[Rd] Unexpected argument-matching when some are missing

[Ista Zahn]

On Thu, Nov 29, 2018 at 5:09 AM Emil Bode <emil.bode using dans.knaw.nl> wrote:
>
> 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')
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>   cat('z=',if(missing(z)) 'missing' else z, '\n')
>
> }
>
> myfun(x=, y=, "z's value")
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>
>
> gives:
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>
>
> # myfun(x = "z's value")
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> # 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”.

Interesting. I would expect it to throw an error, since "x=" is not
syntactically complete. What does "x=" mean anyway? It looks like R
interprets it as "x was not set to anything, i.e., is missing". That
seems reasonable, though I think the example itself is pathological
and would prefer that it produced an error.

--Ista
>
> 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
>
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