Re: [Haskell-cafe] Alternative versus Monoid

On Wed, Dec 21, 2011 at 14:10, Bas van Dijk wrote:

On 16 December 2011 16:26, Yves Parès wrote:

...

"1) What about the First type? Do we {-# DEPRECATE #-} it?"

Personnaly, I'm in favor of following the same logic than Int: Int itself is not a monoid. You have to be specific: it's either Sum or Mult.

It should be the same for Maybe: we remove its instance of Monoid, and we only use First and Last.

The reason you need to be specific with Int is that it's not clear which semantics (sum or product) you want. The semantics of Maybe are clear: it's failure-and-prioritized-choice.

Are you sure? There are (at least) four Monoid instances for Maybe [1]. With a direct instance for Maybe and its Dual you have only covered two. Erik [1] https://byorgey.wordpress.com/2011/04/18/monoids-for-maybe/