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diff --git a/compiler/GHC/Data/SmallArray.hs b/compiler/GHC/Data/SmallArray.hs
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+{-# LANGUAGE MagicHash #-}
+{-# LANGUAGE UnboxedTuples #-}
+{-# LANGUAGE BlockArguments #-}
+
+-- | Small-array
+module GHC.Data.SmallArray
+  ( SmallMutableArray (..)
+  , SmallArray (..)
+  , newSmallArray
+  , writeSmallArray
+  , freezeSmallArray
+  , unsafeFreezeSmallArray
+  , indexSmallArray
+  , listToArray
+  )
+where
+
+import GHC.Exts
+import GHC.Prelude
+import GHC.ST
+
+data SmallArray a = SmallArray (SmallArray# a)
+
+data SmallMutableArray s a = SmallMutableArray (SmallMutableArray# s a)
+
+newSmallArray
+  :: Int  -- ^ size
+  -> a    -- ^ initial contents
+  -> State# s
+  -> (# State# s, SmallMutableArray s a #)
+{-# INLINE newSmallArray #-}
+newSmallArray (I# sz) x s = case newSmallArray# sz x s of
+  (# s', a #) -> (# s', SmallMutableArray a #)
+
+writeSmallArray
+  :: SmallMutableArray s a -- ^ array
+  -> Int                   -- ^ index
+  -> a                     -- ^ new element
+  -> State# s
+  -> State# s
+{-# INLINE writeSmallArray #-}
+writeSmallArray (SmallMutableArray a) (I# i) x = writeSmallArray# a i x
+
+
+-- | Copy and freeze a slice of a mutable array.
+freezeSmallArray
+  :: SmallMutableArray s a -- ^ source
+  -> Int                   -- ^ offset
+  -> Int                   -- ^ length
+  -> State# s
+  -> (# State# s, SmallArray a #)
+{-# INLINE freezeSmallArray #-}
+freezeSmallArray (SmallMutableArray ma) (I# offset) (I# len) s =
+  case freezeSmallArray# ma offset len s of
+    (# s', a #) -> (# s', SmallArray a #)
+
+-- | Freeze a mutable array (no copy!)
+unsafeFreezeSmallArray
+  :: SmallMutableArray s a
+  -> State# s
+  -> (# State# s, SmallArray a #)
+{-# INLINE unsafeFreezeSmallArray #-}
+unsafeFreezeSmallArray (SmallMutableArray ma) s =
+  case unsafeFreezeSmallArray# ma s of
+    (# s', a #) -> (# s', SmallArray a #)
+
+
+-- | Index a small-array (no bounds checking!)
+indexSmallArray
+  :: SmallArray a -- ^ array
+  -> Int          -- ^ index
+  -> a
+{-# INLINE indexSmallArray #-}
+indexSmallArray (SmallArray sa#) (I# i) = case indexSmallArray# sa# i of
+  (# v #) -> v
+
+
+-- | Convert a list into an array.
+listToArray :: Int -> (e -> Int) -> (e -> a) -> [e] -> SmallArray a
+{-# INLINE listToArray #-}
+listToArray (I# size) index_of value_of xs = runST $ ST \s ->
+  let
+    index_of' e = case index_of e of I# i -> i
+    write_elems ma es s = case es of
+      []    -> s
+      e:es' -> case writeSmallArray# ma (index_of' e) (value_of e) s of
+                 s' -> write_elems ma es' s'
+  in
+  case newSmallArray# size undefined s of
+    (# s', ma #) -> case write_elems ma xs s' of
+      s'' -> case unsafeFreezeSmallArray# ma s'' of
+        (# s''', a #) -> (# s''', SmallArray a #)