[Rd] Unexpected argument-matching when some are missing

[Emil Bode]

It looks like you're right that somewhere in (presumably) match.call, the named, empty arguments are removed, such that the call plot(x=1:10, y=, 10:1) is translated to plot(x=1:10, 10:1).
But I would have expected it to be the same as plot(x=1:10, , 10:1) (note the ", ,"), which gives an error (10:1 is not a valid plot-type). In this case you get an error straightaway, I find this more interesting:
> options(warn=-1)
> plot(x=1, y=, 'p', ylim=c(0,10))
> plot(x=1, , 'p', ylim=c(0,10))
Both valid (no errors), albeit strange calls, but I'd say the first call is better code, it's clearer you intend to not give any value for y. But exactly this one gives unexpected results: it tries to plot at position (1, 'p'), or (1, NA).

And the behaviour as it is gives rise to some strange inconsistencies. I have gathered some examples below (at the very bottom of the thread, as it got quite extensive), where some variations are surprisingly different from each other.
There are also some issues when using data.frame(...)[i=, j=,...], but at least here you are warned about naming i and j.
But basically, it means any function where arguments like fun(,,) are a valid possibility should throw the same warning, e.g. any R-code replacement of [.matrix or [.array, or as in my examples, for data.table (and related structures)

On 29/11/2018, 19:10, "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.
    
    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
    

Exampled if what I (Emil) found odd:
---------------------------------------------------------------------------------------------------------------------------------------
> library(data.table)
> options(warn=1) # Or 2
> data.table(a=1:2, b=3:4)[1] # As expected
   a b
1: 1 3
> data.table(a=1:2, b=3:4)[, 1] # As expected
   a
1: 1
2: 2
> data.table(a=1:2, b=3:4)[i=, 1] # Huh? We get the first row
   a b
1: 1 3
> data.table(a=1:2, b=3:4)[, 1, 'a'] # As expected
   a V1
1: 1  1
2: 2  1
> data.table(a=1:2, b=3:4)[i=, 1, 'a'] # I would have expected the same result, and definitely more than 1 value
   a
1: 1
> data.table(a=1:2, b=3:4)[i=, 1, by='a'] # And this doesn't work?
Error in `[.data.table`(data.table(a = 1:2, b = 3:4), i = , 1, by = "a") : 
  'by' or 'keyby' is supplied but not j
> myfun <- function(x,y,z) {
+   print(match.call())
+   cat('nargs: ', nargs(), '\n')
+   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") # 5 arguments??
myfun(z = "z's value")
nargs:  5 
x= missing 
y= missing 
z= z's value 
> myfun(x=, y=, , , "z's value", , ) # But any more are not allowed
Error in myfun(x = , y = , , , "z's value", , ) : 
  unused arguments (alist(, ))
> myfun(x2=, y=, "z's value") # And named arguments are ignored, but the names have to be to existing argument-names
Error in myfun(x2 = , y = , "z's value") : unused argument (alist(x2 = ))
> myfun(x=, x=, , "z's value") # And naming it multiple times also gives an error
Error in myfun(x = , x = , , "z's value") : 
  formal argument "x" matched by multiple actual arguments
> myfun(y=, , "z's value", x=3) # Having fun with obfuscation, is this call backwards and forwards compatible?
myfun(x = 3, z = "z's value")
nargs:  4 
x= 3 
y= missing 
z= z's value 
> myfun(y=rlang::missing_arg(), , "z's value", x=3)
Error in myfun(y = rlang::missing_arg(), , "z's value", x = 3) : 
  unused argument ("z's value")
> myfun(y=rlang::missing_arg(), z=, "z's value", x=3) # Now with a named empty argument
myfun(x = 3, y = rlang::missing_arg(), z = "z's value")
nargs:  4 
x= 3 
y=  
z= z's value 
> myfun(y=rlang::missing_arg(), z= "z's value", x=3) # Just a comma removed: same match.call(), different nargs()
myfun(x = 3, y = rlang::missing_arg(), z = "z's value")
nargs:  3 
x= 3 
y=  
z= z's value