[R] Why does 'exists' need a quoted argument?

[Jason Turner]

On Thu, Feb 20, 2003 at 08:52:22AM -0600, Wright, Kevin wrote:
> 
> Some functions in R need quoted arguments.  Consider this list:
> 
> help(rm)
> rm(a)
> is.na(a)
> get("rm")
> exists("rm")
> 
> Can someone explain why 'get' and 'exists' require quoted object names?
...

I just tried it:

 my.exists <- function (x, where = -1, 
        envir = if (missing(frame)) as.environment(where) 
                else sys.frame(frame), 
                frame, mode = "any", inherits = TRUE) {
                   if(!is.character(x)) x <- deparse(substitute(x))
                   .Internal(exists(x, envir, mode, inherits))
}

The problem is the the "if" statement if the object doesn't exist.  R
quite correctly stops when it tries to do a conditional based on a
non-existant object.

If, on the other hand, we don't do that, and deparse(substitute(x))
without the check, it does unexpected things if a string is given to
it.

my.exists <- function (x, where = -1, 
        envir = if (missing(frame)) as.environment(where) 
                else sys.frame(frame), 
                frame, mode = "any", inherits = TRUE) {
                   x <- deparse(substitute(x))
                   .Internal(exists(x, envir, mode, inherits))
}

> my.exists(ls)
[1] TRUE
> my.exists("ls")
[1] FALSE
> deparse(substitute("ls"))
[1] "\"ls\""

So deparse(substitute(x)) correctly passes the quotes, which wasn't
our intention (but it *was* what we asked for... ;).

If we start pulling quotes out using gsub or similar, we're getting
into very kludgy territory.

Jason
-- 
Indigo Industrial Controls Ltd.
64-21-343-545
jasont at indigoindustrial.co.nz