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

Thanks Duncan, I have tried to make a minimalistic example:

myfun = function(...) {
  input = list(...)
  mysum = function(A = c(), B= c()) {
    return(A+B)
  }
  if ("A" %in% names(input) & "B" %in% names(input)) {
    print(mysum(A = input$A, B = input$B))
  }
}

# test:

myfun(A = 1, B = 2, B = 4)

[1] 3

# So, the second B is ignored.



On Mon, 8 Nov 2021 at 17:03, Duncan Murdoch <murdoch.duncan at gmail.com>
wrote:

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