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

[Malcolm Wallace]

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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