Proposal: add `on` to the Prelude

[Erik Hesselink]

On Fri, 13 Sep 2019 at 13:54, Henning Thielemann <
lemming at henning-thielemann.de> wrote:

>
> On Tue, 10 Sep 2019, David Feuer wrote:
>
> > Every time I reach for Data.Function.on, I feel like a total dolt for
> having to import a module to get a function
> > whose implementation is barely longer than the import. And it's a really
> good function too! Can we please add it
> > to the Prelude?
> >   on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
> >   (.*.) `on` f = \x y -> f x .*. f y
>
> Most of the time I would use this I can use the more specific 'equating'
> and 'comparing'.
>

Does 'equating' exist in base now? I've always missed having that function
in the past, and defined it myself many times.

Erik
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