[Haskell-cafe] not possible with monad transformers ?

[Pavel Zolnikov]

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.