[Haskell-cafe] Fwd: Void is not Monoid?

(forgot to reply all, sorry) Hi Will, Thanks for the reply!

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

now I feel dull never thought about that.

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.

Unit does work fine, but I figure using Void is an interesting idea, as I can make sure that no one can use the "W" part of my RWST. Javran On Sun, Jan 13, 2019 at 12:23 AM William Yager wrote:

On Sun, Jan 13, 2019 at 4:00 PM Javran Cheng 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

-- Javran (Fang) Cheng -- Javran (Fang) Cheng