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

[Jon Purdy]

I often define this in my own code (especially arrowful/pointfree code) as
‘dup = join (,)’—a fairly common name for this from the lands of category
theory and concatenative programming, and a common abbreviation of
‘duplicate’ in other contexts like the aforementioned dup(2) for file
descriptors.

It’s fundamental enough a logical operation (contraction) to warrant a
name, in my opinion—we already have ‘const’ for weakening and ‘flip’ for
exchange.

On Wed, Jul 17, 2019, 05:10 Malcolm Wallace via Libraries <
libraries at haskell.org> wrote:

> I think hiding the function (\x->(x,x)) behind any of these indirections
> (dup, join (,), or (id&&&id)) is just wilful obfuscation.  The clearest and
> most direct way to understand the function is to use the lambda - it needs
> no hard-to-remember name.
>
> M.
>
> On 16 Jul 2019, at 20:56, Vladislav Zavialov 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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