R devel - Feb 2013 - Recommended way to call/import functions from a Suggested package

If in my package "Foo" I call a function from another package
"Bar" if it's available, according to R-exts, this sounds like I
should include "Suggests: Bar" in package Foo's description. But
the manual is silent on how to treat Bar's namespace. Should I import it? If
so, should this be conditional or unconditional? There is a thread from 2008 in
which Duncan Murdoch suggests trying conditionally importing a package if
it's installed, with the caveat "If this is allowed"
(http://tolstoy.newcastle.edu.au/R/e5/devel/08/10/0488.html). This appears to
work in current release of R, 2.15.2, but I'm still not clear if it's
officially allowed, much less recommended.

The manual also says:
> If a package only needs a few objects from another package it can use a
fully qualified variable reference in the code instead of a formal import. A
fully qualified reference to the function f in package foo is of the form
foo::f. This is slightly less efficient than a formal import and also loses the
advantage of recording all dependencies in the NAMESPACE file, so this approach
is usually not recommended. Evaluating foo::f will cause package foo to be
loaded, but not attached, if it was not loaded already—this can be an advantage
in delaying the loading of a rarely used package.
> 

Would this be a better solution than importing when calling into a suggested
package?

Davor


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Hi Davor,

To the best of my knowledge, there's only one way to use functions
from a suggested package: with require:

if (require("suggested_package")) {
  function_from_suggested_package()
} else {
  stop("suggested package not installed")
}

Unfortunately I don't think there's any way to use a suggested package
without polluting the search path.

Hadley

On Fri, Feb 22, 2013 at 6:26 PM, Davor Cubranic <cubranic at stat.ubc.ca>
wrote:
> If in my package "Foo" I call a function from another package
"Bar" if it's available, according to R-exts, this sounds like I
should include "Suggests: Bar" in package Foo's description. But
the manual is silent on how to treat Bar's namespace. Should I import it? If
so, should this be conditional or unconditional? There is a thread from 2008 in
which Duncan Murdoch suggests trying conditionally importing a package if
it's installed, with the caveat "If this is allowed"
(http://tolstoy.newcastle.edu.au/R/e5/devel/08/10/0488.html). This appears to
work in current release of R, 2.15.2, but I'm still not clear if it's
officially allowed, much less recommended.
>
> The manual also says:
>
>> If a package only needs a few objects from another package it can use a
fully qualified variable reference in the code instead of a formal import. A
fully qualified reference to the function f in package foo is of the form
foo::f. This is slightly less efficient than a formal import and also loses the
advantage of recording all dependencies in the NAMESPACE file, so this approach
is usually not recommended. Evaluating foo::f will cause package foo to be
loaded, but not attached, if it was not loaded already?this can be an advantage
in delaying the loading of a rarely used package.
>>
>
>
> Would this be a better solution than importing when calling into a
suggested package?
>
> Davor
>
>
>         [[alternative HTML version deleted]]
>
>
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>


-- 
Chief Scientist, RStudio
http://had.co.nz/