Re: [Haskell-cafe] Alternative versus Monoid

On 21 Dec 2011, at 14:07, Erik Hesselink wrote:

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

The semantics of Maybe are

...

clear: it's failure-and-prioritized-choice.

Are you sure?

Yes.

There are (at least) four Monoid instances for Maybe [1]. With a direct instance for Maybe and its Dual you have only covered two.

Types don't just give data a representation: types evoke structure. The data stored by Maybe can be made into a monoid in several ways, but the failure-management role of Maybe makes just one of them appropriate. Cheers Conor

Erik

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

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