Add Haskell solutions for Problem 25

Haskell/p025.hs

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+-- The Fibonacci sequence is defined by the recurrence relation:
+--
+-- Fn = Fn−1 + Fn−2, where F1 = 1 and F2 = 1.
+-- Hence the first 12 terms will be:
+-- F1 = 1
+-- F2 = 1
+-- F3 = 2
+-- F4 = 3
+-- F5 = 5
+-- F6 = 8
+-- F7 = 13
+-- F8 = 21
+-- F9 = 34
+-- F10 = 55
+-- F11 = 89
+-- F12 = 144
+--
+-- The 12th term, F12, is the first term to contain three digits.
+--
+-- What is the index of the first term in the Fibonacci sequence to contain 1000 digits?
+
+fibs :: [Integer]
+fibs = 0:1:(zipWith (+) fibs (tail fibs))
+
+thousandDigitFib = length $ takeWhile (\x -> (length $ show x) < 1000) $ fibs
+
+main =  do
+    let result = thousandDigitFib
+    putStrLn $ "Project Euler, Problem 25\n"
+        ++ "Answer: " ++ (show result)