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

[Conal Elliott]

On Wed, Jul 17, 2019 at 11:00 AM Edward Kmett <ekmett at gmail.com> wrote:

> +1 for dup either exported by Data.Tuple or via Data.Arrow in greater
> generality.
>

Ditto. +1 for either of these two options.


> On Jul 17, 2019, at 1:49 PM, Jon Purdy <evincarofautumn at gmail.com> wrote:
>
> 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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