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

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 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 wrote:

...

Or 'join (,)'

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

...

id &&& id

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

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