Min & Max without Bounded

[Hécate]

Hi Jon,

While I do not have a particularly strong opinion on the changes you're 
suggesting, I invite you to read the source of this module, which may 
provide some context for the current situation:

https://hackage.haskell.org/package/base-4.14.0.0/docs/src/Data.Functor.Utils.html#%23

Cheers!

Le 10/11/2020 à 22:19, Jon Purdy a écrit :
> The ‘Min’ and ‘Max’ newtypes from ‘Data.Semigroup’ require a ‘Bounded’ 
> constraint for their ‘Monoid’ instances, because they wrap a value of 
> type ‘a’ rather than ‘Maybe a’. The latter is something I’ve needed a 
> few times lately for types that cannot be bounded (or just don’t want 
> to be), and it seems base-worthy to me.
>
> So if it hasn’t been proposed and dismissed already (or if it has been 
> proposed, but just got backburnered), I’d like to suggest adding the 
> following new types, and at least the following instances, to either 
> ‘Data.Semigroup’ or ‘Data.Monoid’:
>
> newtype Minimum a = Minimum { getMinimum :: Maybe a }
>   deriving (Eq, Ord, Monoid, Semigroup)
>     via (Down (Maybe (Down (Max a))))
>
> newtype Maximum a = Maximum { getMaximum :: Maybe a }
>   deriving (Eq, Ord, Monoid, Semigroup)
>     via (Maybe (Max a))
>
> These instances could be specified explicitly instead of with 
> ‘DerivingVia’, but the essential notion here is that ‘Maximum’ uses 
> its empty value ‘Nothing’ as a “negative infinity”, less than all 
> other ‘Just’ values, while ‘Minimum’ reverses that to use ‘Nothing’ as 
> the corresponding “positive infinity”.
>
> I assume these would also want more or less the same array of 
> instances as for the existing ‘Min’ and ‘Max’, and there is the usual 
> opportunity to bikeshed about the naming and how these instances ought 
> to relate to one another. I figure that at least the ‘Ord’ instance 
> should match the ordering that the ‘Semigroup’ instance uses, to avoid 
> some potential for confusion/bugs.
>
> As for use cases, I tend to reach for these things either in 
> configuration records with many fields that I’m combining by semigroup 
> append (Max, Any, set union, and so on) or when I have a 
> possibly-empty container and don’t want to write e.g. ‘maximum (0 : 
> xs)’ to avoid partiality. For example, ‘foldMap (Minimum . Just &&& 
> Maximum . Just)’ computes the minimum and maximum in a single pass, 
> returning Nothing for a null input.
>
> Any thoughts, critiques, questions, rebuttals, edge cases, use cases, 
> &c. are most welcome!
>
>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- 
Hécate ✨
🐦: @TechnoEmpress
IRC: Uniaika
WWW: https://glitchbra.in
RUN: BSD

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20201110/56dce613/attachment.html>