fcoalesce could coalesce on NULL

[MichaelChirico]

Consider trying to extract an element of a list that might not exist:

sapply(l, function(x) x$a)

if a is missing from any element of l, the output will be a list, otherwise it'll be a vector.

There are a few clumsy workarounds e.g.

sapply(l, function(x) if ('a' %chin% names(x)) x$a else 1)
sapply(l, function(x) { y <- x$a; if (is.null(y)) 1 else y})

All are harder to read* than

sapply(l, function(x) fcoalesce(x$a, 1))

The idea would be to essentially treat NULL as NA within fcoalesce. The overhead should be low since we can do this up front. Currently not so:

fcoalesce(NULL, 1)
# NULL
fcoalesce(NA, NULL, 1)
# Error: Item 2 is type NULL but the first item is type logical. Please coerce before coalescing.

*(this is always subjective but I particularly have in mind users who are comfortable with SQL)

[shrektan]

Or, more accurately, fcoalesce() should just ignore the NULL arguments...

One case that I may encounter this issue in reality is:

args <- list(x, y, z, ...) # one of them may or may not be NULL 
do.call(fcoalesce, args) # should work, otherwise the user has to call `args <- Filter(Negate(is.null), args)` first.

[venom1204]

Hi @MichaelChirico,
I would like to work on this issue.

My proposed approach would be to modify the C implementation with the following logic:

• Find the first non-NULL argument: This will be implemented using a while loop to efficiently find the index of the first non-NULL argument. This replaces the current logic which assumes ...[[1]] is the base vector. A while loop is ideal here as it's a direct and performant way to scan the arguments without any memory overhead. This found vector will then be used to determine the output type and length. If the loop completes without finding a valid argument, the function will correctly return NULL.
• Skip intermediate NULL arguments: During the subsequent loops for type validation and data processing, any argument that is NULL will be skipped with a simple continue statement. This will prevent errors and crashes.

If this plan sounds good to you, I'm happy to proceed with preparing a pull request.

[MichaelChirico]

These days you should just use %||% in {base} for NULL. I'm loath to add it to fcoalesce() since NULL is not the same as NA.