From 98329e0a3dd4f78b5d815ac3896272ec70904901 Mon Sep 17 00:00:00 2001
From: Eugen Wissner <belka@caraus.de>
Date: Thu, 11 Dec 2025 10:28:11 +0100
Subject: Add remaining haskell book exercises

---
 Haskell-book/28/DifferenceList/src/Data/DList.hs | 40 ++++++++++++++++++++++++
 1 file changed, 40 insertions(+)
 create mode 100644 Haskell-book/28/DifferenceList/src/Data/DList.hs

(limited to 'Haskell-book/28/DifferenceList/src/Data/DList.hs')

diff --git a/Haskell-book/28/DifferenceList/src/Data/DList.hs b/Haskell-book/28/DifferenceList/src/Data/DList.hs
new file mode 100644
index 0000000..12d3a53
--- /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 #-}
-- 
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