R devel - Sep 2014 - requireNamespace() questions

I am trying to follow directions at 
http://cran.r-project.org/doc/manuals/r-patched/R-exts.html#Suggested-packages 
regarding handling suggested packages with requireNamespace() rather 
than require(), and I have some questions.

1/ When I do requireNamespace() in a function is the loading of the 
namespace only effective within the function?

2/ At the link above in the manual it says "Note the use of rgl:: as 
that object would not necessarily be visible...".   When the required 
package is loading methods, will the method be found when I reference 
the generic, which is not in the package, or do I need to do something 
different?

3/ In some packages I have functions that return an object defined in 
the suggested package being required. For example, a function does 
require("zoo") and then returns a zoo object. So, to work with the 
returned object I am really expecting that zoo will be available in the 
session afterwards. Is it recommended that I just check if the package 
is available on the search path the user has set rather than use 
require() or requireNamespace()?.

Regarding checking the path without actually attaching the package to 
the search path, is there something better than "package:zoo" %in% 
search() or is that the best way?

4/ I have a function in a package that Depends on DBI and suggests 
RMySQL, RPostgreSQL, RSQLite. The function uses dbDriver() in DBI which 
uses do.call(). If I use requireNamespace() in place of require() I get

 > requireNamespace("RMySQL")
Loading required namespace: RMySQL
 > m <- dbDriver("MySQL")
Error in do.call(as.character(drvName), list(...)) :
   could not find function "MySQL"

 > require("RMySQL")
Loading required package: RMySQL
 > m <- dbDriver("MySQL")
 >

Is there a different way to handle this without altering the search path?

The function do.call() does not seem to work with an argument like
    do.call("RMySQL::MySQL", list())
even at the top level, and this situation may be more complicated when 
it is in a required package. What am I missing?

Thanks,
Paul

On Fri, Sep 12, 2014 at 3:13 PM, Paul Gilbert <pgilbert902 at gmail.com>
wrote:
>
> I am trying to follow directions at
>
http://cran.r-project.org/doc/manuals/r-patched/R-exts.html#Suggested-packages
> regarding handling suggested packages with requireNamespace() rather than
> require(), and I have some questions.
>
> 1/ When I do requireNamespace() in a function is the loading of the
> namespace only effective within the function?
Loading the namespace is (effectively) permanent. But the only thing
loading the namespace does is trigger the .onLoad() if present; it
does not affect the search path.
> 2/ At the link above in the manual it says "Note the use of rgl:: as
that
> object would not necessarily be visible...".   When the required
package is
> loading methods, will the method be found when I reference the generic,
> which is not in the package, or do I need to do something different?
Method should be found correctly.
> 3/ In some packages I have functions that return an object defined in the
> suggested package being required. For example, a function does
> require("zoo") and then returns a zoo object. So, to work with
the returned
> object I am really expecting that zoo will be available in the session
> afterwards. Is it recommended that I just check if the package is available
> on the search path the user has set rather than use require() or
> requireNamespace()?.
That may be a reasonable place to require(), in my opinion.
> 4/ I have a function in a package that Depends on DBI and suggests RMySQL,
> RPostgreSQL, RSQLite. The function uses dbDriver() in DBI which uses
> do.call(). If I use requireNamespace() in place of require() I get
>
>> requireNamespace("RMySQL")
> Loading required namespace: RMySQL
>> m <- dbDriver("MySQL")
> Error in do.call(as.character(drvName), list(...)) :
>   could not find function "MySQL"
>
>> require("RMySQL")
> Loading required package: RMySQL
>> m <- dbDriver("MySQL")
>>
>
> Is there a different way to handle this without altering the search path?
I think it's best to do

m <- dbConnect(RMySQL::MySQL, ...)

and avoid do.call() where possible. (I'm pretty sure it is possible,
but it may require changes to the DBI backends), many of which assume
the package to be on the search path.

do.call() probably doesn't do what you think it does:
http://rpubs.com/hadley/do-call2

Hadley

-- 
http://had.co.nz/