The splicing operator !!! operates both in values contexts like
list2() and dots_list(), and in metaprogramming contexts like
expr(), enquos(), or inject(). While the end result looks the
same, the implementation is different and much more efficient in
the value cases. This difference in implementation may cause
performance issues for instance when going from:
xs <- list(2, 3)
list2(1, !!!xs, 4)
to:
inject(list2(1, !!!xs, 4))
In the former case, the performant value-splicing is used. In the latter case, the slow metaprogramming splicing is used.
A common practical case where this may occur is when code is
wrapped inside a tidyeval context like dplyr::mutate(). In this
case, the metaprogramming operator !!! will take over the
value-splicing operator, causing an unexpected slowdown.
To avoid this in performance-critical code, use splice() instead
of !!!:
# These both use the fast splicing:
list2(1, splice(xs), 4)
inject(list2(1, splice(xs), 4))
splice(x)is_spliced(x)
is_spliced_bare(x)
A list or vector to splice non-eagerly.