R double dot special identifiers -


can point doc double dot special identifiers? below context described. example how produce double dot:

l <- list() f <- function(x, y, z){   l <<- c(l, list(match.call()))   x[y][z] } invisible(lapply(list(1:2,3:4), f, c(true,true), c(true,false))) l # [[1]] # fun(x = x[[i]], y = ..1, z = ..2) #  # [[2]] # fun(x = x[[i]], y = ..1, z = ..2)

i've found them mentioned in r-lang without explanation.
assume points index of argument passed in ... read more it.

i trying use sys.call , later in post-processing use match.call on captured call in above example ends losing lapply ... arguments, , in example function primitive generic end error ... used in situation not exist.
match.call used directly in function without going sys.call , later match.call seems way catch info arguments or avoid error in such cases.
prefer understand more deeply, without going r source.

ps. match.call updated in r 3.2.0. codes above produced on r 3.2.0.


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