=head1 NAME
-perlfaq7 - General Perl Language Issues ($Revision: 1.21 $, $Date: 2005/01/21 12:10:22 $)
+perlfaq7 - General Perl Language Issues ($Revision: 1.23 $, $Date: 2005/04/07 21:39:34 $)
=head1 DESCRIPTION
This is like this
------------ ---------------
- $foo{line} $foo{"line"}
- bar => stuff "bar" => stuff
+ $foo{line} $foo{'line'}
+ bar => stuff 'bar' => stuff
The final semicolon in a block is optional, as is the final comma in a
list. Good style (see L<perlstyle>) says to put them in except for
no warnings; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
}
-
+
Additionally, you can enable and disable categories of warnings.
You turn off the categories you want to ignore and you can still
get other categories of warnings. See L<perllexwarn> for the
objects. See L<perlsub/"Pass by Reference"> for this particular
question, and L<perlref> for information on references.
-See ``Passing Regexes'', below, for information on passing regular
+See "Passing Regexes", below, for information on passing regular
expressions.
=over 4
=head2 How do I create a static variable?
-As with most things in Perl, TMTOWTDI. What is a "static variable" in
-other languages could be either a function-private variable (visible
-only within a single function, retaining its value between calls to
-that function), or a file-private variable (visible only to functions
-within the file it was declared in) in Perl.
+(contributed by brian d foy)
-Here's code to implement a function-private variable:
+Perl doesn't have "static" variables, which can only be accessed from
+the function in which they are declared. You can get the same effect
+with lexical variables, though.
+
+You can fake a static variable by using a lexical variable which goes
+of scope. In this example, you define the subroutine C<counter>, and
+it uses the lexical variable C<$count>. Since you wrap this in a BEGIN
+block, C<$count> is defined at compile-time, but also goes out of
+scope at the end of the BEGIN block. The BEGIN block also ensures that
+the subroutine and the value it uses is defined at compile-time so the
+subroutine is ready to use just like any other subroutine, and you can
+put this code in the same place as other subroutines in the program
+text (i.e. at the end of the code, typically). The subroutine
+C<counter> still has a reference to the data, and is the only way you
+can access the value (and each time you do, you increment the value).
+The data in chunk of memory defined by C<$count> is private to
+C<counter>.
BEGIN {
- my $counter = 42;
- sub prev_counter { return --$counter }
- sub next_counter { return $counter++ }
+ my $count = 1;
+ sub counter { $count++ }
}
-Now prev_counter() and next_counter() share a private variable $counter
-that was initialized at compile time.
+ my $start = count();
-To declare a file-private variable, you'll still use a my(), putting
-the declaration at the outer scope level at the top of the file.
-Assume this is in file Pax.pm:
+ .... # code that calls count();
- package Pax;
- my $started = scalar(localtime(time()));
+ my $end = count();
- sub begun { return $started }
+In the previous example, you created a function-private variable
+because only one function remembered its reference. You could define
+multiple functions while the variable is in scope, and each function
+can share the "private" variable. It's not really "static" because you
+can access it outside the function while the lexical variable is in
+scope, and even create references to it. In this example,
+C<increment_count> and C<return_count> share the variable. One
+function adds to the value and the other simply returns the value.
+They can both access C<$count>, and since it has gone out of scope,
+there is no other way to access it.
+
+ BEGIN {
+ my $count = 1;
+ sub increment_count { $count++ }
+ sub return_count { $count }
+ }
-When C<use Pax> or C<require Pax> loads this module, the variable will
-be initialized. It won't get garbage-collected the way most variables
-going out of scope do, because the begun() function cares about it,
-but no one else can get it. It is not called $Pax::started because
-its scope is unrelated to the package. It's scoped to the file. You
-could conceivably have several packages in that same file all
-accessing the same private variable, but another file with the same
-package couldn't get to it.
+To declare a file-private variable, you still use a lexical variable.
+A file is also a scope, so a lexical variable defined in the file
+cannot be seen from any other file.
-See L<perlsub/"Persistent Private Variables"> for details.
+See L<perlsub/"Persistent Private Variables"> for more information.
+The discussion of closures in L<perlref> may help you even though we
+did not use anonymous subroutines in this answer. See
+L<perlsub/"Persistent Private Variables"> for details.
=head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
by everyone
=end comment
-
+
=cut
# program continues
=head2 What does "bad interpreter" mean?
+(contributed by brian d foy)
+
The "bad interpreter" message comes from the shell, not perl. The
actual message may vary depending on your platform, shell, and locale
settings.
If you see "bad interpreter - no such file or directory", the first
line in your perl script (the "shebang" line) does not contain the
-right path to perl (or any other program capable of running scripts).
+right path to perl (or any other program capable of running scripts).
Sometimes this happens when you move the script from one machine to
another and each machine has a different path to perl---/usr/bin/perl
-versus /usr/local/bin/perl for instance.
+versus /usr/local/bin/perl for instance. It may also indicate
+that the source machine has CRLF line terminators and the
+destination machine has LF only: the shell tries to find
+/usr/bin/perl<CR>, but can't.
If you see "bad interpreter: Permission denied", you need to make your
script executable.
https://perl5.git.perl.org / perl5.git / blobdiff