I wonder if it would be helpful to mention in ?stopifnot that
stopifnot(logical(0)) does not give an error (for background on why this
is the case, see [1]). For example, ?all explicitly mentions the
following:
That all(logical(0)) is true is a useful convention
and includes an example:
all(logical(0)) # true, as all zero of the elements are true.
I think it would be nice to give examples in ?stopifnot of calls that
are not ideal uses of the function, such as the poorly written
stopifnot() call that I recently wrote:
x <- 1:5
# does not give an error
stopifnot(ncol(x) == 2)
# gives an error
stopifnot(identical(ncol(x), 2L))
Or this code from [2]:
li <- list()
li$item <- 1
# Does not give an error, because
# "item" is misspelled and "NULL == 0" returns logical(0)
stopifnot(li$tem == 0)
I think that a useful way to teach users how to use a function is to
teach them how not to use it.
Would a patch for the documentation along these lines be considered?
By the way, there are some regression tests in base R that rely on the
behavior of stopifnot(logical(0)), where the logical(0) results from
`==`. I can make a list of these tests if someone thinks it would be a
good idea to double-check them and possibly improve them (e.g., convert
them to use identical() instead of `==`). I'm guessing it's not worth
the time.
Scott
[1]
https://urldefense.proofpoint.com/v2/url?u=https-3A__stat.ethz.ch_pipermail_r-2Dhelp_2015-2DDecember_434610.html&d=DwIBAg&c=pZJPUDQ3SB9JplYbifm4nt2lEVG5pWx2KikqINpWlZM&r=1fpq0SJ48L-zRWX2t0llEVIDZAHfU8S-4oINHlOA0rk&m=G8tEZpMWPL4vxGGinNsRHdfXpDqiFEownNAdY_AOiUk&s=wxOygcK0MIUDAQhkzjgfT-4edxWNCWluOEgAyR-xCC0&e[2]
https://urldefense.proofpoint.com/v2/url?u=https-3A__stackoverflow.com_questions_33670060_how-2Dto-2Dhave-2Dstopifnot-2Dreturn-2Dan-2Derror-2Dwhen-2Dcalled-2Don-2Da-2Dmissing-2Dnull-2Delement&d=DwIBAg&c=pZJPUDQ3SB9JplYbifm4nt2lEVG5pWx2KikqINpWlZM&r=1fpq0SJ48L-zRWX2t0llEVIDZAHfU8S-4oINHlOA0rk&m=G8tEZpMWPL4vxGGinNsRHdfXpDqiFEownNAdY_AOiUk&s=ZCSz07Z1Gz4pAWgw75UUn9wIMI-wCv2Srfkn2MGYYlI&e
--
Scott Kostyshak
Assistant Professor of Economics
University of Florida
https://people.clas.ufl.edu/skostyshak/
