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

[Vincent van Hees]

Thanks for your replies, I only just noticed them as I had daily-digest =
true and someone seems to have removed my e-mail address when replying to
the list. Never mind, I have now switched my daily-digest to false.

Yes - The issue is that "myfun(A = 1, B = 2, B = 4)" in the example below
is ambiguous and only the end-user knows whether B should be 2 or 4.
Either way, I will just implement my own solution. Just wanted to report
that I ran into this situation and maybe others too.

Best, Vincent



On Mon, 8 Nov 2021 at 17:24, Vincent van Hees <vincentvanhees using gmail.com>
wrote:

> 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 using 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
>>
>

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