Add remaining haskell book exercises

1 files changed, 24 insertions, 0 deletions

diff --git a/Haskell-book/18/Instance/src/Identity.hs b/Haskell-book/18/Instance/src/Identity.hs
new file mode 100644
index 0000000..13cd1b2
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+++ b/Haskell-book/18/Instance/src/Identity.hs
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+module Identity where
+
+import Test.QuickCheck
+import Test.QuickCheck.Checkers
+
+newtype Identity a = Identity a
+    deriving (Eq, Ord, Show)
+
+instance Functor Identity where
+    fmap f (Identity x) = Identity $ f x
+
+instance Applicative Identity where
+    pure = Identity
+    (Identity f) <*> x = fmap f x
+
+instance Monad Identity where
+    return = pure
+    (Identity x) >>= f = f x
+
+instance Arbitrary a => Arbitrary (Identity a) where
+    arbitrary = fmap Identity $ arbitrary
+
+instance Eq a => EqProp (Identity a) where
+    (=-=) = eq