[Rd] conditionally import a namespace?

[Felix Andrews]

2008/10/31 Duncan Murdoch <murdoch at stats.uwo.ca>:
> 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.

Many thanks Duncan, that is just what I wanted. The only problem now
is that it fails R CMD check:

* checking package dependencies ... ERROR
Namespace dependencies not required:
  RGtk2



>
> Duncan Murdoch
>
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>



-- 
Felix Andrews / 安福立
http://www.neurofractal.org/felix/
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