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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)


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))







A list or vector to splice non-eagerly.