[Haskell-cafe] Map Monoid instance (was commutative monoid?)
hesselink at gmail.com
Sat Jun 25 20:15:20 CEST 2011
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  which also
uses the semigroup operation on the values, but treats Nothing as
failure, returning Nothing.
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