ARROW-12669:[C++][Python] Implement a new scalar function: list_element

[aucahuasi]

https://issues.apache.org/jira/browse/ARROW-12669

[github-actions]

https://issues.apache.org/jira/browse/ARROW-12669

[aucahuasi]

Tomorrow I'll work on the python and R bindings for this new function
cc @nealrichardson@edponce@ianmcook

[lidavidm]

Just a quick thought, why is this a vector kernel instead of a scalar one? (Though with one input it's maybe not a big difference.)

[lidavidm]

Oh wait nevermind, the index here is a parameter and not an options.

[nealrichardson]

why is this a vector kernel instead of a scalar one?

I think it has to be a scalar kernel if we want to be able to call it in a projection node, which is why we're doing this now--right?

[lidavidm]

why is this a vector kernel instead of a scalar one?

I think it has to be a scalar kernel if we want to be able to call it in a projection node, which is why we're doing this now--right?

Ah, right - plus expressions aren't evaluatable unless they're scalar:

arrow/cpp/src/arrow/compute/exec/expression.cc

Lines 503 to 506 in 9cf2372

	if (!expr.IsScalarExpression()) {
	returnStatus::Invalid(
	"ExecuteScalarExpression cannot Execute non-scalar expression ", expr.ToString());
	}

I think most kernels like this pass the second argument via options, not as an argument. I was also debating whether to comment about this, but might as well: there's no need to generate kernels for different index types, especially if we just make the index an option instead of a parameter. (It'll also limit the number of special cases to account for in type coverage testing.)

[jorisvandenbossche]

I think most kernels like this pass the second argument via options, not as an argument.

Why can't this be a scalar kernel while still having index as a second argument?
Having the index as argument keeps the flexibility to be able to get a different index of each row (I don't know if this would be an important functionality though, but there are probably use cases)

[lidavidm]

I think most kernels like this pass the second argument via options, not as an argument.

Why can't this be a scalar kernel while still having index as a second argument?
Having the index as argument keeps the flexibility to be able to get a different index of each row (I don't know if this would be an important functionality though, but there are probably use cases)

Ah, you're right. So this should just be a scalar kernel, then.

[lidavidm]

Thanks! This looks good to me, I just left one note - the scalar kernel doesn't need to be templated on the list type, so you can save yourself some generated code there.

[lidavidm]

LGTM, thanks!

[edponce]

Thanks for resolving this issue!