=head1 NAME
-perlfaq7 - Perl Language Issues ($Revision: 1.28 $, $Date: 1999/05/23 20:36:18 $)
+perlfaq7 - General Perl Language Issues
=head1 DESCRIPTION
They are type specifiers, as detailed in L<perldata>:
- $ for scalar values (number, string or reference)
- @ for arrays
- % for hashes (associative arrays)
- & for subroutines (aka functions, procedures, methods)
- * for all types of that symbol name. In version 4 you used them like
- pointers, but in modern perls you can just use references.
+ $ for scalar values (number, string or reference)
+ @ for arrays
+ % for hashes (associative arrays)
+ & for subroutines (aka functions, procedures, methods)
+ * for all types of that symbol name. In version 4 you used them like
+ pointers, but in modern perls you can just use references.
There are couple of other symbols that you're likely to encounter that aren't
really type specifiers:
- <> are used for inputting a record from a filehandle.
- \ takes a reference to something.
+ <> are used for inputting a record from a filehandle.
+ \ takes a reference to something.
Note that <FILE> is I<neither> the type specifier for files
nor the name of the handle. It is the C<< <> >> operator applied
to the handle FILE. It reads one line (well, record--see
-L<perlvar/$
/>) from the handle FILE in scalar context, or I<all> lines
+L<perlvar/$
E<sol>>) from the handle FILE in scalar context, or I<all> lines
in list context. When performing open, close, or any other operation
besides C<< <> >> on files, or even when talking about the handle, do
I<not> use the brackets. These are correct: C<eof(FH)>, C<seek(FH, 0,
subroutine) and the left-hand operand to the C<< => >> operator both
count as though they were quoted:
- This is like this
- ------------ ---------------
- $foo{line} $foo{"line"}
- bar => stuff "bar" => stuff
+ This is like this
+ ------------ ---------------
+ $foo{line} $foo{'line'}
+ bar => stuff 'bar' => stuff
The final semicolon in a block is optional, as is the final comma in a
list. Good style (see L<perlstyle>) says to put them in except for
one-liners:
- if ($whoops) { exit 1 }
- @nums = (1, 2, 3);
+ if ($whoops) { exit 1 }
+ @nums = (1, 2, 3);
- if ($whoops) {
- exit 1;
- }
- @lines = (
+ if ($whoops) {
+ exit 1;
+ }
+
+ @lines = (
"There Beren came from mountains cold",
"And lost he wandered under leaves",
- );
+ );
=head2 How do I skip some return values?
One way is to treat the return values as a list and index into it:
- $dir = (getpwnam($user))[7];
+ $dir = (getpwnam($user))[7];
Another way is to use undef as an element on the left-hand-side:
- ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
+ ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
+
+You can also use a list slice to select only the elements that
+you need:
+
+ ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];
=head2 How do I temporarily block warnings?
allows fine control of what warning are produced.
See L<perllexwarn> for more details.
- {
+ {
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
+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:
- {
+ {
local $^W = 0; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
- }
+ }
Note that like all the punctuation variables, you cannot currently
use my() on C<$^W>, only local().
A common mistake is to write:
- unlink $file || die "snafu";
+ unlink $file || die "snafu";
This gets interpreted as:
- unlink ($file || die "snafu");
+ unlink ($file || die "snafu");
To avoid this problem, either put in extra parentheses or use the
super low precedence C<or> operator:
- (unlink $file) || die "snafu";
- unlink $file or die "snafu";
+ (unlink $file) || die "snafu";
+ unlink $file or die "snafu";
The "English" operators (C<and>, C<or>, C<xor>, and C<not>)
deliberately have precedence lower than that of list operators for
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.
produces an lvalue. This assigns $x to either $a or $b, depending
on the trueness of $maybe:
- ($maybe ? $a : $b) = $x;
+ ($maybe ? $a : $b) = $x;
=head2 How do I declare/create a structure?
anonymous) hash reference. See L<perlref> and L<perldsc> for details.
Here's an example:
- $person = {}; # new anonymous hash
- $person->{AGE} = 24; # set field AGE to 24
- $person->{NAME} = "Nat"; # set field NAME to "Nat"
+ $person = {}; # new anonymous hash
+ $person->{AGE} = 24; # set field AGE to 24
+ $person->{NAME} = "Nat"; # set field NAME to "Nat"
If you're looking for something a bit more rigorous, try L<perltoot>.
=head2 How do I create a module?
-A module is a package that lives in a file of the same name. For
-example, the Hello::There module would live in Hello/There.pm. For
-details, read L<perlmod>. You'll also find L<Exporter> helpful. If
-you're writing a C or mixed-language module with both C and Perl, then
-you should study L<perlxstut>.
+(contributed by brian d foy)
-Here's a convenient template you might wish you use when starting your
-own module. Make sure to change the names appropriately.
+L<perlmod>, L<perlmodlib>, L<perlmodstyle> explain modules
+in all the gory details. L<perlnewmod> gives a brief
+overview of the process along with a couple of suggestions
+about style.
- package Some::Module; # assumes Some/Module.pm
+If you need to include C code or C library interfaces in
+your module, you'll need h2xs. h2xs will create the module
+distribution structure and the initial interface files
+you'll need. L<perlxs> and L<perlxstut> explain the details.
- use strict;
- use warnings;
+If you don't need to use C code, other tools such as
+ExtUtils::ModuleMaker and Module::Starter, can help you
+create a skeleton module distribution.
- BEGIN {
- use Exporter ();
- our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
+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.
- ## set the version for version checking; uncomment to use
- ## $VERSION = 1.00;
-
- # if using RCS/CVS, this next line may be preferred,
- # but beware two-digit versions.
- $VERSION = do{my@r=q$Revision: 1.28 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r};
-
- @ISA = qw(Exporter);
- @EXPORT = qw(&func1 &func2 &func3);
- %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
-
- # your exported package globals go here,
- # as well as any optionally exported functions
- @EXPORT_OK = qw($Var1 %Hashit);
- }
- our @EXPORT_OK;
+=head2 How do I adopt or take over a module already on CPAN?
- # exported package globals go here
- our $Var1;
- our %Hashit;
+(contributed by brian d foy)
- # non-exported package globals go here
- our @more;
- our $stuff;
+The easiest way to take over a module is to have the current
+module maintainer either make you a co-maintainer or transfer
+the module to you.
- # initialize package globals, first exported ones
- $Var1 = '';
- %Hashit = ();
+If you can't reach the author for some reason (e.g. email bounces),
+the PAUSE admins at modules@perl.org can help. The PAUSE admins
+treat each case individually.
- # then the others (which are still accessible as $Some::Module::stuff)
- $stuff = '';
- @more = ();
+=over 4
- # all file-scoped lexicals must be created before
- # the functions below that use them.
+=item
- # file-private lexicals go here
- my $priv_var = '';
- my %secret_hash = ();
+Get a login for the Perl Authors Upload Server (PAUSE) if you don't
+already have one: http://pause.perl.org
- # here's a file-private function as a closure,
- # callable as &$priv_func; it cannot be prototyped.
- my $priv_func = sub {
- # stuff goes here.
- };
+=item
- # make all your functions, whether exported or not;
- # remember to put something interesting in the {} stubs
- sub func1 {} # no prototype
- sub func2() {} # proto'd void
- sub func3($$) {} # proto'd to 2 scalars
+Write to modules@perl.org explaining what you did to contact the
+current maintainer. The PAUSE admins will also try to reach the
+maintainer.
- # this one isn't exported, but could be called!
- sub func4(\%) {} # proto'd to 1 hash ref
+=item
- END { } # module clean-up code here (global destructor)
+Post a public message in a heavily trafficked site announcing your
+intention to take over the module.
- 1; # modules must return true
+=item
-The h2xs program will create stubs for all the important stuff for you:
+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
+private communication or the public post, a PAUSE admin can transfer
+it to you.
- % h2xs -XA -n My::Module
+=back
=head2 How do I create a class?
+X<class, creation> X<package>
-See L<perltoot> for an introduction to classes and objects, as well as
-L<perlobj> and L<perlbot>.
+(contributed by brian d foy)
-=head2 How can I tell if a variable is tainted?
+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).
-See L<perlsec/"Laundering and Detecting Tainted Data">. Here's an
-example (which doesn't use any system calls, because the kill()
-is given no processes to signal):
+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>.
- sub is_tainted {
- return ! eval { join('',@_), kill 0; 1; };
- }
+=head2 How can I tell if a variable is tainted?
-This is not C<-w> clean, however. There is no C<-w> clean way to
-detect taintedness--take this as a hint that you should untaint
-all possibly-tainted data.
+You can use the tainted() function of the Scalar::Util module, available
+from CPAN (or included with Perl since release 5.8.0).
+See also L<perlsec/"Laundering and Detecting Tainted Data">.
=head2 What's a closure?
Closures are documented in L<perlref>.
I<Closure> is a computer science term with a precise but
-hard-to-explain meaning. 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 binding).
-
-Closures make sense in any programming language where you can have the
-return value of a function be itself a function, as you can in Perl.
-Note that some languages provide anonymous functions but are not
-capable of providing proper closures: the Python language, for
+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
+binding).
+
+Closures are most often used in programming languages where you can
+have the return value of a function be itself a function, as you can
+in Perl. Note that some languages provide anonymous functions but are
+not capable of providing proper closures: the Python language, for
example. For more information on closures, check out any textbook on
functional programming. Scheme is a language that not only supports
but encourages closures.
-Here's a classic function-generating function:
+Here's a classic non-closure function-generating function:
- sub add_function_generator {
- return sub { shift + shift };
- }
+ sub add_function_generator {
+ return sub { shift() + shift() };
+ }
- $add_sub = add_function_generator();
- $sum = $add_sub->(4,5); # $sum is 9 now.
+ $add_sub = add_function_generator();
+ $sum = $add_sub->(4,5); # $sum is 9 now.
-The closure works as a I<function template> with some customization
-slots left out to be filled later. The anonymous subroutine returned
-by add_function_generator() isn't technically a closure because it
-refers to no lexicals outside its own scope.
+The anonymous subroutine returned by add_function_generator() isn't
+technically a closure because it refers to no lexicals outside its own
+scope. Using a closure gives you a I<function template> with some
+customization slots left out to be filled later.
Contrast this with the following make_adder() function, in which the
returned anonymous function contains a reference to a lexical variable
that Perl return a proper closure, thus locking in for all time the
value that the lexical had when the function was created.
- sub make_adder {
- my $addpiece = shift;
- return sub { shift + $addpiece };
- }
+ sub make_adder {
+ my $addpiece = shift;
+ return sub { shift() + $addpiece };
+ }
- $f1 = make_adder(20);
- $f2 = make_adder(555);
+ $f1 = make_adder(20);
+ $f2 = make_adder(555);
Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas
C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece
Closures are often used for less esoteric purposes. For example, when
you want to pass in a bit of code into a function:
- my $line;
- timeout( 30, sub { $line = <STDIN> } );
+ my $line;
+ timeout( 30, sub { $line = <STDIN> } );
If the code to execute had been passed in as a string,
C<< '$line = <STDIN>' >>, there would have been no way for the
hypothetical timeout() function to access the lexical variable
$line back in its caller's scope.
+Another use for a closure is to make a variable I<private> to a
+named subroutine, e.g. a counter that gets initialized at creation
+time of the sub and can only be modified from within the sub.
+This is sometimes used with a BEGIN block in package files to make
+sure a variable doesn't get meddled with during the lifetime of the
+package:
+
+ BEGIN {
+ my $id = 0;
+ sub next_id { ++$id }
+ }
+
+This is discussed in more detail in L<perlsub>, see the entry on
+I<Persistent Private Variables>.
+
=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. ;)
- my $f = "foo";
- sub T {
- while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
- }
- T;
- print "Finally $f\n";
+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';
+ sub T {
+ while ($i++ < 3) { my $f = $f; $f .= "bar"; 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
-expressions.
+See "Passing Regexes", later in L<perlfaq7>, for information on
+passing regular expressions.
=over 4
Regular variables and functions are quite easy to pass: just pass in a
reference to an existing or anonymous variable or function:
- func( \$some_scalar );
+ func( \$some_scalar );
- func( \@some_array );
- func( [ 1 .. 10 ] );
+ func( \@some_array );
+ func( [ 1 .. 10 ] );
- func( \%some_hash );
- func( { this => 10, that => 20 } );
+ func( \%some_hash );
+ func( { this => 10, that => 20 } );
- func( \&some_func );
- func( sub { $_[0] ** $_[1] } );
+ func( \&some_func );
+ func( sub { $_[0] ** $_[1] } );
=item Passing Filehandles
-To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations.
-These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles">
-and especially L<perlsub/"Pass by Reference"> for more information.
-
-Here's an excerpt:
+As of Perl 5.6, you can represent filehandles with scalar variables
+which you treat as any other scalar.
-If you're passing around filehandles, you could usually just use the bare
-typeglob, like *STDOUT, but typeglobs references would be better because
-they'll still work properly under C<use strict 'refs'>. For example:
+ open my $fh, $filename or die "Cannot open $filename! $!";
+ func( $fh );
- splutter(\*STDOUT);
- sub splutter {
- my $fh = shift;
- print $fh "her um well a hmmm\n";
- }
+ sub func {
+ my $passed_fh = shift;
- $rec = get_rec(\*STDIN);
- sub get_rec {
- my $fh = shift;
- return scalar <$fh>;
- }
+ my $line = <$passed_fh>;
+ }
-If you're planning on generating new filehandles, you could do this:
-
- sub openit {
- my $name = shift;
- local *FH;
- return open (FH, $path) ? *FH : undef;
- }
- $fh = openit('< /etc/motd');
- print <$fh>;
+Before Perl 5.6, you had to use the C<*FH> or C<\*FH> notations.
+These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles">
+and especially L<perlsub/"Pass by Reference"> for more information.
=item Passing Regexes
Here's an example of how to pass in a string to be regex compared
using C<qr//>:
- sub compare($$) {
- my ($val1, $regex) = @_;
- my $retval = $val1 =~ /$regex/;
+ sub compare($$) {
+ my ($val1, $regex) = @_;
+ my $retval = $val1 =~ /$regex/;
return $retval;
- }
- $match = compare("old McDonald", qr/d.*D/i);
-
-Notice how C<qr//> allows flags at the end. That pattern was compiled
-at compile time, although it was executed later. The nifty C<qr//>
-notation wasn't introduced until the 5.005 release. Before that, you
-had to approach this problem much less intuitively. For example, here
-it is again if you don't have C<qr//>:
-
- sub compare($$) {
- 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>.
+ }
+ $match = compare("old McDonald", qr/d.*D/i);
=item Passing Methods
To pass an object method into a subroutine, you can do this:
- call_a_lot(10, $some_obj, "methname")
- sub call_a_lot {
- my ($count, $widget, $trick) = @_;
- for (my $i = 0; $i < $count; $i++) {
- $widget->$trick();
- }
- }
+ call_a_lot(10, $some_obj, "methname")
+ sub call_a_lot {
+ my ($count, $widget, $trick) = @_;
+ for (my $i = 0; $i < $count; $i++) {
+ $widget->$trick();
+ }
+ }
Or, you can use a closure to bundle up the object, its
method call, and arguments:
- my $whatnot = sub { $some_obj->obfuscate(@args) };
- func($whatnot);
- sub func {
- my $code = shift;
- &$code();
- }
+ my $whatnot = sub { $some_obj->obfuscate(@args) };
+ func($whatnot);
+ sub func {
+ my $code = shift;
+ &$code();
+ }
You could also investigate the can() method in the UNIVERSAL class
(part of the standard perl distribution).
=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)
+
+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; $counter++ }
+
+You can fake a static variable by using a lexical variable which goes
+out of scope. In this example, you define the subroutine C<counter>, and
+it uses the lexical variable C<$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 $count = 1;
+ sub counter { $count++ }
+ }
- BEGIN {
- my $counter = 42;
- sub prev_counter { return --$counter }
- sub next_counter { return $counter++ }
- }
+ my $start = counter();
-Now prev_counter() and next_counter() share a private variable $counter
-that was initialized at compile time.
+ .... # code that calls counter();
-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 = counter();
- 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()?
For instance:
- sub visible {
- print "var has value $var\n";
- }
+ 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 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)
- }
+ sub lexical {
+ my $var = 'private'; # new private variable, $var
+ visible(); # (invisible outside of sub scope)
+ }
- $var = 'global';
+ $var = 'global';
- visible(); # prints global
- dynamic(); # prints local
- lexical(); # prints global
+ visible(); # prints global
+ dynamic(); # prints local
+ lexical(); # prints global
Notice how at no point does the value "private" get printed. That's
because $var only has that value within the block of the lexical()
variables. It gives a global variable a temporary value. my() is
what you're looking for if you want private variables.
-See L<perlsub/"Private Variables via my()"> and L<perlsub/"Temporary
-Values via local()"> for excruciating details.
+See L<perlsub/"Private Variables via my()"> and
+L<perlsub/"Temporary Values via local()"> for excruciating details.
=head2 How can I access a dynamic variable while a similarly named lexical is in scope?
-You can do this via symbolic references, provided you haven't set
-C<use strict "refs">. So instead of $var, use C<${'var'}>.
+If you know your package, you can just mention it explicitly, as in
+$Some_Pack::var. Note that the notation $::var is B<not> the dynamic $var
+in the current package, but rather the one in the "main" package, as
+though you had written $main::var.
- local $var = "global";
- my $var = "lexical";
+ use vars '$var';
+ local $var = "global";
+ my $var = "lexical";
- print "lexical is $var\n";
+ print "lexical is $var\n";
+ print "global is $main::var\n";
- no strict 'refs';
- print "global is ${'var'}\n";
+Alternatively you can use the compiler directive our() to bring a
+dynamic variable into the current lexical scope.
-If you know your package, you can just mention it explicitly, as in
-$Some_Pack::var. Note that the notation $::var is I<not> the dynamic
-$var in the current package, but rather the one in the C<main>
-package, as though you had written $main::var. Specifying the package
-directly makes you hard-code its name, but it executes faster and
-avoids running afoul of C<use strict "refs">.
+ require 5.006; # our() did not exist before 5.6
+ use vars '$var';
+
+ local $var = "global";
+ my $var = "lexical";
+
+ print "lexical is $var\n";
+
+ {
+ our $var;
+ print "global is $var\n";
+ }
=head2 What's the difference between deep and shallow binding?
are effectively shallowly bound. Consider this just one more reason
not to use them. See the answer to L<"What's a closure?">.
-=head2 Why doesn't "my($foo) = <FILE>;" work right?
+=head2 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?
C<my()> and C<local()> give list context to the right hand side
of C<=>. The <FH> read operation, like so many of Perl's
To enforce scalar context in this particular case, however, you need
merely omit the parentheses:
- local($foo) = <FILE>; # WRONG
- local($foo) = scalar(<FILE>); # ok
- local $foo = <FILE>; # right
+ local($foo) = <FILE>; # WRONG
+ local($foo) = scalar(<FILE>); # ok
+ local $foo = <FILE>; # right
You should probably be using lexical variables anyway, although the
issue is the same here:
- my($foo) = <FILE>; # WRONG
- my $foo = <FILE>; # right
+ my($foo) = <FILE>; # WRONG
+ my $foo = <FILE>; # right
=head2 How do I redefine a builtin function, operator, or method?
=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 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<@_>:
-When you call your function as C<&foo()>, then you I<do> get a new @_,
-but prototyping is still circumvented.
+ Args in foo are: a b c
-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.
+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:
+
+ 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?
-This is explained in more depth in the L<perlsyn>. Briefly, there's
-no official case statement, because of the variety of tests possible
-in Perl (numeric comparison, string comparison, glob comparison,
-regex matching, overloaded comparisons, ...). Larry couldn't decide
-how best to do this, so he left it out, even though it's been on the
-wish list since perl1.
-
-The general answer is to write a construct like this:
-
- for ($variable_to_test) {
- if (/pat1/) { } # do something
- elsif (/pat2/) { } # do something else
- elsif (/pat3/) { } # do something else
- else { } # default
- }
-
-Here's a simple example of a switch based on pattern matching, this
-time lined up in a way to make it look more like a switch statement.
-We'll do a multi-way conditional based on the type of reference stored
+In Perl 5.10, use the C<given-when> construct described in L<perlsyn>:
+
+ use 5.010;
+
+ given ( $string ) {
+ when( 'Fred' ) { say "I found Fred!" }
+ when( 'Barney' ) { say "I found Barney!" }
+ when( /Bamm-?Bamm/ ) { say "I found Bamm-Bamm!" }
+ default { say "I don't recognize the name!" }
+ };
+
+If one wants to use pure Perl and to be compatible with Perl versions
+prior to 5.10, the general answer is to use C<if-elsif-else>:
+
+ for ($variable_to_test) {
+ if (/pat1/) { } # do something
+ elsif (/pat2/) { } # do something else
+ elsif (/pat3/) { } # do something else
+ else { } # default
+ }
+
+Here's a simple example of a switch based on pattern matching,
+lined up in a way to make it look more like a switch statement.
+We'll do a multiway conditional based on the type of reference stored
in $whatchamacallit:
SWITCH: for (ref $whatchamacallit) {
}
-See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other
-examples in this style.
+See L<perlsyn> for other examples in this style.
Sometimes you should change the positions of the constant and the variable.
For example, let's say you wanted to test which of many answers you were
one takes precedence over another, as C<"SEND"> has precedence over
C<"STOP"> here:
- chomp($answer = <>);
- if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
- elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
- elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
- elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
- elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
-
-A totally different approach is to create a hash of function references.
-
- my %commands = (
- "happy" => \&joy,
- "sad", => \&sullen,
- "done" => sub { die "See ya!" },
- "mad" => \&angry,
- );
+ chomp($answer = <>);
+ if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
+ elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
+ elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
+ elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
+ elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
+
+A totally different approach is to create a hash of function references.
+
+ my %commands = (
+ "happy" => \&joy,
+ "sad", => \&sullen,
+ "done" => sub { die "See ya!" },
+ "mad" => \&angry,
+ );
+
+ print "How are you? ";
+ chomp($string = <STDIN>);
+ if ($commands{$string}) {
+ $commands{$string}->();
+ } else {
+ print "No such command: $string\n";
+ }
- print "How are you? ";
- chomp($string = <STDIN>);
- if ($commands{$string}) {
- $commands{$string}->();
- } else {
- print "No such command: $string\n";
- }
+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,
+as it's implemented as a source filter, it doesn't always work as intended
+when complex syntax is involved.
-=head2 How can I catch accesses to undefined variables/functions/methods?
+=head2 How can I catch accesses to undefined variables, functions, or methods?
The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and
L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to
undefined functions and methods.
When it comes to undefined variables that would trigger a warning
-under C<-w>, you can use a handler to trap the pseudo-signal
-C<__WARN__> like this:
-
- $SIG{__WARN__} = sub {
+under C<use warnings>, you can promote the warning to an error.
- for ( $_[0] ) { # voici un switch statement
-
- /Use of uninitialized value/ && do {
- # promote warning to a fatal
- die $_;
- };
-
- # other warning cases to catch could go here;
-
- warn $_;
- }
-
- };
+ use warnings FATAL => qw(uninitialized);
=head2 Why can't a method included in this same file be found?
L<perlobj>.
Make sure to read about creating modules in L<perlmod> and
-the perils of indirect objects in L<perlobj/"WARNING">.
+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";
+ );
+
+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.
+
+=head2 How can I comment out a large block of Perl code?
+
+(contributed by brian d foy)
-=head2 How can I comment out a large block of perl code?
+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:
-Use embedded POD to discard it:
+ =pod
- # program is here
+ my $object = NotGonnaHappen->new();
- =for nobody
- This paragraph is commented out
+ ignored_sub();
- # program continues
+ $wont_be_assigned = 37;
- =begin comment text
+ =cut
- all of this stuff
+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.
- here will be ignored
- by everyone
+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:
- =end comment text
+ =begin comment
- =cut
+ my $object = NotGonnaHappen->new();
-This can't go just anywhere. You have to put a pod directive where
-the parser is expecting a new statement, not just in the middle
-of an expression or some other arbitrary yacc grammar production.
+ ignored_sub();
+
+ $wont_be_assigned = 37;
+
+ =end comment
+
+ =cut
+
+For more information on Pod, check out L<perlpod> and L<perlpodspec>.
=head2 How do I clear a package?
Use this code, provided by Mark-Jason Dominus:
- sub scrub_package {
- no strict 'refs';
- my $pack = shift;
- die "Shouldn't delete main package"
- if $pack eq "" || $pack eq "main";
- my $stash = *{$pack . '::'}{HASH};
- my $name;
- foreach $name (keys %$stash) {
- my $fullname = $pack . '::' . $name;
- # Get rid of everything with that name.
- undef $$fullname;
- undef @$fullname;
- undef %$fullname;
- undef &$fullname;
- undef *$fullname;
+ sub scrub_package {
+ no strict 'refs';
+ my $pack = shift;
+ die "Shouldn't delete main package"
+ if $pack eq "" || $pack eq "main";
+ my $stash = *{$pack . '::'}{HASH};
+ my $name;
+ foreach $name (keys %$stash) {
+ my $fullname = $pack . '::' . $name;
+ # Get rid of everything with that name.
+ undef $$fullname;
+ undef @$fullname;
+ undef %$fullname;
+ undef &$fullname;
+ undef *$fullname;
+ }
}
- }
-Or, if you're using a recent release of Perl, you can
+Or, if you're using a recent release of Perl, you can
just use the Symbol::delete_package() function instead.
=head2 How can I use a variable as a variable name?
Beginners often think they want to have a variable contain the name
of a variable.
- $fred = 23;
- $varname = "fred";
- ++$$varname; # $fred now 24
+ $fred = 23;
+ $varname = "fred";
+ ++$$varname; # $fred now 24
This works I<sometimes>, but it is a very bad idea for two reasons.
(like C<%main::>) instead of a user-defined hash. The solution is to
use your own hash or a real reference instead.
- $fred = 23;
- $varname = "fred";
- $USER_VARS{$varname}++; # not $$varname++
+ $USER_VARS{"fred"} = 23;
+ $varname = "fred";
+ $USER_VARS{$varname}++; # not $$varname++
There we're using the %USER_VARS hash instead of symbolic references.
Sometimes this comes up in reading strings from the user with variable
reading a string and expanding it to the actual contents of your program's
own variables:
- $str = 'this has a $fred and $barney in it';
- $str =~ s/(\$\w+)/$1/eeg; # need double eval
+ $str = 'this has a $fred and $barney in it';
+ $str =~ s/(\$\w+)/$1/eeg; # need double eval
it would be better to keep a hash around like %USER_VARS and have
variable references actually refer to entries in that hash:
- $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
+ $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
That's faster, cleaner, and safer than the previous approach. Of course,
you don't need to use a dollar sign. You could use your own scheme to
make it less confusing, like bracketed percent symbols, etc.
- $str = 'this has a %fred% and %barney% in it';
- $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
+ $str = 'this has a %fred% and %barney% in it';
+ $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
Another reason that folks sometimes think they want a variable to
contain the name of a variable is because they don't know how to build
wanted two hashes in their program: %fred and %barney, and that they
wanted to use another scalar variable to refer to those by name.
- $name = "fred";
- $$name{WIFE} = "wilma"; # set %fred
+ $name = "fred";
+ $$name{WIFE} = "wilma"; # set %fred
- $name = "barney";
- $$name{WIFE} = "betty"; # set %barney
+ $name = "barney";
+ $$name{WIFE} = "betty"; # set %barney
This is still a symbolic reference, and is still saddled with the
problems enumerated above. It would be far better to write:
- $folks{"fred"}{WIFE} = "wilma";
- $folks{"barney"}{WIFE} = "betty";
+ $folks{"fred"}{WIFE} = "wilma";
+ $folks{"barney"}{WIFE} = "betty";
And just use a multilevel hash to start with.
In those cases, you would turn off C<strict 'refs'> temporarily so you
can play around with the symbol table. For example:
- @colors = qw(red blue green yellow orange purple violet);
- for my $name (@colors) {
- no strict 'refs'; # renege for the block
- *$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
- }
+ @colors = qw(red blue green yellow orange purple violet);
+ for my $name (@colors) {
+ no strict 'refs'; # renege for the block
+ *$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
+ }
All those functions (red(), blue(), green(), etc.) appear to be separate,
but the real code in the closure actually was compiled only once.
For scalars, arrays, and hashes, though--and usually for subroutines--
you probably only want to use hard references.
+=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).
+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. 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.
+
+In either case, you should still be able to run the scripts with perl
+explicitly:
+
+ % perl script.pl
+
+If you get a message like "perl: command not found", perl is not in
+your PATH, which might also mean that the location of perl is not
+where you expect it so you need to adjust your shebang line.
+
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
-All rights reserved.
+Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and
+other authors as noted. All rights reserved.
-When included as part of the Standard Version of Perl, or as part of
-its complete documentation whether printed or otherwise, this work
-may be distributed only under the terms of Perl's Artistic License.
-Any distribution of this file or derivatives thereof I<outside>
-of that package require that special arrangements be made with
-copyright holder.
+This documentation is free; you can redistribute it and/or modify it
+under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain. You are permitted and
https://perl5.git.perl.org / perl5.git / blobdiff