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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