Gcd.scala

import scalaz._
import Scalaz._

object Gcd {
  def main(args: Array[String]): Unit = {
    def gcd(a: Int, b: Int): Writer[List[String], Int] =
      if (b == 0) for {
        _ <- List("Finished with " + a.shows).tell
      } yield a
      else
        List(a.shows + " mod " + b.shows + " = " + (a % b).shows).tell >>= { _ => gcd(b, a % b) }

    println(gcd(8, 3).run)
  }
}

gcdwriter.hs

import Control.Monad.Writer

gcd' :: Int -> Int -> Writer [String] Int
gcd' a b
    | b == 0 = do
        tell ["Finished with " ++ show a]
        return a
    | otherwise = do
        tell [show a ++ " mod " ++ show b ++ " = " ++ show (a `mod` b)]
        gcd' b (a `mod` b)

main = do
    print $ fst $ runWriter (gcd' 8 3)
    mapM_ putStrLn $ snd $ runWriter (gcd' 8 3)