jonkeane commented on issue #45098:
URL: https://github.com/apache/arrow/issues/45098#issuecomment-2564326346
> My assumption here is that acero works by static analysis -- read the AST,
apply known translations, i.e. analogous to {dbplyr}
Yup, at a high level this is what's going on. You might have already found
these, but here are some pointers around `case_when` that might be helpful: we
[register the
binding](https://github.com/apache/arrow/blob/352b710f337c52485b6e50ba678a94a13c0e4113/r/R/dplyr-funcs-conditional.R#L100-L101)
which does some validation and then uses
[`arrow_eval`](https://github.com/apache/arrow/blob/352b710f337c52485b6e50ba678a94a13c0e4113/r/R/dplyr-eval.R#L18-L87)
each of the case formulas. This `arrow_eval` is to create arrow expressions
(you might not need it if you're not operating on expressions that reference
columns in a data.frame(-like) object. Then the arrow expression itself is
returned](https://github.com/apache/arrow/blob/352b710f337c52485b6e50ba678a94a13c0e4113/r/R/dplyr-funcs-conditional.R#L136-L146).
If `fcase()` don't need the tidy evaluation semantics for selecting columns,
the binding might be as simple as just using a similar `Expression$create()` on
the expressions themselv
es (there's probably a bit more to that, but much of the complication with the
dplyr bindings is getting the tidy evaluation working).
The[ test for
`case_when`](https://github.com/apache/arrow/blob/352b710f337c52485b6e50ba678a94a13c0e4113/r/tests/testthat/test-dplyr-funcs-conditional.R#L178-L218)
use a helper that is called
[`compare_dplyr_binding`](https://github.com/apache/arrow/blob/352b710f337c52485b6e50ba678a94a13c0e4113/r/tests/testthat/helper-expectation.R#L70-L83),
which in this case is a little ill-named, but should work, so long as the
input data.frame is `.input` in the test code.
All of that said, because we are looking through the AST to find bindings,
if `fcase` had an AST that aws expressions that already had bindings (either
base R or even a remap to `case_when`) in this circumstance, it _should_ work
with arrow without even needing any changes in the arrow package itself.
Something like the following _should_ work:
```
fcase_arrow <- function(..., default = NA) {
dots <- rlang::list2(...)
# for each pair of arguments, make a formula expression
formulae <- lapply(seq(1, length(dots), by = 2), function(i)
rlang::expr(dots[[!!i]] ~ dots[[!!(i + 1)]]))
dplyr::case_when(!!!formulae, .default = default)
}
```
You could build up the acero with `Expression$create("case_when", ...)`
expressions rather than using `rlang`'s expression and dots management there.
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