[Haskell-cafe] Map Monoid instance (was commutative monoid?)

[Erik Hesselink]

On Sat, Jun 25, 2011 at 19:07, Evan Laforge <qdunkan at gmail.com> wrote:
> On Sat, Jun 25, 2011 at 9:00 AM, Jens Blanck <jens.blanck at gmail.com> wrote:
>>> > So there's a range of possible Monoid instances for each type,
>>>
>>> More for some types than for others. For Maybe there are three:
>>>
>>>  * always take the first/left value;
>>>  * always take the last/right value;
>>>  * or, use a semigroup operation defined on the values.

Brent Yorgey recently blogged about a fourth instance [1] which also
uses the semigroup operation on the values, but treats Nothing as
failure, returning Nothing.

Erik

[1] https://byorgey.wordpress.com/2011/04/18/monoids-for-maybe/