[Rd] conditionally import a namespace?

[Gabor Grothendieck]

I noticed there is also an experimental interface that can be used
from R (as opposed to the NAMESPACE file).  Can't tell from docs
whether it allows conditionals:

?.Import


On Thu, Oct 30, 2008 at 10:44 AM, Duncan Murdoch <murdoch at stats.uwo.ca> 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.
>
> Duncan Murdoch
>
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