[Haskell-cafe] not possible with monad transformers ?

[Mike Gunter]

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 ‘in parallel’. 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:
>
> Let’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 won’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.
>
>
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