in reply to Re^6: Puzzled by value of $overload::ops{binary}
in thread Puzzled by value of $overload::ops{binary}

I keep looking at this piece of overload.pm documentation: 
    *    *Assignments*

             +=  -=  *=  /=  %=  **=  <<=  >>=  x=  .=
             &=  |=  ^=  &.=  |.=  ^.=

         [snip]

         The subroutine for the assignment variant of an operator is
         required only to return the result of the operation.

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That seems to me to be asserting that these operations (unlike '++' and '--') do require a value to be returned.
And it also looks to me that overload.pm is somehow enforcing that policy.

This following rewrite of the simplified overload_add_eq C and XS functions works fine: 
void overload_add_eq(pTHX_ SV * a, SV * b, SV * third) {
     dXSARGS;
     PERL_UNUSED_ARG(third);
     mpz_add_ui(*(INT2PTR(mpz_t *, SvIVX(SvRV(a)))), *(INT2PTR(mpz_t *
+, SvIVX(SvRV(a)))), SvUV(b));
     XSRETURN(1);
}

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and 
void
overload_add_eq (a, b, third)
    SV *    a
    SV *    b
    SV *    third
        CODE:
        overload_add_eq (aTHX_ a, b, third);
        XSRETURN(1);

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But I wonder if that achieves anything significant over the implementation that has been working fine for the last umpteen years.

Anyway, I don't want to get too bogged down in this - though it's certainly interesting enough to encourage a bit more investigation.

Cheers,
Rob

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Re^8: Puzzled by value of $overload::ops{binary}
by etj (Priest) on Jun 26, 2024 at 22:08 UTC
    It definitely doesn't look right that you're doing 
         dXSARGS;
/* ... */
     XSRETURN(1);

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    in your alleged C function, which should either mutate things and let the XS function do all the Perl stack stuff, or return a value but strictly in C-land.
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[d/l]
      I think that the overloading procedure views '+=' as an assignment operation ($x = $x + $addon), not as an inplace mutation ($x += $addon).
      The former requires a value to be returned; the latter does not.
      At least, it's behaving as though that's the case.

      That's why that documentation snippet I posted goes on to say: 
               The subroutine for the assignment variant of an operator is
         required only to return the result of the operation. It is
         permitted to change the value of its operand (this is safe be
+cause
         Perl calls the copy constructor first), but this is optional 
+since
         Perl assigns the returned value to the left-hand operand anyw
+ay.

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      Note that it's specifically stating that it's "permitted" (but not mandatory) "to change the value of its operand ".
      If it was being deemed to be an inplace mutation, then it would have to change the value of its operand.

      AFAICS, there aren't many modules that overload '+=', but Math::BigFloat does: 
        '+='    =>      sub { $_[0] -> badd($_[1]); },

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      which I manually altered to: 
        '+='    =>      sub { $_[0] -> badd($_[1]); 42 },

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      I then get: 
      D:\>perl -MDevel::Peek -MMath::BigFloat -le "$x = Math::BigFloat->new(
+20); $x += 100; print $x; Dump $x;"
42
SV = PVIV(0x1bc99137430) at 0x1bc9913a450
  REFCNT = 1
  FLAGS = (IOK,pIOK,pPOK)
  IV = 42
  PV = 0x1bc9b44fbe0 "42"\0
  CUR = 2
  LEN = 16

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      In that pure perl environment it's clearly the returned result, not the result of the inplace addition, that's being seen.

      I'm now feeling more confident that the need for '+=' overloading to return the new value is intended (though unexpected) behaviour.

      Cheers,
      Rob
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[d/l]
[select]
        Glad to hear it works properly when you do what the documentation says is necessary.

        I was talking about the division of labour between your XS function, which quite rightly does Perl-stack and SV stuff, and your C function, which also does both but is wrong to do so. I am saying you should either incorporate the C function into the XS function entirely, or make the C function operate only on C values, and return those to the XS function to use for Perl stuff.

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