GH-36264: [R] Add scalar() function

[thisisnic]

Fixes #36264 36264

• Closes: [R] Add scalar() function #36264

[thisisnic]

I feel like there may be a reason we haven't done this previously, though a cursory glance to see if it's an exported function from other packages (I just ran ??scalar locally) didn't turn up any results.

[paleolimbot]

Definitely worth having! Perhaps it didn't happen because we also have Expression$scalar() which is what all the bindings use.

It's true that scalar() is a tiny bit generic...the closest thing I found was rlang::is_scalar_xx() and assertthat::is.scalar() (which helpfully returns TRUE for an arrow::Scalar. I think it's fine! There's always arrow_scalar() if you're worried.

[thisisnic]

It's true that scalar() is a tiny bit generic...the closest thing I found was rlang::is_scalar_xx() and assertthat::is.scalar() (which helpfully returns TRUE for an arrow::Scalar. I think it's fine! There's always arrow_scalar() if you're worried.

Thanks for looking for those! If you're happy with this as-is, I'm happy too. Will merge once CI is green.

[github-actions]

⚠️ GitHub issue #36264 has been automatically assigned in GitHub to PR creator.

[conbench-apache-arrow]

Conbench analyzed the 6 benchmark runs on commit ed996933.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details.