Hello, I just noticed that the GHC/Hugs "standard" libraries have acquired a list monad transformer, which is broken, because it does not satisfy the associativity law when applied to non-commutative monads. I am not referring to some corner-case strictness problem, but rather a fairly well known result. The associativity law is important, because without it the 'do' notation can be very confusing. Bellow is an example, for people not familiar with the porblem. -Iavor import Control.Monad.List say :: String -> ListT IO () say x = lift (putStr x) one_two, ex1,ex2 :: ListT IO () one_two = say "1 " `mplus` say "2 " ex1 = do one_two say "3 " say "4 " ex2 = do do one_two say "3 " say "4 " main = runListT ex1 >> putStrLn " or " >> runListT ex2