[Rd] structure(<primitive function>, ...) is sticky: a bug, or should it be an error?

[Henrik Bengtsson]

> I'm pretty convinced we should fix it by checking for primitive
> functions inside the C code of `attributes<-` :
> arguably the bug is really there, rather than in structure().
>
> Patches are welcome (via R's Bugzilla or just here).

Thank you Martin. I'll make sure I create a brief BugZilla report on
this, and hopefully a follow with a patch later on.

One question on urgency or not: Is it too late to get such a change in
for the R 4.5.0 release? I suspect so, because it has a potential of
breaking existing packages. But if there's a possibility of fixing
this in R 4.5.0, I'll make this a top priority.  Please let me know.

/Henrik

On Thu, Mar 20, 2025 at 3:31 AM Martin Maechler
<maechler using stat.math.ethz.ch> wrote:
>
> >>>>> Henrik Bengtsson
> >>>>>     on Wed, 19 Mar 2025 10:58:46 -0700 writes:
>
>     > Hello.
>     > I just (re-)discovered that structure(sum, init = 100) is "sticky",
>     > i.e. it stays with base::sum(). Here's an minimal example:
>
>     > $ R --vanilla --quiet
>     >> void <- structure(sum, some_attr = TRUE)
>     >> str(sum)
>     > function (..., na.rm = FALSE)
>     > - attr(*, "some_attr")= logi TRUE
>
>     >> From my very basic troubleshooting, it looks like this is happening
>     > for primitive functions. I think I understand that this comes down to
>     > primitive functions cannot be copied and baseenv() being special, i.e.
>     > in structure() there will be no copy made of the primitive function,
>     > and then attributes()<- ends up modifying the original primitive
>     > function. Even if this is a documented feature, I believe, it is a
>     > silent feature with risky side effects. We might already have code out
>     > there that silently produces incorrect results, because of this
>     > behavior. For example, I was about to a custom reduce() function where
>     > I control the initial value via an "init" attribute of the reducer
>     > function, e.g.
>
>     > x <- 1:10
>     > sum1 <- reduce(x, `+`)
>     > sum2 <- reduce(x, structure(`+`, init = 100))  # == 100 + sum1
>
>     > If I then call:
>
>     > sum3 <- reduce(x, `+`)
>
>     > the 'init' attribute that was set in the sum2 statement will affect
>     > sum3 such that sum3 == sum2, not sum3 == sum1 as one would expect.
>
>
>     > SUGGESTIONS:
>
>     > If this is a bug, then I think it needs to be fixed. If it cannot be
>     > fixed, maybe this could be protected against, e.g.
>
>     >> void <- structure(sum, some_attr = TRUE)
>     > Error: You must not set attributes on a primitive function: sum
>
>     > Maybe it's sufficient to implement a protection against this in
>     > attr()<-, attributes()<-, and class()<-.
>
>     > Comments?
>
>     > /Henrik
>
> Yes, this is a bug -- a version of your code above:
>
> > void <- structure(sum, foo = TRUE)
> > identical(void, sum)
> [1] TRUE
> > sum
> function (..., na.rm = FALSE)  .Primitive("sum")
> attr(,"foo")
> [1] TRUE
> >
>
> Above, you are already looking at ways to deal with it; and that
> *is* more delicate, indeed:
>
> One thing we (R core) found previously (a couple of years ago)
>  was that the  `structure(..)` function was
> already too "slow" for it to be used in some base R functions,
> and adding another  if(..) to it will not help making it faster ...
>
> OTOH, structure() being a pure R function (no direct .Internal(), .Call() ..)
> is also important I think,  as it keeps its code nicely self documenting.
>
> I'm pretty convinced we should fix it by checking for primitive
> functions inside the C code of `attributes<-` :
> arguably the bug is really there, rather than in structure().
>
> Patches are welcome (via R's Bugzilla or just here).
>
> Martin