Euler12.hs

{-# LANGUAGE NoMonomorphismRestriction #-}
module Main where
import Data.List (group,find)
import Data.Maybe (fromJust)

primes  = 2:filter prime [3..]
prime n = all ((0/=) . mod n) $ takeWhile ((n>=) . s (*) id) primes
s f g x = f x (g x)


factor n = factor' n primes'
    where
      factor' m []               = [m]
      factor' m pss@(p:ps)
              | m <= 1             = []
              | (m `mod` p) == 0   = p:(factor' (m `div` p) pss)
              | otherwise        = factor' m ps
      primes' = takeWhile (<bound) primes
      bound   = floor . sqrt . fromIntegral $ n

divisors :: (Integral a, Integral b) => a ->b
divisors n = product $ map (\x->fromIntegral(x+1)) $  map length $ group . factor $ n


euler12 =   take 1 [ x | x<-tri, (>=500) . divisors $ x ]
       where
         satisfy =  (find (>=500)) . (map divisors)
         tri = scanl1 (+) $  [1..]

main = print euler12

参考にしたページ
http://buchiyamato.nomaki.jp/5jou2soinsuubunnkai.html