[Rd] Usage of PROTECT_WITH_INDEX in R-exts

[Martin Maechler]

>>>>> Kirill Müller <kirill.mueller at ivt.baug.ethz.ch>
>>>>>     on Mon, 5 Jun 2017 17:30:20 +0200 writes:

    > Hi I've noted a minor inconsistency in the documentation:
    > Current R-exts reads

    > s = PROTECT_WITH_INDEX(eval(OS->R_fcall, OS->R_env), &ipx);

    > but I believe it has to be

    > PROTECT_WITH_INDEX(s = eval(OS->R_fcall, OS->R_env), &ipx);

    > because PROTECT_WITH_INDEX() returns void.

Yes indeed, thank you Kirill!

note that the same is true for its partner function|macro REPROTECT()

However, as  PROTECT() is used a gazillion times  and
PROTECT_WITH_INDEX() is used about 100 x less, and PROTECT()
*does* return the SEXP,
I do wonder why PROTECT_WITH_INDEX() and REPROTECT() could not
behave the same as PROTECT()
(a view at the source code seems to suggest a change to be trivial).
I assume usual compiler optimization would not create less
efficient code in case the idiom   PROTECT_WITH_INDEX(s = ...)
is used, i.e., in case the return value is not used ?

Maybe this is mainly a matter of taste,  but I find the use of

   SEXP s = PROTECT(........);

quite nice in typical cases where this appears early in a function.
Also for that reason -- but even more for consistency -- it
would also be nice if  PROTECT_WITH_INDEX()  behaved the same.

Martin

    > Best regards
    > Kirill