[Haskell-cafe] MonadPlus or Alternative or ...?

[Sean Leather]

On Sun, May 2, 2010 at 12:07, Sebastian Fischer wrote:

>
> On May 2, 2010, at 11:10 AM, Sean Leather wrote:
>
>  Or should I make my own class?
>>
>> Then, there obviously won't be any instances for existing types other than
>> your own (which might be a good or bad thing). You may want to do this, if
>> you don't want to require either (>>=) or (<*>), which may not be supported
>> for reasonable instances of your own class.
>>
>> I don't really want to do this option, because it increases the
>> understanding overhead needlessly, I think. Everything else that is provided
>> by Applicative/Monad doesn't matter. I require only the three operations
>> above and do not disallow extra operations.
>>
>
> By not making your own class, you prohibit types that do not support <*>
> (or >>=). That's fine, but an additional burden to instance writers. By not
> listing extra operations in your own class you would not disallow them. You
> require them by not making your own class without them.
>

Understood and agreed. Having my own class would be the most flexible in
what it allows and disallows.

But still, your wish to reuse existing classes may be a fine reason to
> impose an additional burden.


There is an additional maintenance burden that I've recently become aware
of, considering I haven't done a very good job of maintaining my own code.
;) To use my own class, I should reasonably provide instances for everything
that I possibly can. This would include (at least) lists, Maybe,
WrappedAlternative, and WrappedMonadPlus.

Who knows? I may change my mind in a while. In that case, would you have any
suggestions on a name for such a class or names for the methods? ;)

class C f where
  zero :: f a
  one :: a -> f a
  append :: f a -> f a -> f a

Sean
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20100502/876f96bf/attachment.html