in reply to Puzzling $| behavior

Yesterday I wrote the following "opinion":

| print gets its "arguments in reverse order", similar to C.
| The above call would (in C) look like
|(pseudocode): 
| ...
| XPUSH( "\n" );
| XPUSH( $| );
| $|--;               # plus true/false magic
| XPUSH( "second=" );
| XPUSH( $| );
| XPUSH( "first=" );
| PUTBACK;
| call_pv("print", G_ARRAY);
| ...
| (please correct here me if I'm wrong)

which turned out to be completely wrong.
After reading the other posts and looking further
into the topic, I came to the conclusion that perl
evaluates the argument list before calling into a sub-
routine always left-to-right.

This can be shown by reading the deparsed output
from B::Bblock /1/. If used on the following lines: 
1:     $| = 1;
2:     print "first=", $|, " second=", $|--, "\n";

[download]
the important output will be: 
  OP (0x824de58) enter
  COP (0x81f3290) nextstate
#1   $| = 1
  SVOP (0x824dc80) const [6] IV (0x8167cdc) 1
  PADOP (0x818ae48) gvsv  GV (0x8168894) *|
  BINOP (0x8189398) sassign
#
  COP (0x824c638) nextstate
  OP (0x8250178) pushmark
# 2
  SVOP (0x8189428) const [7] PV (0x816881c) "first="
  PADOP (0x818c6f0) gvsv  GV (0x8168894) *|
  SVOP (0x818de00) const [8] PV (0x8168828) " second="
  PADOP (0x824ff88) gvsv  GV (0x8168894) *|
  UNOP (0x824fe50) postdec [4]
  SVOP (0x824de38) const [9] PV (0x81688b8) "\n"
  LISTOP (0x8250150) print
#
  LISTOP (0x824dac0) leave [1]

[download]
from where it can be learned that the $| decrement
occurs (postdec [4]) where it should.

Sorry for writing such a mess without checking before :-(

Regards

mwa

/1/ perl -MO=Bblock mysource.pl

Replies are listed 'Best First'.
Re^2: Puzzling $| behavior
by blokhead (Monsignor) on Oct 08, 2007 at 05:23 UTC
    (please correct here me if I'm wrong)
    I don't know if perl's documentation defines (as a feature) the order in which arguments are evaluated, but it's simple to check that it really does evaluate them in left-to-right order: 
    $ perl -le 'print sub{print 1; "x"}->(), sub{print 2; "y"}->()'
1
2
xy

    [download]
    The reason the OP's example is weird is because Perl's argument passing uses aliases whenever possible (as ikegami illustrates below). In the example, an alias to $| gets passed as the second argument. When evaluating the fourth argument, $| is changed. When print finally inspects its arguments, the aliased second argument will report the changed value.

    blokhead

[reply]
[d/l]

      I don't know if perl's documentation defines (as a feature) the order in which arguments are evaluated

      It's not, but no one has mentioned a system or version where the arguments are evaluated in any order other than left-to-right in past discussions on the subject.

      It's technically subject to change since it's not documented, but I find it highly unlikely to change in Perl5.

[reply]
        ikegami: no one has mentioned a system or version where the arguments are evaluated in any order other than left-to-right

        My answer to the OP was completely wrong,
        which is what I found out after investigating
        into the topic.

        Thats another case where prejudice brings ill-fated conclusions.
        One simple look into perl -MO=Bblock thisprog.pl reveals
        the left-to-right sequence 
        OP (0x824dc78) enter
COP (0x81f30f0) nextstate
SVOP (0x824daa0) const [6] IV (0x8167cdc) 1
PADOP (0x8193748) gvsv  GV (0x816887c) *|
BINOP (0x8189228) sassign
COP (0x824c460) nextstate
OP (0x824ffa8) pushmark
SVOP (0x81892b8) const [7] PV (0x8168804) "first="
PADOP (0x818c5c0) gvsv  GV (0x816887c) *|
SVOP (0x818dc50) const [8] PV (0x8168810) " second="
PADOP (0x824fdb8) gvsv  GV (0x816887c) *|
UNOP (0x824fc80) postinc [4]
SVOP (0x824dc58) const [9] PV (0x81688a0) "\n"
LISTOP (0x824ff80) print
LISTOP (0x824d8e0) leave [1]

        [download]
        int the perl. Sorry, I was mistaken by the way one
        "programs" Perl in C via its interfaces.

        Thanks to all people who helped clearing this up.

        I will eventually make an addendum to my first post.

        Regards

        mwa
[reply]
[d/l]
[select]
        i think it's matter of the compiler optimizer:
        if i have something like f($x+=2, $x+=3); (and comma between arguments is not a sync point, like in C) then the optimizer could collapse $x+=2 and $x+=3 to $x+=5. It's not only matter of order. 

        $_ = 1;
f($_+=2, $_+=3);
sub f { print shift, ", ", shift, "\n"; }
print `perl -v`;
__________
6, 6

This is perl, v5.6.0 built for darwin
....

        [download]
        Oha
[reply]
[d/l]

In Section Seekers of Perl Wisdom