[R] On-demand importing of a package

[Gábor Csárdi]

On Tue, Nov 22, 2011 at 4:27 PM, Martin Morgan <mtmorgan at fhcrc.org> wrote:
[...]
> No need to Depend:. Use
>
> Imports: Matrix
>
> plus in the NAMESPACE file
>
>  importFrom(Matrix, rowSums)
>
> Why do you not want to do this? Matrix is available for everyone, Imports:
> doesn't influence the package search path. There is a cost associated with
> loading the library in the first place, but...?

Not just loading, installing a package has a cost, too. Dependencies
are bad, they might make my package fail, and I have no control over
them. It's not just 'Matrix', I have this issue with other packages as
well.

Anyway, 'Imports: Matrix' is just a workaround I think. Or is the
example in my initial mail expected to fail? Why is that? Why can I
call some functions from 'Matrix' that way and why can't I call
others?

> I'm more into black-and-white -- it either needs Matrix or not; apparently
> it does.

It's a matter of opinion, I guess. I find it very annoying when I need
to install a bunch of packages from which I don't use any code, just
because some tiny bit of a package I need uses them. I would like to
spare my users from this.

[...]
> In another message you mention
>
>> Matrix:::rowSums(W)
> Error in callGeneric() :
>  'callGeneric' must be called from a generic function or method
>
> but something else is going on -- you don't get to call methods directly;
> you're getting Matrix::rowSums (it's exported, so no need for a :::, see
> getNamespaceExports("Matrix")). Maybe traceback() after the error would be
> insightful?

Another poster suggested this, that's why I tried. It is clear that I
should not call it directly. All I want to do is having a function
like this:

f <- function() {
 if (require(Matrix)) {
   res <- sparseMatrix(dims=c(5, 5), i=1:5, j=1:5, x=1:5)
 } else {
   res <- diag(1:5)
 }
 y <- rowSums(res)
 res / y
}

Setting the subjective bit, about depending or not, aside, is there
really no solution for this? The code in the manual page examples work
fine without importing the package and just loading it if needed and
available. Why doesn't the code within the package?

Thanks for the patience,
Gabor

> Martin
>
[...]

-- 
Gabor Csardi <csardi at rmki.kfki.hu>     MTA KFKI RMKI