Re: Proposal #3339: Add (+>) as a synonym for mappend

The main concern with generalizing (++) is that it was once generalized -- completely differently! -- for mplus in MonadPlus. So whether Monoid's mappend is the natural generalization of (++) or MonadPlus's mplus is, is not entirely clear. Neither one can completely subsume all of the use-cases of the other. A secondary concern is that neither MonadPlus nor Monoid are in the Prelude, so its generalized form would have to be exported from Data.Monoid with a different type signature breaking any pre-existing code that brought in Data.Monoid unqualified and happened to use lists. I'm less sold by the second concern than the first one, but both make me hesitate. Bryan's/Jules's existing proposal of a new operator avoids both of these snarls. -Edward Kmett On Fri, Jul 17, 2009 at 10:53 AM, Alexander Dunlap < alexander.dunlap@gmail.com> wrote:

On Tue, Jun 30, 2009 at 5:37 PM, Bryan O'Sullivan wrote:

...

Ticket: http://hackage.haskell.org/trac/ghc/ticket/3339

From the ticket:

This proposal was, I think, originally suggested by Jules Bean. The idea is to add two functions to the Data.Monoid module, (+>) and (<+), corresponding to different uses of mappend. These should not be methods of the Monoid typeclass, but top-level functions.

I hope (but slightly doubt) that the visual nature of the two operators might help to counter the thought that monoids are just for gluing things together.

(+>) :: (Monoid a) => a -> a -> a a +> b = a `mappend` b

(<+) :: (Monoid a) => a -> a -> a a <+ b = b `mappend` a

infixl 4 +> infixl 4 <+

Proposed deadline: two weeks.

If this looks reasonable, I'll attach darcs patches.

I (and apparently a lot of other people who commented on that ticket) would prefer that (++) be generalized instead of introducing a new operator.

Alex _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries