Unexpected argument-matching when some are missing

When trying out some variations with `[.data.frame` I noticed some (to me)
odd behaviour, 

-----Original Message-----
From: R-devel [mailto:r-devel-bounces at r-project.org] On Behalf Of Emil
Bode
Sent: 29 November 2018 10:09
To: r-devel at r-project.org
Subject: [Rd] Unexpected argument-matching when some are missing

When trying out some variations with `[.data.frame` I noticed some (to me)
odd behaviour, which I found out has nothing to do with `[.data.frame`, but
rather with the way arguments are matched, when mixing named/unnamed
and missing/non-missing arguments. Consider the following example:



myfun <- function(x,y,z) {

? print(match.call())

? cat('x=',if(missing(x)) 'missing' else x, '\n')

? cat('y=',if(missing(y)) 'missing' else y, '\n')

? cat('z=',if(missing(z)) 'missing' else z, '\n')

}

myfun(x=, y=, "z's value")



gives:



# myfun(x = "z's value")

# x= z's value

# y= missing

# z= missing



This seems very counterintuitive to me, I expect the arguments x and y to be
missing, and z to get ?z?s value?.

When I call myfun(,y=,"z's value"), x is missing, and y gets ?z?s value?.

Are my expectations wrong or is this a bug? And if my expectations are
wrong, where can I find more information on argument-matching?

My gut-feeling says to call this a bug, but then I?m surprised no-one else has
encountered it before.



And I don?t have multiple installations to work from, so could somebody else
confirm this (if it?s not my expectations that are wrong) for R-devel/other R-
versions/other platforms?

My setup: R 3.5.1, MacOS 10.13.6, both Rstudio 1.1.453 and R --vanilla from
Bash



Best regards,

Emil Bode