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/18/Instance/src/List.hs | 44 ++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)
 create mode 100644 Haskell-book/18/Instance/src/List.hs

(limited to 'Haskell-book/18/Instance/src/List.hs')

diff --git a/Haskell-book/18/Instance/src/List.hs b/Haskell-book/18/Instance/src/List.hs
new file mode 100644
index 0000000..bd3a06b
--- /dev/null
+++ b/Haskell-book/18/Instance/src/List.hs
@@ -0,0 +1,44 @@
+module List where
+
+import Test.QuickCheck
+import Test.QuickCheck.Checkers
+
+data List a =
+    Nil
+  | Cons a (List a)
+  deriving (Eq, Show)
+
+instance Functor List where
+    fmap f Nil = Nil
+    fmap f (Cons x xs) = Cons (f x) (fmap f xs)
+
+append :: List a -> List a -> List a
+append Nil ys = ys
+append (Cons x xs) ys = Cons x $ xs `append` ys
+
+fold :: (a -> b -> b) -> b -> List a -> b
+fold _ b Nil = b
+fold f b (Cons h t) = f h (fold f b t)
+
+concat' :: List (List a) -> List a
+concat' = fold append Nil
+
+flatMap :: (a -> List b) -> List a -> List b
+flatMap f as = concat' $ fmap f as
+
+instance Applicative List where
+    pure f = Cons f Nil
+    Nil <*> _ = Nil
+    _ <*> Nil = Nil
+    f <*> x = flatMap (\f' -> fmap f' x) f
+
+instance Monad List where
+    return = pure
+    x >>= f = concat' $ fmap f x
+
+instance Arbitrary a => Arbitrary (List a) where
+    arbitrary = frequency [(1, pure Nil),
+                           (5, Cons <$> arbitrary <*> arbitrary)]
+
+instance Eq a => EqProp (List a) where
+    (=-=) = eq
-- 
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