1 files changed, 40 insertions, 0 deletions
diff --git a/Haskell-book/28/DifferenceList/src/Data/DList.hs b/Haskell-book/28/DifferenceList/src/Data/DList.hs
new file mode 100644
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--- /dev/null
+++ b/Haskell-book/28/DifferenceList/src/Data/DList.hs
@@ -0,0 +1,40 @@
+module Data.DList
+    ( DList(..)
+    , empty
+    , singleton
+    , toList
+    , cons
+    , snoc
+    , append
+    ) where
+
+newtype DList a = DL { unDL :: [a] -> [a] }
+
+empty :: DList a
+empty = DL ([] ++)
+{-# INLINE empty #-}
+
+singleton :: a -> DList a
+singleton x = DL ([x] ++)
+{-# INLINE singleton #-}
+
+toList :: DList a -> [a]
+toList xsf = unDL xsf []
+{-# INLINE toList #-}
+
+-- Prepend a single element to a dlist.
+infixr `cons`
+cons :: a -> DList a -> DList a
+cons x xs = DL ((x:) . unDL xs)
+{-# INLINE cons #-}
+
+-- Append a single element to a dlist.
+infixl `snoc`
+snoc :: DList a -> a -> DList a
+snoc xs x = append xs $ singleton x
+{-# INLINE snoc #-}
+
+-- Append dlists.
+append :: DList a -> DList a -> DList a
+append xsf ysf = DL $ (unDL xsf) . (unDL ysf)
+{-# INLINE append #-}