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Re: ref to read-only alias ... why? (notabug)

by tye (Cardinal)
on Jan 06, 2012 at 05:13 UTC ( #946525=note: print w/ replies, xml ) Need Help??


in reply to ref to read-only alias ... why?

Whether you get an alias or a copy of a read-only value is a bit of an optimization concern and it has changed previously. I'm not terribly surprised that it has changed again (and seems to change based on features of Perl you include and may have changed several times in just recent versions). I would consider Perl code that depends on this behavior to be broken much more so than I consider either behavior to be a bug in Perl.

You really want it to be a Perl bug unless for(1..3) makes $_ read-write while for(1,2,3) makes $_ read-only?

- tye        


Comment on Re: ref to read-only alias ... why? (notabug)
Re^2: ref to read-only alias ... why? (notabug)
by dk (Chaplain) on Jan 06, 2012 at 15:48 UTC
    I want it documented, and consistent. I agree that the for() case can be seen as stretched ( I like my for(1) to make an alias to $_ so I can do all magic with it inside the for block, but that's another story).

    But anyway, what would you say about this then? Is this a bug or not?

    sub x { my $ref = \ $_[0]; $$ref ++; } x(1);
    I'd say that even if this behavior will be decided on as not a bug, then it has to be documented, at the very least.

      I'd document it pretty much as I already described it. Passing a read-only value or a literal constant to something that makes aliases (for, a sub) may decide to make an alias to the read-only value or may decide to make a copy of it. The decision might even be different in those two case (a literal constant vs. some other read-only scalar). The choice is a matter of optimization and subtle edge cases and Perl code should not depend on either specific behavior. Both behaviors have existed in many different versions of Perl.

      No, I don't consider it a bug that some versions of Perl don't die in the face of:

      sub add1 { return ++$_[0]; } my $two= add1(1);

      Despite the use of "++" over "1+" there being questionable. It does have the interesting and perhaps useful side effect of allowing: add1($count). Yes, it is a contrived example. As is yours.

      I don't find it hard to imagine cases where either result would be preferred. I do find it hard to imagine cases where either result is a serious problem that I wouldn't just address with a better interface for the subroutine.

      - tye        

        Thanks for commenting. Here's a case with a serious problem: a person writes code that doesn't fail, which then gets executed on another installation, and fails. I'd say it's bad enough. I believe I'd file a bug after all (unless you have reasons for me not to).
      Your right it's not consistent.

      But IMHO consistency will most likely break legacy code.

      I think that there should at least be a warning.

      Since it's not always clear if a variable is an alias, this can cause very hard to detect errors in subs which are meant to modify call-by-reference parameters. (not to be confused with Perl references)

      Did you check if there are already older bug-reports regarding this?

      Cheers Rolf

      UPDATE:

      DB<108> sub inc_a { $_[0]++; return } DB<109> $x=1; inc_a($x); print $x 2 DB<110> inc_a(1) Modification of a read-only value attempted at (eval 13)[/usr/share/pe +rl/5.10/perl5db.pl:638] line 2.

      but

      DB<111> sub inc_b { my $r=\ $_[0]; $$r++; return } DB<112> $x=1; inc_b($x); print $x 2 DB<113> inc_b(1) DB<114>

      When passing an aliasing variable instead of 1 it's the same problem. With inc_b non-aliasing vars will increment, but aliases will silently fail to increment.

        Heh, it's a 8-year old (!!!!) bug: https://rt.perl.org/rt3/Public/Bug/Display.html?id=21979 . With resolution that it's indeed a bug ... well, that situation doesn't seem good to me, when noone wants to touch a bug in 8 years. I wonder why.

        Your right it's not consistent. But IMHO consistency will most likely break legacy code.

        I think you're saying that because you envision it being consistently broken (always dying) instead of it being consistently fixed (never dying).

        There are two ways of fixing it:

        • Have literal constants always return new values. That's inefficient.
        • Have literal constants return copy-on-write values. There's currently no such mechanism, and that's a lot of work to implement.

        Check out this bug that would also be fixed by either of the above two fixes:

        for (1..2) { for (1..3) { print $_++; } # 123 234 print "\n"; }

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