Data commit

Task/Test-a-function/Haskell/test-a-function.hs

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+import Test.QuickCheck
+
+isPalindrome :: String -> Bool
+isPalindrome x = x == reverse x
+
+{- There is no built-in definition of how to generate random characters;
+   here we just specify ASCII characters. Generating strings then automatically
+   follows from the definition of String as list of Char. -}
+instance Arbitrary Char where
+  arbitrary = choose ('\32', '\127')
+
+--                                            /------------------------- the randomly-generated parameters
+--                                            |      /------------------ the constraint on the test values
+--                                            |      |                /- the condition which should be true
+--                                            v      v                v
+main = do
+  putStr "Even palindromes: " >> quickCheck (\s   ->                  isPalindrome (s ++ reverse s))
+  putStr "Odd palindromes:  " >> quickCheck (\s   -> not (null s) ==> isPalindrome (s ++ (tail.reverse) s))
+  putStr "Non-palindromes:  " >> quickCheck (\i s -> not (null s) && 0 <= i && i < length s && i*2 /= length s
+                                                                  ==> not (isPalindrome (take i s ++ "•" ++ drop i s)))