=head1 NAME
-perlfaq7 - General Perl Language Issues ($Revision: 10100 $)
+perlfaq7 - General Perl Language Issues
=head1 DESCRIPTION
if ($whoops) { exit 1 }
@nums = (1, 2, 3);
-
+
if ($whoops) {
exit 1;
}
just such situations as the one above.
Another operator with surprising precedence is exponentiation. It
-binds more tightly even than unary minus, making C<-2**2> product a
+binds more tightly even than unary minus, making C<-2**2> produce a
negative not a positive four. It is also right-associating, meaning
that C<2**3**2> is two raised to the ninth power, not eight squared.
$person = {}; # new anonymous hash
$person->{AGE} = 24; # set field AGE to 24
$person->{NAME} = "Nat"; # set field NAME to "Nat"
-
+
If you're looking for something a bit more rigorous, try L<perltoot>.
=head2 How do I create a module?
current maintainer. The PAUSE admins will also try to reach the
maintainer.
-=item
+=item
Post a public message in a heavily trafficked site announcing your
intention to take over the module.
=item
Wait a bit. The PAUSE admins don't want to act too quickly in case
-the current maintainer is on holiday. If there's no response to
+the current maintainer is on holiday. If there's no response to
private communication or the public post, a PAUSE admin can transfer
it to you.
=back
=head2 How do I create a class?
+X<class, creation> X<package>
+
+(contributed by brian d foy)
-See L<perltoot> for an introduction to classes and objects, as well as
-L<perlobj> and L<perlbot>.
+In Perl, a class is just a package, and methods are just subroutines.
+Perl doesn't get more formal than that and lets you set up the package
+just the way that you like it (that is, it doesn't set up anything for
+you).
+
+The Perl documentation has several tutorials that cover class
+creation, including L<perlboot> (Barnyard Object Oriented Tutorial),
+L<perltoot> (Tom's Object Oriented Tutorial), L<perlbot> (Bag o'
+Object Tricks), and L<perlobj>.
=head2 How can I tell if a variable is tainted?
hard-to-explain meaning. Usually, closures are implemented in Perl as
anonymous subroutines with lasting references to lexical variables
outside their own scopes. These lexicals magically refer to the
-variables that were around when the subroutine was defined (deep
+variables that were around when the subroutine was defined (deep
binding).
Closures are most often used in programming languages where you can
my $addpiece = shift;
return sub { shift() + $addpiece };
}
-
+
$f1 = make_adder(20);
$f2 = make_adder(555);
}
$match = compare("old McDonald", qr/d.*D/i);
-Notice how C<qr//> allows flags at the end. That pattern was compiled
-at compile time, although it was executed later. The nifty C<qr//>
-notation wasn't introduced until the 5.005 release. Before that, you
-had to approach this problem much less intuitively. For example, here
-it is again if you don't have C<qr//>:
-
- sub compare($$) {
- my ($val1, $regex) = @_;
- my $retval = eval { $val1 =~ /$regex/ };
- die if $@;
- return $retval;
- }
-
- $match = compare("old McDonald", q/($?i)d.*D/);
-
-Make sure you never say something like this:
-
- return eval "\$val =~ /$regex/"; # WRONG
-
-or someone can sneak shell escapes into the regex due to the double
-interpolation of the eval and the double-quoted string. For example:
-
- $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
-
- eval "\$string =~ /$pattern_of_evil/";
-
-Those preferring to be very, very clever might see the O'Reilly book,
-I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's
-Build_MatchMany_Function() is particularly interesting. A complete
-citation of this book is given in L<perlfaq2>.
-
=item Passing Methods
To pass an object method into a subroutine, you can do this:
(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.
+In Perl 5.10, declare the variable with C<state>. The C<state>
+declaration creates the lexical variable that persists between calls
+to the subroutine:
+
+ sub counter { state $count = 1; $counter++ }
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
my $count = 1;
sub counter { $count++ }
}
-
+
my $start = counter();
-
+
.... # code that calls counter();
-
+
my $end = counter();
In the previous example, you created a function-private variable
sub visible {
print "var has value $var\n";
}
-
+
sub dynamic {
local $var = 'local'; # new temporary value for the still-global
visible(); # variable called $var
}
-
+
sub lexical {
my $var = 'private'; # new private variable, $var
visible(); # (invisible outside of sub scope)
}
-
+
$var = 'global';
-
+
visible(); # prints global
dynamic(); # prints local
lexical(); # prints global
=head2 What's the difference between calling a function as &foo and foo()?
-When you call a function as C<&foo>, you allow that function access to
-your current @_ values, and you bypass prototypes.
-The function doesn't get an empty @_--it gets yours! While not
-strictly speaking a bug (it's documented that way in L<perlsub>), it
-would be hard to consider this a feature in most cases.
+(contributed by brian d foy)
+
+Calling a subroutine as C<&foo> with no trailing parentheses ignores
+the prototype of C<foo> and passes it the current value of the argument
+list, C<@_>. Here's an example; the C<bar> subroutine calls C<&foo>,
+which prints what its arguments list:
+
+ sub bar { &foo }
+
+ sub foo { print "Args in foo are: @_\n" }
+
+ bar( qw( a b c ) );
+
+When you call C<bar> with arguments, you see that C<foo> got the same C<@_>:
+
+ Args in foo are: a b c
-When you call your function as C<&foo()>, then you I<do> get a new @_,
-but prototyping is still circumvented.
+Calling the subroutine with trailing parentheses, with or without arguments,
+does not use the current C<@_> and respects the subroutine prototype. Changing
+the example to put parentheses after the call to C<foo> changes the program:
-Normally, you want to call a function using C<foo()>. You may only
-omit the parentheses if the function is already known to the compiler
-because it already saw the definition (C<use> but not C<require>),
-or via a forward reference or C<use subs> declaration. Even in this
-case, you get a clean @_ without any of the old values leaking through
-where they don't belong.
+ sub bar { &foo() }
+
+ sub foo { print "Args in foo are: @_\n" }
+
+ bar( qw( a b c ) );
+
+Now the output shows that C<foo> doesn't get the C<@_> from its caller.
+
+ Args in foo are:
+
+The main use of the C<@_> pass-through feature is to write subroutines
+whose main job it is to call other subroutines for you. For further
+details, see L<perlsub>.
=head2 How do I create a switch or case statement?
+In Perl 5.10, use the C<given-when> construct described in L<perlsyn>:
+
+ use 5.010;
+
+ given ( $string ) {
+ when( 'Fred' ) { say "I found Fred!" }
+ when( 'Barney' ) { say "I found Barney!" }
+ when( /Bamm-?Bamm/ ) { say "I found Bamm-Bamm!" }
+ default { say "I don't recognize the name!" }
+ };
+
If one wants to use pure Perl and to be compatible with Perl versions
-prior to 5.10, the general answer is to write a construct like this:
+prior to 5.10, the general answer is to use C<if-elsif-else>:
for ($variable_to_test) {
if (/pat1/) { } # do something
"done" => sub { die "See ya!" },
"mad" => \&angry,
);
-
+
print "How are you? ";
chomp($string = <STDIN>);
if ($commands{$string}) {
print "No such command: $string\n";
}
-Note that starting from version 5.10, Perl has now a native switch
-statement. See L<perlsyn>.
-
Starting from Perl 5.8, a source filter module, C<Switch>, can also be
used to get switch and case. Its use is now discouraged, because it's
not fully compatible with the native switch of Perl 5.10, and because,
Make sure to read about creating modules in L<perlmod> and
the perils of indirect objects in L<perlobj/"Method Invocation">.
-=head2 How can I find out my current package?
+=head2 How can I find out my current or calling package?
-If you're just a random program, you can do this to find
-out what the currently compiled package is:
+(contributed by brian d foy)
- my $packname = __PACKAGE__;
+To find the package you are currently in, use the special literal
+C<__PACKAGE__>, as documented in L<perldata>. You can only use the
+special literals as separate tokens, so you can't interpolate them
+into strings like you can with variables:
-But, if you're a method and you want to print an error message
-that includes the kind of object you were called on (which is
-not necessarily the same as the one in which you were compiled):
+ my $current_package = __PACKAGE__;
+ print "I am in package $current_package\n";
- sub amethod {
- my $self = shift;
- my $class = ref($self) || $self;
- warn "called me from a $class object";
- }
+This is different from finding out the package an object is blessed
+into, which might not be the current package. For that, use C<blessed>
+from C<Scalar::Util>, part of the Standard Library since Perl 5.8:
+
+ use Scalar::Util qw(blessed);
+ my $object_package = blessed( $object );
+
+Most of the time, you shouldn't care what package an object is blessed
+into, however, as long as it claims to inherit from that class:
+
+ my $is_right_class = eval { $object->isa( $package ) }; # true or false
+
+If you want to find the package calling your code, perhaps to give better
+diagnostics as C<Carp> does, use the C<caller> built-in:
+
+ sub foo {
+ my @args = ...;
+ my( $package, $filename, $line ) = caller;
+
+ print "I was called from package $package\n";
+ );
-=head2 How can I comment out a large block of perl code?
+By default, your program starts in package C<main>, so you should
+always be in some package unless someone uses the C<package> built-in
+with no namespace. See the C<package> entry in L<perlfunc> for the
+details of empty packages.
-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>.
+=head2 How can I comment out a large block of Perl code?
+
+(contributed by brian d foy)
+
+The quick-and-dirty way to comment out more than one line of Perl is
+to surround those lines with Pod directives. You have to put these
+directives at the beginning of the line and somewhere where Perl
+expects a new statement (so not in the middle of statements like the #
+comments). You end the comment with C<=cut>, ending the Pod section:
+
+ =pod
+
+ my $object = NotGonnaHappen->new();
+
+ ignored_sub();
+
+ $wont_be_assigned = 37;
+
+ =cut
+
+The quick-and-dirty method only works well when you don't plan to
+leave the commented code in the source. If a Pod parser comes along,
+you're multiline comment is going to show up in the Pod translation.
+A better way hides it from Pod parsers as well.
+
+The C<=begin> directive can mark a section for a particular purpose.
+If the Pod parser doesn't want to handle it, it just ignores it. Label
+the comments with C<comment>. End the comment using C<=end> with the
+same label. You still need the C<=cut> to go back to Perl code from
+the Pod comment:
- # program is here
-
=begin comment
-
- all of this stuff
-
- here will be ignored
- by everyone
-
+
+ my $object = NotGonnaHappen->new();
+
+ ignored_sub();
+
+ $wont_be_assigned = 37;
+
=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 grammar production.
+ =cut
-See L<perlpod> for more details.
+For more information on Pod, check out L<perlpod> and L<perlpodspec>.
=head2 How do I clear a package?
=head1 REVISION
-Revision: $Revision: 10100 $
+Revision: $Revision$
-Date: $Date: 2007-10-21 20:59:30 +0200 (Sun, 21 Oct 2007) $
+Date: $Date$
See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2009 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
projects / p5sagit/p5-mst-13.2.git / blobdiff