[Rd] Unexpected argument-matching when some are missing

[Ista Zahn]

On Thu, Nov 29, 2018 at 1:10 PM S Ellison <S.Ellison using lgcgroup.com> wrote:
>
>
> > > plot(x=1:10, y=)
> > > plot(x=1:10, y=, 10:1)
> > >
> > > In both cases, 'y=' is ignored. In the first, the plot is for y=NULL (so not
> > 'missing' y)
> > > In the second case, 10:1 is positionally matched to y despite the intervening
> > 'missing' 'y='
> > >
> > > So it isn't just 'missing'; it's 'not there at all'
> >
> > What exactly is the difference between "missing" and "not there at all"?
>
> A "missing argument" in R means that an argument with no default value was omitted from the call, and that is what I meant by "missing".
> But that is not what is happening here. I was talking about "y=" apparently being treated as not present in the call, rather than the argument y being treated as a missing argument.
>
> In these examples, plot.default has a default value for y (NULL) so y can never be "missing" in the sense of the 'missing argument' error (compare what happens with plot(y=1:10), which reports x as 'missing').
> In the first example, y was (from the plot behaviour) taken as NULL - the default - so was not considered a missing argument. In the second, it was taken as 10:1 - again, non-missing, despite 10:1 being in the normal position for the (character) argument "type".
> But neither call did anything at all with "y=". Instead, the behaviour is consistent with what would have happened if 'y=' were "not present at all" when counting position or named argument list, rather than if 'y' were an absent required argument.
> It _looks_ as if the initial call parsing silently ignored the malformed expression "y=" before any argument matching - positional or by name - takes place.

Yes, I think all of that is correct. But y _is_ missing in this sense:

> debug(plot)
> plot(1:10, y=)
debugging in: plot(1:10, y = )
debug: UseMethod("plot")
Browse[2]> missing(y)
[1] TRUE

though this does not explain the behavior since

> plot( , , "l")
debugging in: plot(, , "l")
debug: UseMethod("plot")
Browse[2]> missing(y)
[1] TRUE

--Ista
>
> But I'm thinking that it'll take an R-core guru to explain what's going on here, so I was going to wait and see.
>
> Steve Ellison
>
>
>
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