conditional dependencies & loading

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Hi!

I'm working on a package that must convert data to and from JSON.  For this,
it can use either the rjson package, or preferably, the faster RJSONIO
package.

I have two related questions about this.

First, how can I specify that the package depends on *either* RJSONIO *or*
rjson?  (I.e. both are not required.)

I don't think you can do that.  You can say it suggests both, but it 
will not automatically fail to install if they are not found.  You need 
to put a check into your .onLoad function to fail with an error if 
neither is found.
Second, what's the best-practice R idiom for such conditional loading?  I.e.
I'm looking for R's equivalent of Python's

try:

...     import json
... except ImportError:
...     import simplejson as json

You would use something like

if (!require(RJSONIO) && !require(rjson)) stop("Need one of RJSONIO or 
rjson")

in your .onLoad function.

Duncan Murdoch
Thanks!

kynn

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