count.hs

count :: Int -> IO Int
count it
 | it <= 0   = pure 0
 | otherwise = 
    do 
      n <- count (it - 1)
      pure (n + 1)

count.scala

import cats.effect.IO

val count: Int => IO[Int] = {
  case 0 => IO.pure(0)
  case it =>
    for {
     n <- count (it - 1)
    } yield (n + 1)
}

count(10000).unsafeRunSync

This does not stack overflow with even large numbers like 1000000. Why?