-- not tail recursive
sum1 1 = 1
sum1 x = x + sum1 (x - 1)


-- should be tail recursive
-- because of lazy evaluation, can still overflow the stack!
-- ghc optimizations will alleviate this, but ghci won't
sum2 x =
  let
    sum2' x accum =
      if x == 0
        then accum
        else sum2' (x - 1) (accum + x)
  in sum2' x 0

main = putStr (show (sum2 1000000))