[R] silently testing for data from another package for .Rd examples

[Michael Friendly]

In an .Rd example for a package, I want to use data from another 
package, but avoid loading the entire
package and avoid errors/warnings if that other package is not available.

If I don't care about loading the other package, I can just do:

if (require("ElemStatLearn", quietly=TRUE)) {
     data(prostate)
  #  rest of example
}

I'd rather just be able to do something like:

if (data(prostate, package="ElemStatLearn")) {
   #  rest of example
}

but it appears that data() doesn't return anything useful (like FALSE or 
NULL) in case the named data
set doesn't exist, or the package cannot be found.  Below are some test 
cases in a fresh R 2.13.1 session.

Is there someway I can incorporate such a data example silently without 
errors or warnings if the
package doesn't exist, as is the case with require()?

 > data(prostate, package="ElemStatLearn")
 > dd <- data(prostate, package="ElemStatLearn")
 > dd
[1] "prostate"
 > dd2 <- data(xxxxx, package="ElemStatLearn")
Warning message:
In data(xxxxx, package = "ElemStatLearn") : data set 'xxxxx' not found
 > dd2
[1] "xxxxx"
 > dd2 <- data(xxxxx, package="ElemStatLearn", verbose=FALSE)
Warning message:
In data(xxxxx, package = "ElemStatLearn", verbose = FALSE) :
   data set 'xxxxx' not found
 >
 > dd3 <- data(zzzzz, package="foobar")
Error in find.package(package, lib.loc, verbose = verbose) :
   there is no package called 'foobar'
 > dd3
Error: object 'dd3' not found
 >

try() doesn't seem to help here:

 > ddtry <- try(data(zzzzz, package="foobar"))
Error in find.package(package, lib.loc, verbose = verbose) :
   there is no package called 'foobar'
 > ddtry
[1] "Error in find.package(package, lib.loc, verbose = verbose) : \n  
there is no package called 'foobar'\n"
attr(,"class")
[1] "try-error"
 >

-- 
Michael Friendly     Email: friendly AT yorku DOT ca
Professor, Psychology Dept.
York University      Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street    Web:   http://www.datavis.ca
Toronto, ONT  M3J 1P3 CANADA