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

Thanks, Duncan, for the kind advice.

In my understanding, this is what I have done (or I tried to do).  My code is as follows:

   opt.methods <- c("Nelder-Mead", "BFGS", "nlminb")
   if(requireNamespace("nloptr", quietly = TRUE)) {
     nloptr.methods <- c("newuoa", "bobyqa", "cobyla")
     if(opt.method %in%  nloptr.methods) require(nloptr, quietly=TRUE)
     opt.methods <- c(opt.methods, nloptr.methods)
    }

<skip>

    if(opt.method %in% nloptr.methods) {
       pos <- match("package:nloptr", search())
       nloptr.method <-  get(paste("nloptr", opt.method, sep="::"), pos=pos)
       opt <- nloptr.method(<args>)
       }
        
In the DESCRIPTION file there is
    Suggests: ggplot2, survival, nloptr

However, when I run ?R CMD check <etc>?, I get the following message
* checking package dependencies ... ERROR
Namespace dependency missing from DESCRIPTION Imports/Depends entries: ?nloptr?

See section ?The DESCRIPTION file? in the ?Writing R Extensions?
manual.
* DONE

And - admittedly - I have no idea about how to insert appropriate import statements in NAMESPACE.


Best regards,

Adelchi

On 4 Dec 2024, at 20:56, Duncan Murdoch <murdoch.duncan at gmail.com> wrote:

On 2024-12-04 1:25 p.m., Adelchi Azzalini wrote:

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?

The usual way to do this is to list the package in Suggests, and then wrap any use of it in `if (requireNamespace("pkg")) { ... }` blocks. This doesn't quite import the functions, you would need to use the `pkg::fn` syntax to access the functions.

If you really want to simulate importing so that you don't need the `pkg::` prefix, you could do it this way:  In the `.onLoad` function of your package, you would have code like

  if (requireNamespace("pkg")) {
    foo <- pkg::foo
    bar <- pkg::bar
  } else {
    foo <- stub
    bar <- stub
  }

where `stub` is a function that says "you need `pkg` to use this function".

Duncan Murdoch