(from discussion in ARROW-9489 with @maartenbreddels)

A general "where" kernel like np.where (https://numpy.org/doc/stable/reference/generated/numpy.where.html) seems a generally useful kernel to have, and could also help mimicking some other python (setitem-like) operations.

The concrete use case in ARROW-9489 is to basically do a fill_null(array[string], array[string]) which could be expressed as where(is_null(arr), arr2, arr).

Reporter: Joris Van den Bossche / @jorisvandenbossche
Assignee: Niranda Perera / @nirandaperera

Related issues:

• [C++] Add additional type support for if_else kernel (is a parent of)
• [C++][Python] if_else kernel doesn't support upcasting (is a parent of)
• [C++] Add a 'choose' kernel/scalar compute function (relates to)
• [C++] Add "replace" kernel (is related to)
• [C++] Add "replace" kernel (supercedes)
• [R] Add bindings for ifelse() and if_else() (is depended upon by)

PRs and other links:

• GitHub Pull Request #10377
• GitHub Pull Request #10410

_{Note: This issue was originally created as ARROW-10640. Please see the migration documentation for further details.}