in reply to my (0?$a:$b): a koan

I think my(EXPR?$x:$y) = 3; is equivalent to: 
my $x = 3 if EXPR;
my $y = 3 if !EXPR;

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which is a known issue, and may do unexpected things if EXPR is false (although there has been code out there that (mis)uses this). Since 5.10 there has been a warning on 
my $x if EXPR;

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is EXPR can be determined to be false at compile time (constant folded to 0).

I wouldn't worry about my(0?$x:$y) too much. It's not something people will be do by accident - it isn't documented to work, so people who do it shouldn't be surprised if their code no longer works after upgrading Perl.

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Re^2: my (0?$a:$b): a koan
by educated_foo (Vicar) on May 05, 2011 at 14:42 UTC
    I wouldn't worry about my(0?$x:$y) too much.
    Oh, I'm not "worried" -- there's no need to either deprecate or document it. I just think: (1) it is surprising that it parses at all, since I thought "my" took a list of variables; and (2) it shows the compile- and run-time effects of "my" in an interesting way.
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      it is surprising that it parses at all, since I thought "my" took a list of variables

      I suspect that it takes any expression, then checks if the expression is valid. 

      >perl -e"my ( f($x, $y.) );"
syntax error at -e line 1, near ".) "
Execution of -e aborted due to compilation errors.

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      The catch here is that that constant folding happens before the check.

      Another allowed expression: 

      >perl -e"my(my(my(my $sharona)))"

>

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        I suspect that it takes any expression, then checks if the expression is valid.
        Yeah, but even then, it will reject most expressions: 
        $perl -we 'my(f($x))'
Can't declare subroutine entry in "my" at -e line 1, at EOF
Execution of -e aborted due to compilation errors.

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        Now, my does return an lvalue, and ?: can, but that isn't enough in itself for it to be accepted by my: 
        $ perl -we 'my($x = 3)'
Can't declare scalar assignment in "my" at -e line 1, at EOF
Execution of -e aborted due to compilation errors.

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        What the reason is that my(my $x) and my(0?$x:$y) are accept isn't clear to me.

        This one is interesting: 

        $ perl -we 'my(local $x)'
Can't localize lexical variable $x at -e line 1.
$ perl -we 'perl -we 'local(my $x)'
Can't localize lexical variable $x at -e line 1.
$

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        Regardless of the nesting of local and my, we get the same error message.

        Mixing state and my: 

        $ perl -wE 'sub f {state (my $x); say ++$x} f; f;'
1
1
$ perl -wE 'sub f {my (state $x); say ++$x} f; f;'
1
2
$ perl -wE 'sub f {state my $x; say ++$x} f; f;'
No such class my at -e line 1, near "{state my"
Execution of -e aborted due to compilation errors.

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        Mixing our and local: 
        $ perl -wE 'sub f {our local $x; say ++$x} f; f;'
No such class local at -e line 1, near "{our local"
Execution of -e aborted due to compilation errors.
$ perl -wE 'sub f {our (local $x); say ++$x} f; f;'
1
1
$ perl -wE 'sub f {local our $x; say ++$x} f; f;'
1
1
$

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        So we can have our (local $x), local our $x, but our local $x doesn't parse.
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In Section Meditations