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
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.
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