Add solution for problem 27.

Problem027.hs

@@ -0,0 +1,23 @@
+-- Considering quadratics of the form:
+--
+--   n² + an + b, where |a| < 1000 and |b| < 1000
+--
+-- Find the product of the coefficients, a and b, for the quadratic expression
+-- that produces the maximum number of primes for consecutive values of n,
+-- starting with n = 0.
+
+import Euler
+
+isPrime :: Integer -> Bool
+isPrime n = n >= 2 && not (any (\p -> n `mod` p == 0) $ takeWhile (< n) primes)
+
+nPrimes a b = length $ takeWhile isPrime $ map (\n -> n^2 + a*n + b) [0..]
+
+main = print $ uncurry (*) $ snd $ maximumBy (compare `on` fst) $
+       [ (n, (a, b))
+       | a <- [-999..999]
+       , b <- [-999..999]
+       , isPrime (40^2 + a*40 + b)
+       , isPrime (30^2 + a*30 + b)
+       , let n = nPrimes a b
+       ]