Find numbers that are both square and triangle numbers. To use, start ghci and execute `:l tri_squares.hs`, followed by `tri_squares`

tri_squares.hs

-- find numbers that are both a square and a triangle number.
-- https://www.youtube.com/watch?v=Gh8h8MJFFdI

squares = map (\x -> x * x) [0..]

triangle_number x = div (x * (x + 1)) 2
triangle_numbers = map triangle_number [0..]

-- Check if you can arrange x points in a triangle with an integer side length
is_triangle x =
    x == last (takeWhile (<= x) triangle_numbers)

tri_squares = [i | i <- squares, is_triangle i]