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

[Javran Cheng]

That's a valid point. come to think of it, is there any use case where Void
appears on a positive position?

On Sun, Jan 13, 2019 at 1:15 AM Isaac Elliott <isaace71295 at gmail.com> wrote:

> If you had an `RWST r Void s m a`, then you would be able to produce an `m
> (a, s, Void)`, which is `absurd`.
>
> On Sun, 13 Jan. 2019, 6:43 pm Javran Cheng, <javran.c at gmail.com> wrote:
>
>> (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 <will.yager at gmail.com>
>> wrote:
>>
>>> 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
>>>
>>
>>
>> --
>> Javran (Fang) Cheng
>>
>>
>> --
>> Javran (Fang) Cheng
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>
>

-- 
Javran (Fang) Cheng
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