1 files changed, 30 insertions, 0 deletions
diff --git a/Haskell-book/14/qc/tests/Idempotence.hs b/Haskell-book/14/qc/tests/Idempotence.hs
new file mode 100644
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+++ b/Haskell-book/14/qc/tests/Idempotence.hs
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+module Main where
+
+import Data.Char
+import Data.List
+import Test.QuickCheck
+
+capitalizeWord :: String -> String
+capitalizeWord [] = []
+capitalizeWord (x:xs) = toUpper x : xs
+
+twice :: (a -> a) -> (a -> a)
+twice y = y . y
+
+fourTimes :: (a -> a) -> (a -> a)
+fourTimes = twice . twice
+
+f :: String -> Bool
+f x = 
+    (capitalizeWord x == twice capitalizeWord x)
+    && (capitalizeWord x == fourTimes capitalizeWord x)
+
+f' :: Ord a => [a] -> Bool
+f' x =
+    (sort x == twice sort x)
+    && (sort x == fourTimes sort x)
+
+main :: IO ()
+main = do
+    quickCheck f
+    quickCheck (f' :: String -> Bool)
+\ No newline at end of file