[Rd] conditionally import a namespace?

[Duncan Murdoch]

On 10/30/2008 10:15 AM, Martin Maechler wrote:
>>>>>> "FA" == Felix Andrews <felix at nfrac.org>
>>>>>>     on Thu, 30 Oct 2008 17:40:17 +1100 writes:
> 
>     FA> Dear R-devel,
>     FA> I have a problem defining the dependencies for a package.
> 
>     FA> My package (latticist, not yet released) "Suggests" RGtk2, but
>     FA> specifically does not require it. If RGtk2 is available, the user can
>     FA> call a function that builds a GUI with RGtk2. However, I do not want
>     FA> to attach the RGtk2 namespace, because it is irrelevant to the user
>     FA> and exports about 7500 symbols.
> 
>     FA> Last time I asked a similar question to this, Professor Ripley noted
>     FA> that the usual method to get around this situation is the use an
>     FA> explicit package prefix to function calls (the `::` operator). But
>     FA> this is not so easy with non-standard functions. Take this chunk of
>     FA> code:
> 
>     FA> widg <- gtkComboBoxEntryNewText()
>     FA> widg$show()
>     FA> widg["width-request"] <- 100
> 
>     FA> The first call is easy to prefix, as RGtk2::gtkComboBoxEntryNewText()
>     FA> but the others, `$.RGtkObject` and `[<-.RGtkObject` are not.
> 
> indeed.
> 
>     FA> While I *could* rewrite all the code to use explicit functions, I
>     FA> think, the resulting code would be much less clear.
> 
>     FA> Essentially what I want to do is conditionally import the RGtk2 namespace.
>     FA> Any suggestions?
> 
> Maybe something along the line of
> 
> if(is.function(try(RGtk2::gtkComboBoxEntryNewText))) {
>    library(RGtk2)
>    ....
>    ....
> }
> 
> ??

I think the problem is that that puts the namespace on the user's search 
list, which is what Felix wanted to avoid.  He would like to 
import(RGtk2), but only if it exists.

There are conditionals allowed in NAMESPACE files, but I don't know if 
they are allowed to be used to control imports.  They are not well 
documented -- they were called "experimental" when introduced in 1.9.0.

If this is allowed, then something like

if( "RGtk2" %in% rownames(installed.packages()) ) {
   import(RGtk2)
}

should work.

Duncan Murdoch