[Haskell-cafe] Re: not possible with monad transformers ?
You are absolutely right! I missed that âletâ. And it does work, thanks. On the other hand it is still not what I intended, so I decided not to use Monads in this particular case. I wanted to represent values that can fail and that have complete history of errors. So that if I calculate c = (liftM2 (+)) a b and if *both* a and b have failed, c would also fail plus it would have history of errors form a and b (and potentially itâs own). I tried to do this by combining Maybe and Output monads. Now I realize that as long as I am using liftM2 it is not possible to have history of errors from a and b - only form a. And as I moved to custom lift functions I realized I didnât need monads in the first place. Anyway, thanks for your help, it is very much appreciated. Regards, Pavel. I didnât know you can have âletâ without âinâ in do expressions, nice! ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: not possible with monad transformers ?
Thanks for response; unfortunately my problem was a bit deeper. Note to self: try to verify all code snippets before posting ;-) The problem is that code on line 4 is useless. If one of the arguments a or b is Nothing, computation will just return Nothing and will not bother to execute lines 4-5: 1 foo a b = do 2 output "before" 3 let r = liftM2 (+) a b 4 when (r == Nothing) $ output "error" 5 return r -- ??? "lift r" I guess it does make perfect sense. It just was not intuitive to me that as soon as I use Maybe monad in my monad transformation I will loose ability to do output *after* error occurred. But, anyway, I think I was able to find a nice solution to that: type M2 a = OuptutMonadT Maybe String a whenError:: M2 a -> M2 a -> M2 a â 1 foo a b = do 2 output "before" 3 let r = liftM2 (+) a b 4 `whenError` $ reportError "error" 5 return r whenError combines two computations so that if first fails it will use second instead. Thanks, Pavel. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] not possible with monad transformers ?
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
