keep() and discard() err on NA ... perhaps unintentionally?

[mmuurr]

I haven't checked previous {purrr} versions to see if this is new(ish) behavior, but for some reason this surprised me:

keep(c(TRUE, FALSE, NA), identity)
# Error in `keep()`:
# ℹ In index: 3.
# Caused by error:

(Same for discard().)

When inspecting keep(), however, we see that:

keep
# function(.x, .p, ...) {
#   where <- where_if(.x, .p, ...)
#   .x[!is.na(where) & where]
# }

... suggesting that it's intended to handle NA values (same for discard()), but it's purrr:::where_if() that's actually throwing the error.

Is this intended?

While I tend to try to opt for explicitness as much as possible, I think the NA-handling as coded in keep() and discard() make sense and align with dplyr::filter()'s logic. I also think that enabling NA-handling as coded again would be backwards compatible, since any existing code must be coercing (or otherwise explicitly-handling) NAs anyhow.

FWIW, the documentation doesn't explicitly mention NA-handling, but states:

• for keep: "all elements where .p evaluates to TRUE", which would be valid as coded
• for discard: "all elements where .p evaluates to FALSE", which is not exactly valid as coded, as discard()'s definition also retains NAs (if where_if() didn't throw).

Alternatively, adding something like .na = c(TRUE, FALSE) parameter (with match.arg) would be a clear & compact way to make such choices explicit, while removing common coercion cruft in .p.

Cheers!

{purrr} v1.0.2