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