R devel - Jun 2004 - Problem with hasArg and the ... argument (PR#7027)

Full_Name: Jelle Goeman
Version: 1.9.0
OS: mingw32, windows 2000
Submission from: (NULL) (145.88.209.33)


Hi Everyone,

I get very strange results using the function hasArg with the ... function
argument. In my own function: 
> gt <- globaltest(X,Y)
> sampling(gt)
works fine, but
> sampling(globaltest(X,Y))
results in:

Error in eval(expr, envir, enclos) : "missing" illegal use of missing

I've tracked down the problem. Define the simple function:

xory <- function(x, ...) if (hasArg(y)) y else x

then 

x <- 1:10
xx <- xory(x)
plot(x, xx)

works fine, but

plot(x, xory(x))

gives the same error. The problem is that the plot function also has an argument
y, which somehow interferes with the hasArg function. Is there an alternative to
hasArg that really checks if an argument y was supplied for the xy function
itself?

Jelle

On Mon, 28 Jun 2004 11:12:28 +0200 (CEST), j.j.goeman@lumc.nl wrote:
>Full_Name: Jelle Goeman
>Version: 1.9.0
>OS: mingw32, windows 2000
>Submission from: (NULL) (145.88.209.33)
>I've tracked down the problem. Define the simple function:
>
>xory <- function(x, ...) if (hasArg(y)) y else x
>
>then 
>
>x <- 1:10
>xx <- xory(x)
>plot(x, xx)
>
>works fine, but
>
>plot(x, xory(x))
>
>gives the same error. The problem is that the plot function also has an
argument
>y, which somehow interferes with the hasArg function. Is there an
alternative to
>hasArg that really checks if an argument y was supplied for the xy function
>itself?
This is still present in 1.9.1 and r-patched (on Windows).  It appears
to be a bug in or misuse of sys.function: within hasArg,
sys.function(0) returns hasArg as expected, but sys.function(1)
returns plot.  xory() seems to get lost.

Duncan Murdoch

j.j.goeman@lumc.nl writes:
> xory <- function(x, ...) if (hasArg(y)) y else x
> 
> then 
> 
> x <- 1:10
> xx <- xory(x)
> plot(x, xx)
> 
> works fine, but
> 
> plot(x, xory(x))
> 
> gives the same error. The problem is that the plot function also has
> an argument y, which somehow interferes with the hasArg function. Is
> there an alternative to hasArg that really checks if an argument y
> was supplied for the xy function itself?
No, but hasArg has issues: 

    fnames <- names(formals(sys.function(1)))

is getting the names of plot() rather than xory(). I almost thought I
had caught the Mighty John blundering there, but this actually looks
more like sys.function is not behaving as documented and counts frames
in the wrong direction. 

Checking... Yep, the logic in R_sysfunction() is to give the function
of frame #n:

    if (n > 0)
        n = framedepth(cptr) - n;
    else
        n = - n;
    if (n < 0 )
        errorcall(R_GlobalContext->call, "illegal frame number");
    while (cptr->nextcontext != NULL) {
        if (cptr->callflag & CTXT_FUNCTION ) {
            if (n == 0)
                return duplicate(cptr->callfun);  /***** do we need to DUP?
*/
            else
                n--;
        }
        cptr = cptr->nextcontext;
    }


whereas the documentation has

     'sys.function' gives the definition of the function currently
     being evaluated in the frame 'n' generations back.

However, we're using the current convention in quite a few places
(sys.function(sys.parent())) and, worse, who know what packages might
do. It is tempting just to change the documentation, but it is part of
a grouped documentation of sys.whatever, which has

   which: the frame number if non-negative, the number of generations
          to go back if negative. (See the Details section.)

       n: the number of frame generations to go back.

and sys.function is documented with argument 'n', which we'd have to
change to 'which', but the default is n=0 for "current
function" which
is unlike 'which' which has 0 meaning .GlobalEnv. Argh...

My take is that we need to fix sys.function to behave according to
docs, change what we can in the R internals, and face the consequences
for package maintainers.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907