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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