   :  experimental
 -- Portability :  non-portable
 --
--- $Id: ST.hs,v 1.1 2001/06/28 14:15:02 simonmar Exp $
---
--- Mutable boxed/unboxed arrays in the ST monad.
+-- Mutable boxed and unboxed arrays in the 'Control.Monad.ST.ST' monad.
 --
 -----------------------------------------------------------------------------
 
 module Data.Array.ST (
-   module Data.Array.MArray,
+
+   -- * Boxed arrays
    STArray,            -- instance of: Eq, MArray
+   runSTArray,
+
+   -- * Unboxed arrays
    STUArray,           -- instance of: Eq, MArray
+   runSTUArray,
    castSTUArray,       -- :: STUArray s i a -> ST s (STUArray s i b)
+
+   -- * Overloaded mutable array interface
+   module Data.Array.MArray,
  ) where
 
 import Prelude
 
 import Data.Array.MArray
-import Data.Array.Base
+import Data.Array.Base ( STUArray, castSTUArray, UArray, unsafeFreezeSTUArray )
+import Control.Monad.ST        ( ST, runST )
 
-#ifdef __GLASGOW_HASKELL__
-import GHC.Arr
-import GHC.ST
+#ifdef __HUGS__
+import Hugs.Array      ( Array )
+import Hugs.ST         ( STArray, unsafeFreezeSTArray )
+#endif
 
-castSTUArray :: STUArray s ix a -> ST s (STUArray s ix b)
-castSTUArray (STUArray l u marr#) = return (STUArray l u marr#)
+#ifdef __GLASGOW_HASKELL__
+import GHC.Arr         ( STArray, Array, unsafeFreezeSTArray )
 #endif
+
+-- | 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 :: (Ix i)
+          => (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 :: (Ix i)
+          => (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).