[Rd] Unexpected argument-matching when some are missing

[S Ellison]

> > 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.

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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