On Mon, Oct 31, 2011 at 04:21:18PM +0200, Meir Guttman wrote:
> Hey Mongers!
>
> Goggling around, I could not determine whether or not Perl LABELs are
> subject to the usual lexical scoping rules as are "my" declared variables.
Labels are not varriable and not affected by my.
The "if" statement is
straightforward. Because BLOCKs are always bounded by curly brackets,
there is never any ambiguity about which "if" an "else" goes with. If you
use "unless" in place of "if", the sense of the test is reversed. Like
"if", "unless" can be followed by "else". "unless" can even be followed
by one or more "elsif" statements, though you may want to think twice
before using that particular language construct, as everyone reading
your code will have to think at least twice before they can understand
what's going on.
The "while" statement executes the block as long as the expression
is true. The "until" statement executes the block as long as the
expression is false. The LABEL is optional, and if present,
consists of an identifier followed by a colon. The LABEL
identifies the loop for the loop control statements "next",
"last", and "redo". If the LABEL is omitted, the loop control
statement refers to the innermost enclosing loop. This may
include dynamically looking back your call-stack at run time to
find the LABEL. Such desperate behavior triggers a warning if
you use the "use warnings" pragma or the -w flag.
If there is a "continue" BLOCK, it is always executed just before
the conditional is about to be evaluated again. Thus it can be
used to increment a loop variable, even when the loop has been
continued via the "next" statement.
Extension modules can also hook into the Perl parser to define
new kinds of compound statement. These are introduced by a
keyword which the extension recognises, and the syntax following
the keyword is defined entirely by the extension. If you are an
implementor, see "PL_keyword_plugin" in perlapi for the mechanism.
If you are using such a module, see the module's documentation
for details of the syntax that it defines.
> I
> almost always label "for"/"foreach"/"while" etc. blocks so it should be
> clear where will "next"/"last"/"redo" go. And sometimes the natural name for
> the loop is the same, e.g., "EMPLOYEE:". So I wonder if I can reuse the
> same label if it is clearly in a different scope.
>
> Simple tests were successful, but I would like to have a more authoritative
> answer. The closest I came to finding any reference to the issue is in
> <http://perldoc.perl.org/perlsyn.html#Goto> "perlsyn" GOTO:
>
> The goto <http://perldoc.perl.org/functions/goto.html> -EXPR form expects a
> label name, whose scope will be resolved dynamically. This allows for
> computed goto <http://perldoc.perl.org/functions/goto.html> s per FORTRAN,
> but isn't necessarily recommended if you're optimizing for maintainability:
>
> 1. goto <http://perldoc.perl.org/functions/goto.html> (("FOO", "BAR",
> "GLARCH")[$i]);
>
> But although this does suggest a scope for a label, it does not answer IMHO
> my question beyond doubt.
>
> Can anybody elucidate?
>
> Regards,
> Meir
>
>
>
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