Using applicatives to track the number of steps in a computation.

gistfile1.hs

module Main where

import Data.Monoid
import Control.Applicative
import Data.Functor.Compose

type SteppedIO a = Compose ((,) (Sum Int)) IO a

step :: IO a -> SteppedIO a
step cmd = Compose (Sum 1, cmd)

countSteps :: SteppedIO a -> Int
countSteps = getSum . fst . getCompose

exec :: SteppedIO a -> IO a
exec =  snd . getCompose

program :: SteppedIO () 
program =
    step (putStrLn "aaa") 
    *>  
    step (putStrLn "bbb")
    *>
    step (putStrLn "ccc")

main :: IO ()
main = do
    putStrLn $ "Number of steps: " ++ show (countSteps program)
    exec program