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

On 6/25/11 2:15 PM, Erik Hesselink wrote:

On Sat, Jun 25, 2011 at 19:07, Evan Laforge wrote:

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On Sat, Jun 25, 2011 at 9:00 AM, Jens Blanck wrote:

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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/

Ah, yes. I forgot about that one. Though technically that one actually requires a monoid, not just a semigroup, since mempty = Just mempty. Which furthers the point that most types have an abundance of monoid instances. -- Live well, ~wren