On 30 Nov 2004, at 20:55, Pavel Zolnikov wrote:
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 didn’t need monads in the first place.
It's not liftM2 that's the problem, it's rather the strategy of the Maybe monad. The strategy of the maybe monad is very roughly 'abort on error'. As you correctly observe, it doesn't make it at all easy to collect error messages, let alone tree-shaped error graphs. However, your problem *does* have a natural underlying monad, if you care to use it. It's a kind of Error monad, but not the standard one, since you would have to implement your chosen rules for combining errors. I'm not quite sure exactly what you want, probably either a list of errors, or a tree of errors, depending how much structure you're trying to preserve. You may not find the monad approach easier than just coding by hand, though... although if you have a large amount of code, I would expect monads to make it a bit shorter and clearer. Jules