=head1 NAME
-perlfaq7 - General Perl Language Issues ($Revision: 1.14 $, $Date: 2003/01/31 17:38:14 $)
+perlfaq7 - General Perl Language Issues ($Revision: 1.27 $, $Date: 2005/10/28 17:38:32 $)
=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
$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
+complete details, including the category names and hierarchy.
+
+ {
+ no warnings 'uninitialized';
+ $a = $b + $c;
+ }
+
If you have an older version of Perl, the C<$^W> variable (documented
in L<perlvar>) controls runtime warnings for a block:
=head2 How do I create a module?
-A module is a package that lives in a file of the same name. For
-example, the Hello::There module would live in Hello/There.pm. For
-details, read L<perlmod>. You'll also find L<Exporter> helpful. If
-you're writing a C or mixed-language module with both C and Perl, then
-you should study L<perlxstut>.
+(contributed by brian d foy)
+
+L<perlmod>, L<perlmodlib>, L<perlmodstyle> explain modules
+in all the gory details. L<perlnewmod> gives a brief
+overview of the process along with a couple of suggestions
+about style.
-The C<h2xs> program will create stubs for all the important stuff for you:
+If you need to include C code or C library interfaces in
+your module, you'll need h2xs. h2xs will create the module
+distribution structure and the initial interface files
+you'll need. L<perlxs> and L<perlxstut> explain the details.
- % h2xs -XA -n My::Module
+If you don't need to use C code, other tools such as
+ExtUtils::ModuleMaker and Module::Starter, can help you
+create a skeleton module distribution.
-The C<-X> switch tells C<h2xs> that you are not using C<XS> extension
-code. The C<-A> switch tells C<h2xs> that you are not using the
-AutoLoader, and the C<-n> switch specifies the name of the module.
-See L<h2xs> for more details.
+You may also want to see Sam Tregar's "Writing Perl Modules
+for CPAN" ( http://apress.com/book/bookDisplay.html?bID=14 )
+which is the best hands-on guide to creating module
+distributions.
=head2 How do I create a class?
Here's a classic function-generating function:
sub add_function_generator {
- return sub { shift + shift };
+ return sub { shift() + shift() };
}
$add_sub = add_function_generator();
sub make_adder {
my $addpiece = shift;
- return sub { shift + $addpiece };
+ return sub { shift() + $addpiece };
}
$f1 = make_adder(20);
=head2 What is variable suicide and how can I prevent it?
-Variable suicide is when you (temporarily or permanently) lose the
-value of a variable. It is caused by scoping through my() and local()
-interacting with either closures or aliased foreach() iterator
-variables and subroutine arguments. It used to be easy to
-inadvertently lose a variable's value this way, but now it's much
-harder. Take this code:
+This problem was fixed in perl 5.004_05, so preventing it means upgrading
+your version of perl. ;)
+
+Variable suicide is when you (temporarily or permanently) lose the value
+of a variable. It is caused by scoping through my() and local()
+interacting with either closures or aliased foreach() iterator variables
+and subroutine arguments. It used to be easy to inadvertently lose a
+variable's value this way, but now it's much harder. Take this code:
- my $f = "foo";
+ my $f = 'foo';
sub T {
- while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
+ while ($i++ < 3) { my $f = $f; $f .= $i; print $f, "\n" }
}
T;
print "Finally $f\n";
+If you are experiencing variable suicide, that C<my $f> in the subroutine
+doesn't pick up a fresh copy of the C<$f> whose value is <foo>. The output
+shows that inside the subroutine the value of C<$f> leaks through when it
+shouldn't, as in this output:
+
+ foobar
+ foobarbar
+ foobarbarbar
+ Finally foo
+
The $f that has "bar" added to it three times should be a new C<$f>
-(C<my $f> should create a new local variable each time through the loop).
-It isn't, however. This was a bug, now fixed in the latest releases
-(tested against 5.004_05, 5.005_03, and 5.005_56).
+C<my $f> should create a new lexical variable each time through the loop.
+The expected output is:
+
+ foobar
+ foobar
+ foobar
+ Finally foo
=head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
objects. See L<perlsub/"Pass by Reference"> for this particular
question, and L<perlref> for information on references.
-See ``Passing Regexes'', below, for information on passing regular
+See "Passing Regexes", below, for information on passing regular
expressions.
=over 4
=head2 How do I create a static variable?
-As with most things in Perl, TMTOWTDI. What is a "static variable" in
-other languages could be either a function-private variable (visible
-only within a single function, retaining its value between calls to
-that function), or a file-private variable (visible only to functions
-within the file it was declared in) in Perl.
-
-Here's code to implement a function-private variable:
+(contributed by brian d foy)
+
+Perl doesn't have "static" variables, which can only be accessed from
+the function in which they are declared. You can get the same effect
+with lexical variables, though.
+
+You can fake a static variable by using a lexical variable which goes
+out of scope. In this example, you define the subroutine C<counter>, and
+it uses the lexical variable C<$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();
+
+ .... # code that calls 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:
+ my $end = count();
- package Pax;
- my $started = scalar(localtime(time()));
+In the previous example, you created a function-private variable
+because only one function remembered its reference. You could define
+multiple functions while the variable is in scope, and each function
+can share the "private" variable. It's not really "static" because you
+can access it outside the function while the lexical variable is in
+scope, and even create references to it. In this example,
+C<increment_count> and C<return_count> share the variable. One
+function adds to the value and the other simply returns the value.
+They can both access C<$count>, and since it has gone out of scope,
+there is no other way to access it.
- sub begun { return $started }
+ 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()?
=head2 How can I comment out a large block of perl code?
-You can use embedded POD to discard it. The =for directive
-lasts until the next paragraph (two consecutive newlines).
+You can use embedded POD to discard it. Enclose the blocks you want
+to comment out in POD markers. The <=begin> directive marks a section
+for a specific formatter. Use the C<comment> format, which no formatter
+should claim to understand (by policy). Mark the end of the block
+with <=end>.
# program is here
- =for nobody
- This paragraph is commented out
-
- # program continues
-
-The =begin and =end directives can contain multiple
-paragraphs.
-
- =begin comment text
+ =begin comment
all of this stuff
here will be ignored
by everyone
- =end comment text
+ =end comment
+
+ =cut
+
+ # program continues
The pod directives cannot go just anywhere. You must put a
pod directive where the parser is expecting a new statement,
not just in the middle of an expression or some other
-arbitrary s grammar production.
+arbitrary grammar production.
See L<perlpod> for more details.
(like C<%main::>) instead of a user-defined hash. The solution is to
use your own hash or a real reference instead.
- $fred = 23;
+ $USER_VARS{"fred"} = 23;
$varname = "fred";
$USER_VARS{$varname}++; # not $$varname++
=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.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
-All rights reserved.
+Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
+other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
projects / p5sagit/p5-mst-13.2.git / blobdiff