[R-pkg-devel] conditional import of a package?

[Adelchi Azzalini]

Hi. I am working on the development of an existing package (smof, on CRAN). My current aim is widen the list of possible optimizers from which the user can select one method for optimizing a certain task. Well-known possibilities within the base package are optim (with various options) and nlminb. Besides these, I am thinking of including also those of package nloptr, but without forcing users to install this package which perhaps they don't need for other purposes. Hence, I would like to import nloptr only if it is available on the user system; it not, I can just confine the list of optimizers to optim and nlminb.  

This idea implies a sort of “conditional import” of nloptr. Is this possible? Section 1.1.3.1 "Suggested packages" of
https://translation.r-project.org/man/R-exts/R-exts-ko.html#Suggested-packages
seems to hint at such a possibility.  See the use of requireNamespace in the second paragraph.

After elaborating along this line, I packaged my code, with nloptr listed on the line Suggests of DESCRIPTION.  However this attempt failed a the “R CMD check “ stage with message

Namespace dependency missing from DESCRIPTION Imports/Depends entries: ‘nloptr’

In addition, I have no idea of how to declare a "conditional import” in NAMESPACE. 

Is this idea of “conditional import” totally unfeasible, then?

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Adelchi Azzalini
http://azzalini.stat.unipd.it