Re^4: problem unless I use ".="

That's a good question, but you'll find it in the on-line documentation.

From perlop (under Assignment Operators):

    Assignment operators work as in C. That is,

        $a += 2;

    is equivalent to

        $a = $a + 2;

    although without duplicating any side effects that
    dereferencing the lvalue might trigger
[download]

It's that last part, about avoiding dereferencing side effects, that comes into play here.

When you do:

    $baseball{yankees} = $baseball{yankees} . $baseball{mets};
[download]

you are incurring a side effect of dereferencing $baseball{yankees}, namely testing for definedness, which you won't incur when you use the assignment operator:

    $baseball{yankees} .= $baseball{mets}
[download]

s''(q.S:$/9=(T1';s;(..)(..);$..=substr+crypt($1,$2),2,3;eg;print$..$/

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Re^5: problem unless I use ".=" by demerphq (Chancellor) on May 14, 2007 at 08:50 UTC
Its not clear to me what the "avoiding dereferencing side effects" part of the documentation means exactly, however I feel pretty confident in saying that it isnt relevent here. When you do `$hash{key}.=$str;` [download] there is no dereferencing going on. With `$hash->{key}.=$str;` [download] there is, however it happens before the .= operator is involved, so it doesnt explain the special casing behaviour of .= and other mutator operators. It turns out that the documentation of this behaviour is not to be found in perlop, but rather in perlsyn (in the section Declarations): `Operators such as ++, --, +=, -=, and .=, that operate on undefined le +ft values such as: my $a; $a++; are also always exempt from such warnings.` [download] Hope this clears things up for you. --- $world=~s/war/peace/g	[reply] [d/l] [select]

Replies are listed 'Best First'.

Re^5: problem unless I use ".="
by demerphq (Chancellor) on May 14, 2007 at 08:50 UTC

Its not clear to me what the "avoiding dereferencing side effects" part of the documentation means exactly, however I feel pretty confident in saying that it isnt relevent here. When you do

$hash{key}.=$str;
[download]

there is no dereferencing going on. With

$hash->{key}.=$str;
[download]

there is, however it happens before the .= operator is involved, so it doesnt explain the special casing behaviour of .= and other mutator operators. It turns out that the documentation of this behaviour is not to be found in perlop, but rather in perlsyn (in the section Declarations):

Operators such as ++, --, +=, -=, and .=, that operate on undefined le
+ft values such as:

    my $a;
    $a++;

are also always exempt from such warnings.
[download]

Hope this clears things up for you.

---
$world=~s/war/peace/g

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