Re: [Haskell-cafe] commutative monoid?

On Sat, Jun 25, 2011 at 02:34:47PM -0700, Evan Laforge wrote:

On Sat, Jun 25, 2011 at 2:02 PM, Brent Yorgey wrote:

...

Actually, there are (at least) four: there's also the one where mappend = liftA2 mappend, i.e. introduce potential failure into a monoid operation defined on the values.  I wrote about it here:

   http://byorgey.wordpress.com/2011/04/18/monoids-for-maybe/

Just out of curiosity, what was the problem that wanted this kind of monoid for the solution? I always find concrete examples useful in addition to the abstract explorations and toy examples.

To be honest, I don't remember! -Brent