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
It appears to me that the MonadPlus instance for ListT breaks the
following MonadPlus law
m mzero = mzero
I've tried with every implementation of ListT I could find - the one
in mtl, transformers, List, even ListT done right and it's
alternative. They all seem to violate the above law.
