Modified sequencing monad

Trace.hs

-- See
-- http://stackoverflow.com/questions/5920200/how-to-prevent-common-sub-expression-elimination-cse-with-ghc

module Main
    where

import Debug.Trace

data Eval a = Done a
            | Trace String a

instance Monad Eval where
  return x = Done x

  Done x    >>= k = k x
  Trace s a >>= k = k (trace s a)

runEval :: Eval a -> a
runEval (Done x) = x
runEval (Trace s a) = trace s a

track :: String -> a -> Eval a
track = Trace

main :: IO ()
main = print $ runEval $ do
            t1 <- track "hit 1" 1
            t2 <- track "hit 2" 2
            return (t2 + t1)