Add Haskell solutions for Problems 4, 5, 6, 7

Haskell/p004.hs

@@ -0,0 +1,15 @@
+-- A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
+--
+-- Find the largest palindrome made from the product of two 3-digit numbers.
+
+isPalindrome :: Integer -> Bool
+isPalindrome n = show n == (reverse $ show n)
+
+maxPalindrome :: Integer
+maxPalindrome =
+    maximum $ filter isPalindrome [ x * y | x <- [100..999], y <- [100..999] ]
+
+main = do
+    let result = maxPalindrome
+    putStrLn $ "Project Euler, Problem 4\n"
+        ++ "Answer: " ++ (show result)