First of all, ListT is not a monad transformer, since it breaks the law of associativity of bind: *Control.Monad.List> let one = (lift $ putStrLn "1") :: ListT IO () *Control.Monad.List> let two = (lift $ putStrLn "2") :: ListT IO () *Control.Monad.List> let choice = return 1 `mplus` return 2 :: ListT IO Int *Extensible Control.Monad.List> runListT $ choice >> (one >> two) 1 2 1 2 [(),()] *Extensible Control.Monad.List> runListT $ (choice >> one) >> two 1 1 2 2 [(),()]
It appears to me that the MonadPlus instance for ListT breaks the following MonadPlus law m >> mzero = mzero
The precise set of MonadPlus laws isn't actually agreed upon. The above law is the least agreed upon. Incidentally, this law is certainly violated for _any_ back-tracking monad transformer. Consider (lift <any action in base monad>) >> mzero for example (lift (putStrLn "printed")) >> mzero Is it reasonable to expect that printing should be undone? That the paper should be fed back into the printer and the toner lifted? There are back-tracking monad transformers; for example, LogicT.