Add Haskell solutions for Problems 8 and 15

Haskell/p008.hs

@@ -0,0 +1,56 @@
+-- The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.
+--
+-- 73167176531330624919225119674426574742355349194934
+-- 96983520312774506326239578318016984801869478851843
+-- 85861560789112949495459501737958331952853208805511
+-- 12540698747158523863050715693290963295227443043557
+-- 66896648950445244523161731856403098711121722383113
+-- 62229893423380308135336276614282806444486645238749
+-- 30358907296290491560440772390713810515859307960866
+-- 70172427121883998797908792274921901699720888093776
+-- 65727333001053367881220235421809751254540594752243
+-- 52584907711670556013604839586446706324415722155397
+-- 53697817977846174064955149290862569321978468622482
+-- 83972241375657056057490261407972968652414535100474
+-- 82166370484403199890008895243450658541227588666881
+-- 16427171479924442928230863465674813919123162824586
+-- 17866458359124566529476545682848912883142607690042
+-- 24219022671055626321111109370544217506941658960408
+-- 07198403850962455444362981230987879927244284909188
+-- 84580156166097919133875499200524063689912560717606
+-- 05886116467109405077541002256983155200055935729725
+-- 71636269561882670428252483600823257530420752963450
+--
+-- Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?
+
+import Data.Char
+
+nDigitProduct :: Int -> String -> Int
+nDigitProduct n s
+    | length s < n = -1
+    | otherwise = max (product (map digitToInt (take n s))) (nDigitProduct n (tail s))
+
+main = do
+    let s = "73167176531330624919225119674426574742355349194934"
+            ++ "96983520312774506326239578318016984801869478851843"
+            ++ "85861560789112949495459501737958331952853208805511"
+            ++ "12540698747158523863050715693290963295227443043557"
+            ++ "66896648950445244523161731856403098711121722383113"
+            ++ "62229893423380308135336276614282806444486645238749"
+            ++ "30358907296290491560440772390713810515859307960866"
+            ++ "70172427121883998797908792274921901699720888093776"
+            ++ "65727333001053367881220235421809751254540594752243"
+            ++ "52584907711670556013604839586446706324415722155397"
+            ++ "53697817977846174064955149290862569321978468622482"
+            ++ "83972241375657056057490261407972968652414535100474"
+            ++ "82166370484403199890008895243450658541227588666881"
+            ++ "16427171479924442928230863465674813919123162824586"
+            ++ "17866458359124566529476545682848912883142607690042"
+            ++ "24219022671055626321111109370544217506941658960408"
+            ++ "07198403850962455444362981230987879927244284909188"
+            ++ "84580156166097919133875499200524063689912560717606"
+            ++ "05886116467109405077541002256983155200055935729725"
+            ++ "71636269561882670428252483600823257530420752963450"
+        result = nDigitProduct 13 s
+    putStrLn $ "Project Euler, Problem 8\n"
+        ++ "Answer: " ++ show result

Haskell/p015.hs

@@ -0,0 +1,11 @@
+-- Starting in the top left corner of a 2×2 grid, and only being able to move to the right and down, there are exactly 6 routes to the bottom right corner
+-- How many such routes are there through a 20×20 grid?
+
+factorial :: (Integral a) => a -> a
+factorial 0 = 1
+factorial n = n * factorial (n - 1)
+
+main = do
+    let result = factorial 40 `div` factorial 20 ^ 2
+    putStrLn $ "Project Euler, Problem 15\n"
+        ++ "Answer: " ++ show result