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

[Edward Kmett]

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

Sent from my iPhone

> 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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