splice and !!! don't seem to have identical behaviours

[kieranjmartin]

My understanding from the documentation is that these should behave in the same way, but I have found at least one (slightly convoluted) example where this is not the case.

(this is in R 4.4.1 with rlang 1.1.4)

simpler_inside <- function(c,d){
  c+d
}

simpler <- function(...){
  ldots <- list(...)
  rlang::inject(
    simpler_inside(
      !!!ldots
    )
  )
}
simpler(3,4)

This works, but if you replace simpler with

simpler <- function(...){
  ldots <- list(...)
  rlang::inject(
    simpler_inside(
      rlang::splice(ldots)
    )
  )
}
simpler(3,4)

This now errors with Error in simpler_inside(rlang::splice(ldots)) : argument "d" is missing, with no default. Looking at the debug, it seems like in this example the "splice" call is passed as the first argument to simpler_fun and then unpacks, and ideally should unpack before that.

I appreciate this example is a little artificial (there's no reason for me to turn the dots into a list here); my actual example involves a function factory where I need to extract the dots to pass to a function.

Perhaps this is me not understanding the differences, but I had thought that splice and !!! should act in the same way here.

[lionel-]

yep splice() is lazy and only interpreted by list2().

[kieranjmartin]

Apologies, is this documented anywhere? The only reference to splice explicitly seems to be here which just says "To avoid this in performance-critical code, use splice() instead of !!!:" which led me to think they were perhaps interchangeable.

[lionel-]

I added some docs in ad84ce4

The docs are not great because this is an obscure (and forgotten) part of dynamic dots.

[kieranjmartin]

Thanks, that looks good!