Yusaku Hashimoto wrote:
If I understood your post correctly, you said
- generalizing lookup to MonadPlus or Alternative or such classes are not necessary
In particular, it doesn't become more general, it becomes less general (in a sense).
- use Maybe as usual, we should use adapters as we need
Conor, You have said this many times elsewhere, but unfortunately, I heard it for the first time =) so please correct me if I'm wrong.
I thought generalizing lookup is good example for usage of the MonadPlus as I read in RWH[1], but you said it's not necessary.
Now, I understood there are two positions for such classes. One is using generalizing for it, another is not.
So, I want to know that when such classes should be used from later position.
Heinrich suggested that is for overloading.
To elaborate on generality versus overloading: the function lookupM :: MonadPlus m => k -> Map k a -> m a is not more general than lookup :: k -> Map k a -> Maybe a because you can implement the former with the latter lookupM k = mop . lookup k mop = maybe mzero return In other words, lookupM doesn't introduce new functionality. Rather, it gives you the syntactic convenience of not having to mention mop by overloading the result type. In other words, you can write lookup = lookupM or lookupE :: k -> Map k a -> Either e a lookupE = lookupM
But do any other usages are exist?
I'm not quite sure I understand what you mean here? Regards, apfelmus -- http://apfelmus.nfshost.com