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

Haskell/p006.hs

@@ -0,0 +1,19 @@
+-- The sum of the squares of the first ten natural numbers is,
+--
+-- 1^2 + 2^2 + ... + 10^2 = 385
+--
+-- The square of the sum of the first ten natural numbers is,
+--
+-- (1 + 2 + ... + 10)^2 = 55^2 = 3025
+--
+-- Hence the difference between the sum of the squares of the first ten natural numbers and the square of the sum is 3025 − 385 = 2640.
+--
+--Find the difference between the sum of the squares of the first one hundred natural numbers and the square of the sum.
+
+sumSquareDiff :: Integer -> Integer
+sumSquareDiff n = (sum [1..n]^2) - (sum $ map (^2) [1..n])
+
+main = do
+    let result = sumSquareDiff 100
+    putStrLn $ "Project Euler, Problem 6\n"
+        ++ "Answer: " ++ (show result)