Add remaining haskell book exercises

1 files changed, 25 insertions, 0 deletions

diff --git a/Haskell-book/15/semigroup/src/Combine.hs b/Haskell-book/15/semigroup/src/Combine.hs
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+++ b/Haskell-book/15/semigroup/src/Combine.hs
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+{-# LANGUAGE FlexibleInstances #-}
+module Combine where
+
+import Data.Semigroup
+import Test.QuickCheck
+
+newtype Combine a b =
+    Combine { unCombine :: (a -> b) }
+
+instance Semigroup b => Semigroup (Combine a b) where
+    (Combine f1) <> (Combine f2) = Combine f
+        where f x = (f1 x) <> (f2 x)
+
+instance (Monoid b) => Monoid (Combine a b) where
+    mempty = Combine $ \_ -> mempty
+    mappend (Combine f1) (Combine f2) = Combine f
+        where f x = mappend (f1 x) (f2 x)
+
+instance (Num a, CoArbitrary a, Arbitrary b) => Arbitrary (Combine a b) where
+    arbitrary = do
+        f <- arbitrary
+        return $ Combine (\n -> f n)
+
+instance (Show a, Show b) => Show (Combine a b) where
+    show _ = "a -> b"