Re: [Rd] conditionally import a namespace?

From: Duncan Murdoch <murdoch_at_stats.uwo.ca>
Date: Thu, 30 Oct 2008 11:03:53 -0400

On 10/30/2008 10:44 AM, Duncan Murdoch wrote:

> 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.

I just did a little test, and it does appear to work if you use utils::installed.packages in the condition. And one clarification: those lines go into your NAMESPACE file, not into R code.

Duncan Murdoch



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