[Haskell-cafe] Re: not possible with monad transformers ?

[Pavel Zolnikov]

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 …

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.