Re: eval order of args to a sub

Perl evaluates the arguments from left to right (according to the associtivity of the list seperator).

>perl -le"$i=3; print($i+0, ++$i, $i+0);"
344
[download]

However, because Perl passes all arguments by reference, the order of operation sometimes *appears* unclear.

>perl -le"$i=3; print($i, ++$i, $i);"
444
[download]

The first snippet is similar to

# $i=3; print($i+0, ++$i, $i+0);
$i = 3;
do {
   local @_;
   $anon0 = $i+0;  alias $_[0] = $anon0;
   $i = $i+1;      alias $_[1] = $i;
   $anon1 = $i+0;  alias $_[2] = $anon1;
   &print
};
[download]

The second snippet is similar to

# $i=3; print($i, ++$i, $i);
$i = 3;
do {
   local @_;
   $i;         alias $_[0] = $i;
   $i = $i+1;  alias $_[1] = $i;
   $i;         alias $_[2] = $i;
   &print
};
[download]

Can you guess what the following prints?

perl -le"$i=3; sub { $_[1]++; print @_ }->($i+0, ++$i, $i+0, $i);"
[download]

Comment on Re: eval order of args to a sub Select or Download Code

Replies are listed 'Best First'.
Re^2: eval order of args to a sub by dewey (Pilgrim) on May 30, 2007 at 20:08 UTC
That's very interesting. I could never have guessed that output before this thread. The creation of anonymous values (for lack of a better term) for some expressions makes me want to do some obfuscation... ~dewey	[reply]
Re^3: eval order of args to a sub by ikegami (Patriarch) on May 30, 2007 at 20:22 UTC
All scalar variables are really only references to SVs. (There's a similar relation between arrays and AVs, hashes and HVs, etc.) Sometimes Perl creates SVs to which no variable refers, and it's possible to create more than one variable that refers to the same SV. `+------------+ references (1:1) +------------+ \| Lexical or \| -----------------------> \| \| \| Package \| \| SV \| \| Scalar Var \| referenced by (1:N) \| \| +------------+ <----------------------- +------------+` [download] The "anonymous values" are simply SVs.	[reply] [d/l]
Re^2: eval order of args to a sub by graq (Curate) on May 31, 2007 at 07:25 UTC
Can you guess what the following prints? perl -le"$i=3; sub { $_1++; print @_ }->($i+0, ++$i, $i+0, $i);" That's a sneaky one! I was completely wrong: `syntax error at -e line 1, near "="` PS: Just joking with you :) -=( Graq )=-	[reply] [d/l]
Re^2: eval order of args to a sub by eXile (Priest) on May 31, 2007 at 13:56 UTC
thanks for that explanation, I experimented a bit with that behaviour and found this a fun example: perl -le'$i=3; print($i, $i++, $i);' 434 perl -le'$i=3; print($i, $i+1, $i);' 343	[reply]
Re^2: eval order of args to a sub by mrpeabody (Friar) on Jun 03, 2007 at 09:55 UTC
Further, `$ perl -e'$i=3; sub { $_[0]++; print @_ }->(++$i, $i+0);' 54 $ perl -e'$i=3; sub { $_[1]++; print @_ }->($i+0, ++$i);' 35` [download]	[reply] [d/l]


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