gistfile1.hs

import Control.Monad.Writer
import Control.Monad.Maybe
type FooM a = MaybeT (Writer [String]) a

test =
 do print (runIt warningFail)    -- I want [ 'failure' ]
    print (runIt warningSuccess) -- I want [ 'success' ]
    print (runIt (warningFail >> warningSuccess)) -- I want [ 'failure' ]
    print (runIt (warningSuccess >> warningFail)) -- I want [ 'success', 'failure' ]
 where
  runIt = runWriter . runMaybeT

warningSuccess :: FooM Int
warningSuccess =
 do tell [ "success" ]
    return (42 :: Int)

warningFail :: FooM ()
warningFail =
 do tell [ "failure" ]
    fail "argh"