[Rd] Recommended way to call/import functions from a Suggested package

[Martin Morgan]

On 02/25/2013 01:28 PM, Hadley Wickham wrote:
>> To summarize, it appears that the only way to call functions from a
>> suggested package is by using either 'require' (which will dynamically
>> attach it) or the double colon method. Is this something that should be
>> mentioned in R-exts?
>
> Except the double colon method doesn't work (i.e. does not pass R CMD
> check) unless you also import the package, which means it's no longer
> just a suggestion - it must always be installed.
>
> A simple test case (attached DESCRIPTION and R/test.r) yields this
> warning on R CMD check:
>
> * checking for unstated dependencies in R code ... WARNING
> '::' or ':::' import not declared from: 'MASS'
> See the information on DESCRIPTION files in the chapter 'Creating R
> packages' of the 'Writing R Extensions' manual.

haven't been following fully, but loadNamespace rather than require, with 
Suggests: MASS in DESCRIPTION ?

f = function() {
     ok <- tryCatch({
         loadNamespace("MASS")
         TRUE
     }, error=function(...) FALSE)

     if (ok) {
         MASS::huber(1:10)
         cat("OK\n")
     }
}

loadNamespaces loads but does not attach the package. Suggests: is enough to 
quieten the warning with

~/tmp$ R --version
R Under development (unstable) (2013-02-21 r62017) -- "Unsuffered Consequences"

This is consistent with RShowDoc("R-exts") section 1.1.1

   Namespaces accessed by the ‘::’ and ‘:::’ operators must be listed here, or 
in ‘Suggests’ or ‘Enhances’ (see below).

Martin

>
> Hadley
>
>
>
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