[R] Determine the type of an unevaluated expression

[asfimport]

There would be several benefits to knowing the data type of an expression before its value has actually been computed. Probably the greatest benefit would be that other expressions could then behave dynamically based on the data types of their inputs. Consider how this capability might be implemented.

Reporter: Ian Cook / @ianmcook
Assignee: Neal Richardson / @nealrichardson

Related issues:

• [R] Support tidyselect where() selection helper in dplyr verbs (is depended upon by)

_{Note: This issue was originally created as ARROW-12291. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
I think this just isn't knowable from R without evaluating the expressions, or otherwise reimplementing the C++ kernel logic.

[asfimport]

Ian Cook / @ianmcook:
I wonder whether the function registry in the C++ library could be extended with a function that could be called exactly like arrow::compute::CallFunction but that would return the resultant data type instead of the result itself (and would do so without actually running any computations on the data).

[asfimport]

Neal Richardson / @nealrichardson:
Done in ARROW-12731