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.
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@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@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@augustsson.net> wrote:
>>>
>>> Or 'join (,)'
>>>
>>> On Tue, Jul 16, 2019, 09:26 Zemyla <zemyla@gmail.com> wrote:
>>>>
>>>> id &&& id
>>>>
>>>> On Tue, Jul 16, 2019, 10:58 Ignat Insarov <kindaro@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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