1 files changed, 31 insertions, 0 deletions
diff --git a/Haskell-book/18/Instance/src/Sum.hs b/Haskell-book/18/Instance/src/Sum.hs
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+++ b/Haskell-book/18/Instance/src/Sum.hs
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+module Sum where
+
+import Test.QuickCheck
+import Test.QuickCheck.Checkers
+
+data Sum a b =
+    First a
+  | Second b
+  deriving (Eq, Show)
+
+instance Functor (Sum a) where
+    fmap f (First x) = First x
+    fmap f (Second x) = Second $ f x
+
+instance Applicative (Sum a) where
+    pure x = Second x
+    First f <*> _ = First f
+    Second f <*> x = fmap f x
+
+instance Monad (Sum a) where
+    return = pure
+    (First x) >>= f = First x
+    (Second x) >>= f = f x
+
+instance (Arbitrary a, Arbitrary b) => Arbitrary (Sum a b) where
+    arbitrary = frequency [ (1, fmap First arbitrary)
+                          , (1, fmap Second arbitrary)
+                          ]
+
+instance (Eq a, Eq b) => EqProp (Sum a b) where
+    (=-=) = eq