R devel - Dec 2025 - rep(): Expecting an argument-is-missing error, but ...

What's going on here:

$ R --vanilla --quiet
> f <- function(n) rep("x", times = n)
> f()
[1] "x"

?
> getRversion()
[1] ?4.5.2?

/Henrik

?s 06:40 de 11/12/2025, Henrik Bengtsson escreveu:
> What's going on here:
> 
> $ R --vanilla --quiet
>> f <- function(n) rep("x", times = n)
>> f()
> [1] "x"
> 
> ?
> 
>> getRversion()
> [1] ?4.5.2?
> 
> /Henrik
> 
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
Hello,

I can reproduce this on Windows.


Microsoft Windows [Version 10.0.22631.5039]
(c) Microsoft Corporation. Todos os direitos reservados.

C:\>R --vanilla --quiet
 > f <- function(n) rep("x", times = n)
 > f
function(n) rep("x", times = n)
 > f()
[1] "x"
 > getRversion()
[1] '4.5.2'
 > g <- function(n) rep(1, times = n)
 > g
function(n) rep(1, times = n)
 > g()
[1] 1
 > h <- function(n) rep(1, n)
 > h
function(n) rep(1, n)
 > h()
[1] 1
 > i <- function(n) rep(1, each = n)
 > i()
[1] 1
 > sessionInfo()
R version 4.5.2 (2025-10-31 ucrt)
Platform: x86_64-w64-mingw32/x64
Running under: Windows 11 x64 (build 22631)

Matrix products: default
   LAPACK version 3.12.1

locale:
[1] LC_COLLATE=Portuguese_Portugal.utf8  LC_CTYPE=Portuguese_Portugal.utf8
[3] LC_MONETARY=Portuguese_Portugal.utf8 LC_NUMERIC=C
[5] LC_TIME=Portuguese_Portugal.utf8

time zone: Europe/Lisbon
tzcode source: internal

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

loaded via a namespace (and not attached):
[1] compiler_4.5.2 tools_4.5.2
 > q()


Hope this helps,

Rui Barradas

I think it's a bit of a quirk of rep:
> rep("x")
> [1] "x"
Help of 'rep' does state this:

The default behaviour is as if the call was

       rep(x, times = 1, length.out = NA, each = 1)

So it seems to even be documented.

Peter


Peter

On Thu, Dec 11, 2025 at 2:40?PM Henrik Bengtsson
<henrik.bengtsson at gmail.com> wrote:
>
> What's going on here:
>
> $ R --vanilla --quiet
> > f <- function(n) rep("x", times = n)
> > f()
> [1] "x"
>
> ?
>
> > getRversion()
> [1] ?4.5.2?
>
> /Henrik
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

? Wed, 10 Dec 2025 22:40:29 -0800
Henrik Bengtsson <henrik.bengtsson at gmail.com> ?????:
> What's going on here:
> 
> $ R --vanilla --quiet
> > f <- function(n) rep("x", times = n)
> > f()  
> [1] "x"
Let's take these R functions:

not_rep <- compiler::cmpfun(\(x, times = 1) invisible(times))
wrap_not_rep <- compiler::cmpfun(\(times) not_rep(1, times = times))

In a call from wrap_not_rep() to not_rep(1, times = times), the thing
being matched against the formals is a promise to evaluate 'times' in
the wrap_not_rep()'s environment, so Rf_matchArgs_NR matches it
successfully and not_rep() eventually forces the promise and signals an
argument-is-missing exception.

What about the following function?

wrap_rep <- compiler::cmpfun(\(times) rep(1, times = times))

Being a pritmitive, rep() performs its own argument matching, so in a
call from wrap_rep() to rep(1, times = times), things happen
differently.

First, it receives the "raw" arguments, without them being wrapped
into
promises:

(gdb) call R_inspect(args)
@5555585b2220 02 LISTSXP g1c0 [MARK,REF(2)]
  @5555585a5700 14 REALSXP g1c1 [MARK,REF(65535)] (len=1, tl=0) 1
  TAG: @555557d17458 01 SYMSXP g1c0 [MARK,REF(65535)] "times"
  @555557d17458 01 SYMSXP g1c0 [MARK,REF(65535)] "times"

Then it calls DispatchOrEval(), which evaluates the expressions in the
call, replacing the "times" symbol with R_MissingArg. Only then it
calls Rf_matchArgs_NR:

(gdb) call R_inspect(supplied)
@5555588f74a8 02 LISTSXP g0c0 [STP] 
  @5555585a5700 14 REALSXP g0c1 [REF(65535)] (len=1, tl=0) 1
  TAG: @555557d17458 01 SYMSXP g0c0 [REF(65535)] "times"
  @5555559c6320 01 SYMSXP g0c0 [MARK,REF(7008)] [missing argument]

Rf_matchArgs_NR then faithfully substitutes that missing argument into
the return value together with other missing arguments that it
substitutes for every value that wasn't passed at all:

(gdb) call R_inspect(args)
@5555588fc850 02 LISTSXP g0c0 [STP] 
  @5555585a5700 14 REALSXP g0c1 [REF(65535)] (len=1, tl=0) 1
  # this one was passed as missing
  @5555559c6320 01 SYMSXP g0c0 [MARK,REF(7010)] [missing argument]
  # the following ones weren't passed at all
  @5555559c6320 01 SYMSXP g0c0 [MARK,REF(7010)] [missing argument]
  @5555559c6320 01 SYMSXP g0c0 [MARK,REF(7010)] [missing argument]
  @5555559c6320 01 SYMSXP g0c0 [MARK,REF(7010)] [missing argument]

There doesn't seem to be a way to distinguish between the two kinds of
missing arguments in the return value.

In order to cause an argument-is-missing error, rep() would have to
walk the list of supplied arguments before matching them, replacing the
R_MissingArg with something else, then match the arguments, then
complain about the missing argument replacement being matched to a
non-dots formal argument. "To call this an ugly hack would be to insult
all existing ugly hacks at large in the world." Is there a better way?

-- 
Best regards,
Ivan