[Haskell-cafe] Void is not Monoid?

[William Yager]

On Sun, Jan 13, 2019 at 4:00 PM Javran Cheng <javran.c at gmail.com> wrote:

> Hi Cafe,
>
> I'm wondering why Data.Void does not have a Monoid instance, or, what
> would be the problem if we do "mempty = absurd mempty"?
>

This diverges, does it not?

A monoid has an identity element, and void does not.


> Long story: I was using a monad with some transformers, then I realize I
> can collapse State and Reader into RWST with Void being Writer output.
> (well, I could have just used Unit but I wanna give Void a try...) I know
> beforehand that Void is Semigroup but is a bit surprise it doesn't have
> Monoid instance.
>

How would you write return with void as the writer?

You can accomplish what you want with the free monoid over Void - i.e.
[Void], which is isomorphic to unit. So unit seems like the right choice.

--Will
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20190113/1899bab3/attachment.html>