R devel - Aug 2013 - Confusion about Depends:, Imports:, Enhances:, import(), inportFrom()

In checking my vcdExtra package, the following NOTE newly appeared 
(R-Forge, using R version 3.0.1 Patched (2013-08-20 r63635))

Package in Depends field not imported from: ?gnm?
   These packages needs to imported from for the case when
   this namespace is loaded but not attached.

In the DESCRIPTION file, I have

Depends: R (>= 2.10), vcd, gnm (>= 1.0.3)

In NAMESPACE:

# we are a vcd extension
import(vcd)

I've read 1.1.1 of R-Exts, but it is not clear to me whether I should 
also import gnm or change
the DESCRIPTION file to use

Imports: vcd, gnm (>= 1.0.3)

R-Exts says: The ?Imports? field lists packages whose namespaces are 
imported from (as specified in the
NAMESPACE file) but which do not need to be attached, but how can I tell 
if gnm needs to be attached?

Also, what is the difference between Imports: in DESCRIPTION and 
imports() in NAMESPACE?

-Michael

-- 
Michael Friendly     Email: friendly AT yorku DOT ca
Professor, Psychology Dept. & Chair, Quantitative Methods
York University      Voice: 416 736-2100 x66249 Fax: 416 736-5814
4700 Keele Street    Web:   http://www.datavis.ca
Toronto, ONT  M3J 1P3 CANADA

On Aug 22, 2013, at 9:09, Michael Friendly <friendly at yorku.ca> wrote:
> In checking my vcdExtra package, the following NOTE newly appeared
(R-Forge, using R version 3.0.1 Patched (2013-08-20 r63635))
> 
> Package in Depends field not imported from: ?gnm?
>  These packages needs to imported from for the case when
>  this namespace is loaded but not attached.
> 
> In the DESCRIPTION file, I have
> 
> Depends: R (>= 2.10), vcd, gnm (>= 1.0.3)
> 
> In NAMESPACE:
> 
> # we are a vcd extension
> import(vcd)
> 
> I've read 1.1.1 of R-Exts, but it is not clear to me whether I should
also import gnm or change
> the DESCRIPTION file to use
> 
> Imports: vcd, gnm (>= 1.0.3)
> 
> R-Exts says: The ?Imports? field lists packages whose namespaces are
imported from (as specified in the
> NAMESPACE file) but which do not need to be attached, but how can I tell if
gnm needs to be attached?
I think the current best practice is to use Imports unless gnm provides
functions the end user needs in order to use your package.

Practically, I think this usually comes down to asking whether gnm provides
relevant generics -- if you provide methods only, the end user would only be
able to call them directly (ick!) unless gnm is both loaded and attached
(Depends)

If gnm is tools for your code, but not the user to call directly, Imports is
cleaner.

Does that help?

Michael
> 
> Also, what is the difference between Imports: in DESCRIPTION and imports()
in NAMESPACE?
> 
One enables the other: Imports in DESCRIPTION allows for both imports() and
importsFrom(). It could probably be automatic, but the DESC file is much older
than the NAMESPACE file.

> -Michael
> 
> -- 
> Michael Friendly     Email: friendly AT yorku DOT ca
> Professor, Psychology Dept. & Chair, Quantitative Methods
> York University      Voice: 416 736-2100 x66249 Fax: 416 736-5814
> 4700 Keele Street    Web:   http://www.datavis.ca
> Toronto, ONT  M3J 1P3 CANADA
> 
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