12.hs

import Math.NumberTheory.Primes.Factorisation -- based on arithmoi library
import Data.Set

{-

The sequence of triangle numbers is generated by adding the natural numbers. So the 7th triangle number would be 1 + 2 + 3 + 4 + 5 + 6 + 7 = 28. The first ten terms would be:

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, ...

Let us list the factors of the first seven triangle numbers:

     1: 1
     3: 1,3
     6: 1,2,3,6
    10: 1,2,5,10
    15: 1,3,5,15
    21: 1,3,7,21
    28: 1,2,4,7,14,28

We can see that 28 is the first triangle number to have over five divisors.

What is the value of the first triangle number to have over five hundred divisors?
-}

triangles = tail $ scanl (+) 0 [1..]

euler12 = (head . dropWhile (\x -> (size.divisors) x < 500)) triangles

{-main = do
  print $ head dropWhile (\x -> length x < 500) (map triangle [1..10000])
  -}