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[R-pkg-devel] Conditionally register method with generic in other package

6 messages · Joshua Ulrich, Bill Denney, Duncan Murdoch +1 more

Joshua Ulrich
# 
I have a feeling I know the answer to this question, but I'm asking
with the hope that I learn something new.

Package A defines a S3 generic function, "foo()".  Package B defines a
S3 class "bar", and a foo.bar() method.  This method is not strictly
necessary for the package to function, and package A has many
dependencies.

To avoid excessive dependencies, I would like to only register
foo.bar() if package A is installed at the time package B is
installed.  If package A is installed after package B, then warn the
user when package B is loaded and/or attached, so they can re-install
A and have foo.bar() registered correctly.

I could figure out a way to do this manually, but doubt it would be
CRAN-approved.  What is the recommended way to handle this situation?
I've received one suggestion to split B into 2 packages: one without
the method, and one with.  Are there other alternatives?

Thanks,
Josh
Bill Denney
# 
One simple solution would be to wrap the instantiation of foo.bar in a require test:

if (require(A)) {
  foo.bar <- function(...) {
    print("To be or not to be")
  }
} else {
  message("To use the foo.bar function, please install package A.")
}

Thanks,

Bill
Duncan Murdoch
#
On 06/12/2017 8:44 AM, Bill Denney wrote:
It's usually better to use requireNamespace("A") instead of require("A") 
(and the quotes are needed in requireNamespace(), but optional in 
require()). That tries to load the package, but doesn't mess with the 
search list.

But that'll likely cause warnings, unless the same condition is used in 
the NAMESPACE file where there should be an entry

S3method(foo, bar)

The thing is, I'm not sure if

if (requireNamespace("A"))
   S3method(foo, bar)

is legal in a NAMESPACE file.

Duncan Murdoch
Joshua Ulrich
# 
Bill, Duncan,

Thanks for the replies.

On Wed, Dec 6, 2017 at 12:51 PM, Duncan Murdoch
<murdoch.duncan at gmail.com> wrote:
While perhaps not technically illegal, that doesn't work because you need

if (requireNamespace("A", quietly = TRUE)) {
  importFrom(A, foo)
  S3method(foo, bar)
}

to prevent an error about being able to find object 'foo' when loading
B's namespace.  But the importFrom() causes R CMD check to expect a
hard dependency on package A (and all its dependencies), which is what
I'm trying to avoid...

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Hadley Wickham
    

    

      
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On Thu, Dec 7, 2017 at 2:44 AM, Bill Denney <bill at denney.ws> wrote:

            

      
      
      
    

        
I don't think that will work because the code will be executed at
build time, not package-load time.

Hadley

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Hadley Wickham
    

    

      
      #
    

  


  

      
We use this technique in a couple of places:
https://github.com/tidyverse/dbplyr/blob/6be777d8b23d588f19c98de52f4e58f16c2ef67e/R/zzz.R

Basic idea is to call registerS3method() manually, when needed - it's
just a little tricky because the suggested package may be loaded
either before or after your package.

Hadley

        
On Thu, Dec 7, 2017 at 1:45 AM, Joshua Ulrich <josh.m.ulrich at gmail.com> wrote: