Is there a function \x -> (x, x) in standard libraries?

[David Feuer]

I don't think an obscure GHC IO function needs to be considered with regard
to naming. I don't currently have an opinion about whether to add this.

On Tue, Jul 16, 2019, 6:30 PM Theodore Lief Gannon <tanuki at gmail.com> wrote:

> Note that there is already a function named dup in base [1], albeit in
> what's probably a low-traffic module (GHC.IO.Device).
>
> As an alternative I would suggest twin, which returns no results from
> Stackage Hoogle.
>
> On the other hand, I'm roughly -0.5 on adding this at all; what use case
> does it serve that's not better handled with &&&? (Those 22 occurences of id
> &&& id seem most likely to provide illumination...)
>
> [1]:
> https://hackage.haskell.org/package/base-4.5.1.0/docs/GHC-IO-Device.html#v:dup
>
> On Tue, Jul 16, 2019 at 2:22 PM Dan Burton <danburton.email at gmail.com>
> wrote:
>
>> +1 for "dup", if it is to be included at all.
>>
>> -- Dan Burton
>>
>>
>> On Tue, Jul 16, 2019 at 12:56 PM Vladislav Zavialov <
>> vladislav at serokell.io> wrote:
>>
>>> It is reasonably common. I have grepped Hackage using
>>> https://codesearch.aelve.com/haskell/ and
>>>
>>> * \x->(x,x) occurs 203 times
>>> * join (,) occurs 53 times
>>> * id &&& id occurs 22 times
>>>
>>> It also pops up in discussions as a counter-example of a linear function:
>>>
>>> dup :: a ->. (a,a)
>>> dup x = (x,x)  -- does not typecheck with -XLinearTypes!
>>>
>>> I think it should be added under the name "dup" (short for “duplicate”).
>>>
>>> - Vlad
>>>
>>> > On 16 Jul 2019, at 22:29, Lennart Augustsson <lennart at augustsson.net>
>>> wrote:
>>> >
>>> > It's not harmful, but is it common enough to have a name?
>>> >
>>> > On Tue, Jul 16, 2019, 12:28 Ignat Insarov <kindaro at gmail.com> wrote:
>>> > So, it is not considered harmful. Then I propose we add it to
>>> > Data.Tuple under a humane name.
>>> >
>>> > On Tue, 16 Jul 2019 at 19:41, Lennart Augustsson <
>>> lennart at augustsson.net> wrote:
>>> > >
>>> > > Or 'join (,)'
>>> > >
>>> > > On Tue, Jul 16, 2019, 09:26 Zemyla <zemyla at gmail.com> wrote:
>>> > >>
>>> > >> id &&& id
>>> > >>
>>> > >> On Tue, Jul 16, 2019, 10:58 Ignat Insarov <kindaro at gmail.com>
>>> wrote:
>>> > >>>
>>> > >>> Hello.
>>> > >>>
>>> > >>> I wonder if there is a function like \x -> (x, x) in the standard
>>> libraries.
>>> > >>>
>>> > >>> I looked up in Hoogle. It gave me links:
>>> > >>>
>>> > >>> 1.
>>> https://hackage.haskell.org/package/utility-ht/docs/Data-Tuple-HT.html#v:double
>>> > >>> 2.
>>> https://hackage.haskell.org/package/extra/docs/Data-Tuple-Extra.html#v:dupe
>>> > >>> 3.
>>> https://hackage.haskell.org/package/relude/docs/Relude-Extra-Tuple.html#v:dupe
>>> > >>>
>>> > >>> None of these is to the standard library. I wonder if there is an
>>> > >>> intentional omission for some important reason? If not, I think
>>> this
>>> > >>> function should be added to Data.Tuple.
>>> > >>>
>>> > >>> I know some of the persons that make decisions here like Category
>>> > >>> theory, so I point out this function is mentioned in Categories for
>>> > >>> the Working Mathematician, as δ, the diagonal function, on page 3
>>> > >>> (second edition), although set with angular brackets.
>>> > >>>
>>> > >>> Have a great day.
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>>> > >>
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