Using modify_at() for removing elements

[echasnovski]

Recently on Twitter there was a discussion about a tidy way of removing vector elements given their names (this tweet in particular).
I suggested modify_at(x, names_to_remove, ~NULL), but it failed to work with multiple elements (thanks to this tweet from EFromTheWest user):

x <- list(a = 10, b = 15, c = 18, d = 9)
names_to_drop <- c("a", "b", "c")
purrr::modify_at(x, names_to_drop, ~NULL)
#> $b
#> [1] 15
#> 
#> $d
#> [1] 9

^{Created on 2019-10-26 by the reprex package (v0.3.0)}

This seems to be because internally modify_at() uses modify_if(), which in turn is implemented with iterative modification based on integer indexes (see these lines). This approach fails to work if elements are removed as a result of modification.

This issue is created to notify about this possible confusion and, probably, for you to consider supporting this behavior.

[lionel-]

We could support this via rlang::zap(). However I'm not sure that we should allow the result of modify_ functions to have a different size than the output.

[hadley]

We're generally going to move away from the _at() functions (#874) which means that we won't be adding any new features to them.