Euler4.hs

--Helper and Main
isPalindrome :: Eq a => [a] -> Bool
isPalindrome xs = reverse xs == xs

main = print num4 >> print num4'

--Naive Solution (still finishes in .122 seconds with -O2)
threeDigNums :: [Int]
threeDigNums = [1000,999..100]

num4 = maximum [(a,x,y) | x <- threeDigNums, y <- threeDigNums, let a = x*y, isPalindrome (show a)]

-- Less Naive Solution, takes .05-.06 seconds without any other optimizations

threeDigNums' :: [Int]
threeDigNums' = [1000,999..900]
 
num4' = maximum [(a,x,y) | x <- threeDigNums', y <- threeDigNums', let a = x*y, isPalindrome (show a)]