> On 6 Jun 2017, at 18:50 , Hong Ooi via R-devel <r-devel@r-project.org> wrote: > > This is something I came across just now: > > f <- function(x) missing(x) > z <- quote(expr=) > > f(z) > # TRUE > > The object z contains the equivalent of a missing function argument. Another > method for generating a missing arg would be alist(a=)$a . > > Should f(z) return TRUE in this case? I interpret missing() as checking > whether the parent function call had a value supplied for the given argument. > Here, I have supplied an argument (z), so I would expect f to return FALSE.
Missing values propagate in R, e.g. > f <- function(x) missing(x) > g <- function(y) f(y) > g() [1] TRUE This is technically done by having a "missing" object, which is not really intended to be visible to users, but pops up in a few esoteric constructions. Trying do anything constructive with the missing object usually leads to grief, or at least surprises, e.g.: > z <-quote(expr=) > z <- z Error: argument "z" is missing, with no default -pd -- Peter Dalgaard, Professor, Center for Statistics, Copenhagen Business School Solbjerg Plads 3, 2000 Frederiksberg, Denmark Phone: (+45)38153501 Office: A 4.23 Email: pd....@cbs.dk Priv: pda...@gmail.com ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel