splice
Splice lists
Description
-
splicemarks an object to be spliced. It is equivalent to using!!!in a function taking dynamic dots. -
dots_splice()is likedots_list()but automatically splices list inputs.
Usage
splice(x)
is_spliced(x)
is_spliced_bare(x)
dots_splice(
...,
.ignore_empty = c("trailing", "none", "all"),
.preserve_empty = FALSE,
.homonyms = c("keep", "first", "last", "error"),
.check_assign = FALSE
)Arguments
x |
A list to splice. |
... |
Arguments to collect in a list. These dots are dynamic. |
.ignore_empty |
Whether to ignore empty arguments. Can be one
of |
.preserve_empty |
Whether to preserve the empty arguments that
were not ignored. If |
.homonyms |
How to treat arguments with the same name. The
default, |
.check_assign |
Whether to check for |
Standard splicing versus quoting splicing
-
In quoting functions
!!!disaggregates its argument (let's call itx) into as many objects as there are elements inx. E.g.quo(foo(!!! c(1, 2)))is completely equivalent toquo(foo(1, 2)). The creation of those separate objects has an overhead but is typically not important when manipulating calls because function calls typically take a small number of arguments. -
In standard functions, disaggregating the spliced collection would have a negative performance impact in cases where
dots_list()is used to build up data structures from user inputs. To avoid this spliced inputs are marked withsplice()and the final list is built with (the equivalent of)flatten_if(dots, is_spliced).
Most of the time you should not care about the difference. However
if you use a standard function taking tidy dots within a quoting
function, the !!! operator will disaggregate its argument because
the behaviour of the quasiquoting function has priority. You might
then observe some performance cost in edge cases. Here is one
example where this would happen:
purrr::rerun(10, dplyr::bind_rows(!!! x))
purrr::rerun() is a quoting function and dplyr::bind_rows() is
a standard function. Because bind_rows() is called inside
rerun(), the list x will be disaggregated into a pairlist of
arguments. To avoid this you can use splice() instead:
purrr::rerun(10, dplyr::bind_rows(splice(x)))
Life cycle
-
dots_splice()is in the questioning stage. It is part of our experiments with dots semantics. Compared todots_list(),dots_splice()automatically splices lists. We now lean towards adopting a single type of dots semantics (those ofdots_list()) where splicing is explicit. -
splice()is in the questioning stage. It is not clear whether it is really needed as there are other ways to avoid the performance issue discussed above.