[R-pkg-devel] R feature suggestion: Duplicated function arguments check

[Duncan Murdoch]

On 08/11/2021 10:29 a.m., Vincent van Hees wrote:
> Not sure if this is the best place to post this message, as it is more of a
> suggestion than a question.
> 
> When an R function accepts more than a handful of arguments there is the
> risk that users accidentally provide arguments twice, e.g myfun(A=1, B=2,
> C=4, D=5, A=7), and if those two values are not the same it can have
> frustrating side-effects. To catch this I am planning to add a check for
> duplicated arguments, as shown below, in one of my own functions. I am now
> wondering whether this would be a useful feature for R itself to operate in
> the background when running any R function that has more than a certain
> number of input arguments.
> 
> Cheers, Vincent
> 
> myfun = function(...) {
>    #check input arguments for duplicate assignments
>    input = list(...)
>    if (length(input) > 0) {
>      argNames = names(input)
>      dupArgNames = duplicated(argNames)
>      if (any(dupArgNames)) {
>        for (dupi in unique(argNames[dupArgNames])) {
>          dupArgValues = input[which(argNames %in% dupi)]
>          if (all(dupArgValues == dupArgValues[[1]])) { # double arguments,
> but no confusion about what value should be
>            warning(paste0("\nArgument ", dupi, " has been provided more than
> once in the same call, which is ambiguous. Please fix."))
>          } else { # double arguments, and confusion about what value should
> be,
>            stop(paste0("\nArgument ", dupi, " has been provided more than
> once in the same call, which is ambiguous. Please fix."))
>          }
>        }
>      }
>    }
>    # rest of code...
> }
> 

Could you give an example where this is needed?  If a named argument is 
duplicated, R will catch that and give an error message:

   > f(a=1, b=2, a=3)
   Error in f(a = 1, b = 2, a = 3) :
     formal argument "a" matched by multiple actual arguments

So this can only happen when it is an argument in the ... list that is 
duplicated.  But usually those are passed to some other function, so 
something like

   g <- function(...) f(...)

would also catch the duplication in g(a=1, b=2, a=3):

   > g(a=1, b=2, a=3)
   Error in f(...) :
     formal argument "a" matched by multiple actual arguments

The only case where I can see this getting by is where you are never 
using those arguments to match any formal argument, e.g.

   list(a=1, b=2, a=3)

Maybe this should have been made illegal when R was created, but I think 
it's too late to outlaw now:  I'm sure there are lots of people making 
use of this.

Or am I missing something?

Duncan Murdoch