Project Euler 75 (3 s)

pr75.hs

import Data.List

possibleTriangles :: Integer -> Int
possibleTriangles n = length . filter (\x -> length x == 1) . group . sort . concat $ [ [s,2*s..n] | i <- [1,3..(upperBound n)]
                                                                                                   , j <- [2,4..(upperBound n)-i]
                                                                                                   , gcd i j == 1
                                                                                                   , let s = abs (j*j - i*i) + 2*i*j + i*i + j*j ]

upperBound :: Integer -> Integer
upperBound = truncate . sqrt . fromIntegral

main = print . possibleTriangles $ 1500000