Proposal: add Bounded instances for Maybe and Either

[Edward Kmett]

minBound = Left minBound <= Left a < Right b <= Right maxBound = maxBound

fits with the behavior of the existing Ord instance.

On the other hand, Left (), being strictly less than minBound would rather
drastically undermine the expected interaction of Ord and Bounded.

-Edward


On Tue, May 8, 2018 at 1:06 AM, Ivan Lazar Miljenovic <
ivan.miljenovic at gmail.com> wrote:

> On 8 May 2018 at 13:19, David Feuer <david.feuer at gmail.com> wrote:
> > We can write
> >
> > instance Bounded a => Bounded (Maybe a) where
> >   minBound = Nothing
> >   maxBound = Just maxBound
> >
> > instance (Bounded a, Bounded b) => Bounded (Either a b) where
> >   minBound = Left minBound
> >   maxBound = Right maxBound
> >
> > While Bounded intentionally does not have an Ord superclass, I think it's
> > worth mentioning that these are the instances that arise naturally from
> the
> > Ord instances for the types in question.
>
> Except these may be surprising to people; e.g. someone may expect that
> `minBound = Right minBound` as well.  Though as Maybe and Either don't
> have Enum instances this may be less of an issue.
>
> --
> Ivan Lazar Miljenovic
> Ivan.Miljenovic at gmail.com
> http://IvanMiljenovic.wordpress.com
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