Discussion on: Daily Challenge #199 - List of All Rationals

import Data.Ratio (numerator, denominator, (%))

data Tree a = Tree a (Tree a) (Tree a)

rationalTree :: Tree Rational
rationalTree = build' (1 % 1)
  where build' :: Rational -> Tree Rational
        build' n = let a = numerator n
                       b = denominator n
                   in  Tree n (build' (a % (a + b))) (build' ((a + b) % b))

-- from https://doisinkidney.com/posts/2018-12-18-traversing-graphs.html, idk how it works
flatTree :: Tree a -> [a]
flatTree r = f r b []
  where f (Tree x l r) fw bw = x : fw ([l, r] : bw)

        b [] = []
        b qs = foldl (foldr f) b qs []

allRationals :: [Rational]
allRationals = flatTree rationalTree