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

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 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:d...

On Tue, Jul 16, 2019 at 2:22 PM Dan Burton wrote:

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+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@serokell.io> wrote:

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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

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On 16 Jul 2019, at 22:29, Lennart Augustsson wrote:

It's not harmful, but is it common enough to have a name?

On Tue, Jul 16, 2019, 12:28 Ignat Insarov 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@augustsson.net> wrote:

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Or 'join (,)'

On Tue, Jul 16, 2019, 09:26 Zemyla wrote:

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id &&& id

On Tue, Jul 16, 2019, 10:58 Ignat Insarov

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> > Hello. > > I wonder if there is a function like \x -> (x, x) in the standard

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> > I looked up in Hoogle. It gave me links: > > 1. https://hackage.haskell.org/package/utility-ht/docs/Data-Tuple-HT.html#v:dou... > 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:du... > > None of these is to the standard library. I wonder if there is an > intentional omission for some important reason? If not, I think

wrote: libraries. this

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> 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. > _______________________________________________ > Libraries mailing list > Libraries@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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