Haste might seem like it has TCO, but it doesn't.

Main.hs

{-# LANGUAGE BangPatterns #-}
module Main where

-- This is constant space. Haste's Inlining (via GHC frontend) seems to
-- be able to combine even+odd to give us a single, tail-recursive loop.

even' :: Int -> Bool
even' 0 = True
even' !x = odd' (x - 1)

odd' :: Int -> Bool
odd' 0 = False
odd' !x = even' (x - 1)

-- GHC frontend can't inline these into a tail-recursive loop,
-- So we need full TCO, which Haste/JS doesn't give us.
-- Runs fine with GHC, but w/ Haste we blow the JS stack just on 3,000 :(
-- handwritten Javascript impl of the same takes 14,000 to blow JS stack.

table = [even'', odd'']

even'' :: Int -> Int -> Bool
even'' !lu 0 = True
even'' !lu !x = (table !! lu) 0 (x - 1)

odd'' :: Int -> Int -> Bool
odd'' !lu 0 = False
odd'' !lu !x = (table !! lu) 1 (x - 1)


main :: IO ()
main = do
--  putStrLn (show (even'' 1 3000)) -- blows stack!
  putStrLn (show (even' 1000000)) -- OK!