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

[Dan Burton]

+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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