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Dynamically create a (convenience) function in a package

3 messages · Sigbert Klinke, Iris Simmons, Martin Maechler

Original
Sigbert Klinke
# 
Hi,

n a package, I have a data object with attributes, and I want to 
dynamically create a convenience function to access those attributes. 
This way, instead of using attr(x, "number"), I would like to use number(x).



Because I don't know in advance which attributes the data object may 
have, I've used the following algorithm:

x <- structure(pi, number=exp(1))

a <- attributes(x)

for (n in names(a)) {

   if (!exists(n, mode="function")) {

     f <- eval(parse(text=sprintf("function(x) { attr(x, '%s') } ", n))) 


     assign(n, f, envir=.GlobalEnv)

   }

}

number(x)

However, I believe modifying the global environment with this is not 
allowed by CRAN for a package. Is there a way to implement such 
functionality?

Thanks Sigbert
Iris Simmons
# 
If you don't know the name of the attributes in advance, how can you know
the function name to be able to call it? This seems like a very flawed
approach.

Also, I would discourage the use of eval(parse(text = )), it's almost
always not the right way to do what you want to do. In your case,

eval(bquote(function(x) attr(x, .(n))))

would be better.


On Mon, Oct 30, 2023, 06:28 Sigbert Klinke <sigbert at wiwi.hu-berlin.de>
wrote:

  
  


  


      

    

    

      
      

        


  

        

      Martin Maechler
    

    

      
      #
    

  


  

          

      
      
      
    

        
> If you don't know the name of the attributes in advance,
    > how can you know the function name to be able to call it?
    > This seems like a very flawed approach.

    > Also, I would discourage the use of eval(parse(text = )),
    > it's almost always not the right way to do what you want
    > to do. In your case,

    > eval(bquote(function(x) attr(x, .(n))))

    > would be better.

Indeed!  Thank you, Iris!

... as an old timer, I'd like to raise

  R> fortunes::fortune("eval(parse")

  Personally I have never regretted trying not to underestimate my own future
  stupidity.
     -- Greg Snow (explaining why eval(parse(...)) is often suboptimal, answering
	a question triggered by the infamous fortune(106))
	R-help (January 2007)

  R> fortunes::fortune(106)

  If the answer is parse() you should usually rethink the question.
     -- Thomas Lumley
	R-help (February 2005)

  R> 


Best, Martin



    > On Mon, Oct 30, 2023, 06:28 Sigbert Klinke

        
> <sigbert at wiwi.hu-berlin.de> wrote:

        
>> Hi,
    >> 
    >> n a package, I have a data object with attributes, and I
    >> want to dynamically create a convenience function to
    >> access those attributes.  This way, instead of using
    >> attr(x, "number"), I would like to use number(x).
    >> 
    >> 
    >> 
    >> Because I don't know in advance which attributes the data
    >> object may have, I've used the following algorithm:
    >> 
    >> x <- structure(pi, number=exp(1))
    >> 
    >> a <- attributes(x)
    >> 
    >> for (n in names(a)) {
    >> 
    >> if (!exists(n, mode="function")) {
    >> 
    >> f <- eval(parse(text=sprintf("function(x) { attr(x, '%s')
    >> } ", n)))
    >> 
    >> 
    >> assign(n, f, envir=.GlobalEnv)
    >> 
    >> }
    >> 
    >> }
    >> 
    >> number(x)
    >> 
    >> However, I believe modifying the global environment with
    >> this is not allowed by CRAN for a package. Is there a way
    >> to implement such functionality?
    >> 
    >> Thanks Sigbert
    >> 
    >> --
    >> https://hu.berlin/sk https://www.stat.de/faqs
    >> https://hu.berlin/mmstat https://hu.berlin/mmstat-ar
    >> 
    >> ______________________________________________
    >> R-help at r-project.org mailing list -- To UNSUBSCRIBE and
    >> more, see https://stat.ethz.ch/mailman/listinfo/r-help
    >> PLEASE do read the posting guide
    >> http://www.R-project.org/posting-guide.html and provide
    >> commented, minimal, self-contained, reproducible code.
    >> 

    > 	[[alternative HTML version deleted]]

    > ______________________________________________
    > R-help at r-project.org mailing list -- To UNSUBSCRIBE and
    > more, see https://stat.ethz.ch/mailman/listinfo/r-help
    > PLEASE do read the posting guide
    > http://www.R-project.org/posting-guide.html and provide
    > commented, minimal, self-contained, reproducible code.