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

[Martin Maechler]

>>>>> 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