On Tue, May 25, 2010 at 6:34 PM, Ross Paterson
On Tue, May 25, 2010 at 04:55:09PM -0400, Edward Kmett wrote:
In my perfect world the next version of transformers would provide an entry in Control.Monad.Instances for:
instance Applicative (Either m) instance Alternative (Either m) instance Monad (Either m) instance MonadPlus (Either m)
What definition of empty/mzero do you have in mind?
Thats what I get for dashing off my response half baked. =) Upon actually looking, the code that I have for that Applicative didn't supply an Alternative/MonadPlus instance, since there isn't a suitable 'Default' class that provides a single value without a full monoid, and if it existed, you'd want to use it to support 'fail'
But there are quite possibly too many moving parts to make such a big jump, and then you have the other bikeshedding issue of there being yet another definition for Either that makes sense to default to:
instance Monoid m => Applicative (Either m) instance Monoid m => Alternative (Either m) instance Monoid m => Monad (Either m) instance Monoid m => MonadPlus (Either m)
I can't think of a Monad instance that uses the Monoid constraint. What is the instance you mean?
The Monoid instance only exists to provide mempty to support fail, in a manner consistent with the Alternative and MonadPlus instance as no weaker class exists to use to obtain a Default. Neither answer is really entirely satisfying. -Edward Kmett