Mike Gunter m at ryangunter.com
Mon Nov 29 17:53:41 EST 2004

```
This:

foo a b = do
output "before"
let r = liftM2 (+) a b
when (r == Nothing) \$ output "error"
return r -- ??? "lift r"

seems to address your complains, but I don't understand what you want
well enough to know if this is that.

mike

Pavel Zolnikov <pavelzolnikov at yahoo.com> writes:

> 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 â€˜in 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:
>
> Letâ€™s 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 wonâ€™t 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.
>
>
> _______________________________________________