Hi, I have been playing with Maybe and Output monad transformers and realized that I cannot make the following work nicely:
> foo a b = do > output "before" > r <- (liftM2(+)) a b > when r == Nothing $ output "error" > return r As soon as computation produces Nothing, I am loosing ability to do any output. I think what I need is not wrapping one monad into another with transformer; I need some sort of monad combiner: > newtype MC m1 m2 = MC (m1 a, m2 b) So I could execute both of monads âin parallelâ. But I have no idea how to express this or even if this is doable at all. Any comments? Thanks, Pavel. P.S. Here is summary of what I tried with transformers so far: Letâs say I have following monad transformers: > newtype MaybeMonadT m a = MMT (m a) > newtype OuptutMonadT m o a = OMT (m a, o) And I am trying to use following monads: > type M1 a = MaybeMonadT OutputM a > type M2 a = OuptutMonadT Maybe String a Now, they both wonât quite work in the following example: > foo a b = do > output "before" > r <- (liftM2(+)) a b > when r == Nothing $ output "error" > return r In case of M1, as soon as I get Nothing in r, computation will stop and return without any output gathered. In case of M2, as soon as I get Nothing in r, computation will stop and return only with output gathered so far. That is > output "error" will never be called. _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
