perlfaq7
(1)
Name
perlfaq7 - General Perl Language Issues
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLFAQ7(1)
NAME
perlfaq7 - General Perl Language Issues
DESCRIPTION
This section deals with general Perl language issues that
don't clearly fit into any of the other sections.
Can I get a BNF/yacc/RE for the Perl language?
There is no BNF, but you can paw your way through the yacc
grammar in perly.y in the source distribution if you're
particularly brave. The grammar relies on very smart
tokenizing code, so be prepared to venture into toke.c as
well.
In the words of Chaim Frenkel: "Perl's grammar can not be
reduced to BNF. The work of parsing perl is distributed
between yacc, the lexer, smoke and mirrors."
What are all these $@%&* punctuation signs, and how do I know
when to use them?
They are type specifiers, as detailed in 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.
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.
Note that <FILE> is neither the type specifier for files nor
the name of the handle. It is the "<>" operator applied to
the handle FILE. It reads one line (well, record--see "$/"
in perlvar) from the handle FILE in scalar context, or all
lines in list context. When performing open, close, or any
other operation besides "<>" on files, or even when talking
about the handle, do not use the brackets. These are
correct: "eof(FH)", "seek(FH, 0, 2)" and "copying from STDIN
to FILE".
Do I always/never have to quote my strings or use semicolons
and commas?
Normally, a bareword doesn't need to be quoted, but in most
cases probably should be (and must be under "use strict").
But a hash key consisting of a simple word (that isn't the
name of a defined subroutine) and the left-hand operand to
the "=>" operator both count as though they were quoted:
perl v5.12.5 Last change: 2012-11-03 1
Perl Programmers Reference Guide PERLFAQ7(1)
This is like this
------------ ---------------
$foo{line} $foo{'line'}
bar => stuff 'bar' => stuff
The final semicolon in a block is optional, as is the final
comma in a list. Good style (see perlstyle) says to put
them in except for one-liners:
if ($whoops) { exit 1 }
@nums = (1, 2, 3);
if ($whoops) {
exit 1;
}
@lines = (
"There Beren came from mountains cold",
"And lost he wandered under leaves",
);
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];
Another way is to use undef as an element on the left-hand-
side:
($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];
How do I temporarily block warnings?
If you are running Perl 5.6.0 or better, the "use warnings"
pragma allows fine control of what warning are produced.
See 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
perllexwarn for the complete details, including the category
names and hierarchy.
perl v5.12.5 Last change: 2012-11-03 2
Perl Programmers Reference Guide PERLFAQ7(1)
{
no warnings 'uninitialized';
$a = $b + $c;
}
If you have an older version of Perl, the $^W variable
(documented in 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 $^W, only local().
What's an extension?
An extension is a way of calling compiled C code from Perl.
Reading perlxstut is a good place to learn more about
extensions.
Why do Perl operators have different precedence than C
operators?
Actually, they don't. All C operators that Perl copies have
the same precedence in Perl as they do in C. The problem is
with operators that C doesn't have, especially functions
that give a list context to everything on their right, eg.
print, chmod, exec, and so on. Such functions are called
"list operators" and appear as such in the precedence table
in perlop.
A common mistake is to write:
unlink $file || die "snafu";
This gets interpreted as:
unlink ($file || die "snafu");
To avoid this problem, either put in extra parentheses or
use the super low precedence "or" operator:
(unlink $file) || die "snafu";
unlink $file or die "snafu";
The "English" operators ("and", "or", "xor", and "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
perl v5.12.5 Last change: 2012-11-03 3
Perl Programmers Reference Guide PERLFAQ7(1)
minus, making "-2**2" produce a negative not a positive
four. It is also right-associating, meaning that "2**3**2"
is two raised to the ninth power, not eight squared.
Although it has the same precedence as in C, Perl's "?:"
operator produces an lvalue. This assigns $x to either $a
or $b, depending on the trueness of $maybe:
($maybe ? $a : $b) = $x;
How do I declare/create a structure?
In general, you don't "declare" a structure. Just use a
(probably anonymous) hash reference. See perlref and
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"
If you're looking for something a bit more rigorous, try
perltoot.
How do I create a module?
(contributed by brian d foy)
perlmod, perlmodlib, perlmodstyle explain modules in all the
gory details. perlnewmod gives a brief overview of the
process along with a couple of suggestions about style.
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. perlxs and perlxstut explain the details.
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.
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.
How do I adopt or take over a module already on CPAN?
(contributed by brian d foy)
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.
If you can't reach the author for some reason (e.g. email
bounces), the PAUSE admins at [email protected] can help. The
perl v5.12.5 Last change: 2012-11-03 4
Perl Programmers Reference Guide PERLFAQ7(1)
PAUSE admins treat each case individually.
o Get a login for the Perl Authors Upload Server (PAUSE)
if you don't already have one: http://pause.perl.org
o Write to [email protected] explaining what you did to
contact the current maintainer. The PAUSE admins will
also try to reach the maintainer.
o Post a public message in a heavily trafficked site
announcing your intention to take over the module.
o 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.
How do I create a class?
(contributed by brian d foy)
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 perlboot (Barnyard Object Oriented
Tutorial), perltoot (Tom's Object Oriented Tutorial),
perlbot (Bag o' Object Tricks), and perlobj.
How can I tell if a variable is tainted?
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 "Laundering and Detecting Tainted
Data" in perlsec.
What's a closure?
Closures are documented in perlref.
Closure is a computer science term with a precise but 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
perl v5.12.5 Last change: 2012-11-03 5
Perl Programmers Reference Guide PERLFAQ7(1)
functional programming. Scheme is a language that not only
supports but encourages closures.
Here's a classic non-closure function-generating function:
sub add_function_generator {
return sub { shift() + shift() };
}
$add_sub = add_function_generator();
$sum = $add_sub->(4,5); # $sum is 9 now.
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 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 outside the scope of that function
itself. Such a reference requires 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 };
}
$f1 = make_adder(20);
$f2 = make_adder(555);
Now "&$f1($n)" is always 20 plus whatever $n you pass in,
whereas "&$f2($n)" is always 555 plus whatever $n you pass
in. The $addpiece in the closure sticks around.
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 { $line = <STDIN> } );
If the code to execute had been passed in as a string,
'$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 private to a
named subroutine, e.g. a counter that gets initialized at
creation time of the sub and can only be modified from
perl v5.12.5 Last change: 2012-11-03 6
Perl Programmers Reference Guide PERLFAQ7(1)
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 perlsub, see the entry
on Persistent Private Variables.
What is variable suicide and how can I prevent it?
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 $f = 'foo';
sub T {
while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
}
T;
print "Finally $f\n";
If you are experiencing variable suicide, that "my $f" in
the subroutine doesn't pick up a fresh copy of the $f whose
value is <foo>. The output shows that inside the subroutine
the value of $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 $f "my $f" should create a new lexical variable each
time through the loop. The expected output is:
foobar
foobar
foobar
Finally foo
perl v5.12.5 Last change: 2012-11-03 7
Perl Programmers Reference Guide PERLFAQ7(1)
How can I pass/return a {Function, FileHandle, Array, Hash,
Method, Regex}?
You need to pass references to these objects. See "Pass by
Reference" in perlsub for this particular question, and
perlref for information on references.
Passing Variables and Functions
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_array );
func( [ 1 .. 10 ] );
func( \%some_hash );
func( { this => 10, that => 20 } );
func( \&some_func );
func( sub { $_[0] ** $_[1] } );
Passing Filehandles
As of Perl 5.6, you can represent filehandles with
scalar variables which you treat as any other scalar.
open my $fh, $filename or die "Cannot open $filename! $!";
func( $fh );
sub func {
my $passed_fh = shift;
my $line = <$passed_fh>;
}
Before Perl 5.6, you had to use the *FH or "\*FH"
notations. These are "typeglobs"--see "Typeglobs and
Filehandles" in perldata and especially "Pass by
Reference" in perlsub for more information.
Passing Regexes
Here's an example of how to pass in a string and a
regular expression for it to match against. You
construct the pattern with the "qr//" operator:
sub compare($$) {
my ($val1, $regex) = @_;
my $retval = $val1 =~ /$regex/;
return $retval;
}
$match = compare("old McDonald", qr/d.*D/i);
perl v5.12.5 Last change: 2012-11-03 8
Perl Programmers Reference Guide PERLFAQ7(1)
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();
}
}
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();
}
You could also investigate the can() method in the
UNIVERSAL class (part of the standard perl
distribution).
How do I create a static variable?
(contributed by brian d foy)
In Perl 5.10, declare the variable with "state". The "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 "counter", and it uses the lexical variable
$count. Since you wrap this in a BEGIN block, $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 "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 $count is private to
"counter".
perl v5.12.5 Last change: 2012-11-03 9
Perl Programmers Reference Guide PERLFAQ7(1)
BEGIN {
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 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,
"increment_count" and "return_count" share the variable. One
function adds to the value and the other simply returns the
value. They can both access $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 }
}
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 "Persistent Private Variables" in perlsub for more
information. The discussion of closures in perlref may help
you even though we did not use anonymous subroutines in this
answer. See "Persistent Private Variables" in perlsub for
details.
What's the difference between dynamic and lexical (static)
scoping? Between local() and my()?
"local($x)" saves away the old value of the global variable
$x and assigns a new value for the duration of the
subroutine which is visible in other functions called from
that subroutine. This is done at run-time, so is called
dynamic scoping. local() always affects global variables,
also called package variables or dynamic variables.
"my($x)" creates a new variable that is only visible in the
current subroutine. This is done at compile-time, so it is
called lexical or static scoping. my() always affects
private variables, also called lexical variables or
perl v5.12.5 Last change: 2012-11-03 10
Perl Programmers Reference Guide PERLFAQ7(1)
(improperly) static(ly scoped) variables.
For instance:
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
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() function, and it is hidden from called
subroutine.
In summary, local() doesn't make what you think of as
private, local variables. It gives a global variable a
temporary value. my() is what you're looking for if you
want private variables.
See "Private Variables via my()" in perlsub and "Temporary
Values via local()" in perlsub for excruciating details.
How can I access a dynamic variable while a similarly named
lexical is in scope?
If you know your package, you can just mention it
explicitly, as in $Some_Pack::var. Note that the notation
$::var is not the dynamic $var in the current package, but
rather the one in the "main" package, as though you had
written $main::var.
use vars '$var';
local $var = "global";
my $var = "lexical";
print "lexical is $var\n";
print "global is $main::var\n";
perl v5.12.5 Last change: 2012-11-03 11
Perl Programmers Reference Guide PERLFAQ7(1)
Alternatively you can use the compiler directive our() to
bring a dynamic variable into the current lexical scope.
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";
}
What's the difference between deep and shallow binding?
In deep binding, lexical variables mentioned in anonymous
subroutines are the same ones that were in scope when the
subroutine was created. In shallow binding, they are
whichever variables with the same names happen to be in
scope when the subroutine is called. Perl always uses deep
binding of lexical variables (i.e., those created with
my()). However, dynamic variables (aka global, local, or
package variables) are effectively shallowly bound.
Consider this just one more reason not to use them. See the
answer to "What's a closure?".
Why doesn't "my($foo) = <FILE>;" work right?
"my()" and "local()" give list context to the right hand
side of "=". The <FH> read operation, like so many of
Perl's functions and operators, can tell which context it
was called in and behaves appropriately. In general, the
scalar() function can help. This function does nothing to
the data itself (contrary to popular myth) but rather tells
its argument to behave in whatever its scalar fashion is.
If that function doesn't have a defined scalar behavior,
this of course doesn't help you (such as with sort()).
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
You should probably be using lexical variables anyway,
although the issue is the same here:
my($foo) = <FILE>; # WRONG
my $foo = <FILE>; # right
perl v5.12.5 Last change: 2012-11-03 12
Perl Programmers Reference Guide PERLFAQ7(1)
How do I redefine a builtin function, operator, or method?
Why do you want to do that? :-)
If you want to override a predefined function, such as
open(), then you'll have to import the new definition from a
different module. See "Overriding Built-in Functions" in
perlsub. There's also an example in "Class::Template" in
perltoot.
If you want to overload a Perl operator, such as "+" or
"**", then you'll want to use the "use overload" pragma,
documented in overload.
If you're talking about obscuring method calls in parent
classes, see "Overridden Methods" in perltoot.
What's the difference between calling a function as &foo and
foo()?
(contributed by brian d foy)
Calling a subroutine as &foo with no trailing parentheses
ignores the prototype of "foo" and passes it the current
value of the argument list, @_. Here's an example; the "bar"
subroutine calls &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 "bar" with arguments, you see that "foo" got
the same @_:
Args in foo are: a b c
Calling the subroutine with trailing parentheses, with or
without arguments, does not use the current @_ and respects
the subroutine prototype. Changing the example to put
parentheses after the call to "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 "foo" doesn't get the @_ from its
caller.
Args in foo are:
perl v5.12.5 Last change: 2012-11-03 13
Perl Programmers Reference Guide PERLFAQ7(1)
The main use of the @_ pass-through feature is to write
subroutines whose main job it is to call other subroutines
for you. For further details, see perlsub.
How do I create a switch or case statement?
In Perl 5.10, use the "given-when" construct described in
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
"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) {
/^$/ && die "not a reference";
/SCALAR/ && do {
print_scalar($$ref);
last SWITCH;
};
/ARRAY/ && do {
print_array(@$ref);
last SWITCH;
};
/HASH/ && do {
print_hash(%$ref);
last SWITCH;
};
perl v5.12.5 Last change: 2012-11-03 14
Perl Programmers Reference Guide PERLFAQ7(1)
/CODE/ && do {
warn "can't print function ref";
last SWITCH;
};
# DEFAULT
warn "User defined type skipped";
}
See 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 given, but in a case-
insensitive way that also allows abbreviations. You can use
the following technique if the strings all start with
different characters or if you want to arrange the matches
so that one takes precedence over another, as "SEND" has
precedence over "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,
);
print "How are you? ";
chomp($string = <STDIN>);
if ($commands{$string}) {
$commands{$string}->();
} else {
print "No such command: $string\n";
}
Starting from Perl 5.8, a source filter module, "Switch",
can also be used to get switch and case. Its use is now
discouraged, because it's not fully compatible with the
native switch of Perl 5.10, and because, as it's implemented
as a source filter, it doesn't always work as intended when
perl v5.12.5 Last change: 2012-11-03 15
Perl Programmers Reference Guide PERLFAQ7(1)
complex syntax is involved.
How can I catch accesses to undefined variables, functions, or
methods?
The AUTOLOAD method, discussed in "Autoloading" in perlsub
and "AUTOLOAD: Proxy Methods" in perltoot, lets you capture
calls to undefined functions and methods.
When it comes to undefined variables that would trigger a
warning under "use warnings", you can promote the warning to
an error.
use warnings FATAL => qw(uninitialized);
Why can't a method included in this same file be found?
Some possible reasons: your inheritance is getting confused,
you've misspelled the method name, or the object is of the
wrong type. Check out perltoot for details about any of the
above cases. You may also use "print ref($object)" to find
out the class $object was blessed into.
Another possible reason for problems is because you've used
the indirect object syntax (eg, "find Guru "Samy"") on a
class name before Perl has seen that such a package exists.
It's wisest to make sure your packages are all defined
before you start using them, which will be taken care of if
you use the "use" statement instead of "require". If not,
make sure to use arrow notation (eg., "Guru->find("Samy")")
instead. Object notation is explained in perlobj.
Make sure to read about creating modules in perlmod and the
perils of indirect objects in "Method Invocation" in
perlobj.
How can I find out my current or calling package?
(contributed by brian d foy)
To find the package you are currently in, use the special
literal "__PACKAGE__", as documented in perldata. You can
only use the special literals as separate tokens, so you
can't interpolate them into strings like you can with
variables:
my $current_package = __PACKAGE__;
print "I am in package $current_package\n";
If you want to find the package calling your code, perhaps
to give better diagnostics as "Carp" does, use the "caller"
built-in:
perl v5.12.5 Last change: 2012-11-03 16
Perl Programmers Reference Guide PERLFAQ7(1)
sub foo {
my @args = ...;
my( $package, $filename, $line ) = caller;
print "I was called from package $package\n";
);
By default, your program starts in package "main", so you
should always be in some package unless someone uses the
"package" built-in with no namespace. See the "package"
entry in perlfunc for the details of empty packages.
This is different from finding out the package an object is
blessed into, which might not be the current package. For
that, use "blessed" from "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
And, with Perl 5.10 and later, you don't have to check for
an inheritance to see if the object can handle a role. For
that, you can use "DOES", which comes from "UNIVERSAL":
my $class_does_it = eval { $object->DOES( $role ) }; # true or false
You can safely replace "isa" with "DOES" (although the
converse is not true).
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 "=cut", ending the Pod section:
=pod
my $object = NotGonnaHappen->new();
ignored_sub();
$wont_be_assigned = 37;
perl v5.12.5 Last change: 2012-11-03 17
Perl Programmers Reference Guide PERLFAQ7(1)
=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 "=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 "comment". End the
comment using "=end" with the same label. You still need the
"=cut" to go back to Perl code from the Pod comment:
=begin comment
my $object = NotGonnaHappen->new();
ignored_sub();
$wont_be_assigned = 37;
=end comment
=cut
For more information on Pod, check out perlpod and
perlpodspec.
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 $$fullname;
undef @$fullname;
undef %$fullname;
undef &$fullname;
undef *$fullname;
}
}
perl v5.12.5 Last change: 2012-11-03 18
Perl Programmers Reference Guide PERLFAQ7(1)
Or, if you're using a recent release of Perl, you can just
use the Symbol::delete_package() function instead.
How can I use a variable as a variable name?
Beginners often think they want to have a variable contain
the name of a variable.
$fred = 23;
$varname = "fred";
++$$varname; # $fred now 24
This works sometimes, but it is a very bad idea for two
reasons.
The first reason is that this technique only works on global
variables. That means that if $fred is a lexical variable
created with my() in the above example, the code wouldn't
work at all: you'd accidentally access the global and skip
right over the private lexical altogether. Global variables
are bad because they can easily collide accidentally and in
general make for non-scalable and confusing code.
Symbolic references are forbidden under the "use strict"
pragma. They are not true references and consequently are
not reference counted or garbage collected.
The other reason why using a variable to hold the name of
another variable is a bad idea is that the question often
stems from a lack of understanding of Perl data structures,
particularly hashes. By using symbolic references, you are
just using the package's symbol-table hash (like %main::)
instead of a user-defined hash. The solution is to use your
own hash or a real reference instead.
$USER_VARS{"fred"} = 23;
$varname = "fred";
$USER_VARS{$varname}++; # not $$varname++
There we're using the %USER_VARS hash instead of symbolic
references. Sometimes this comes up in reading strings from
the user with variable references and wanting to expand them
to the values of your perl program's variables. This is
also a bad idea because it conflates the program-addressable
namespace and the user-addressable one. Instead of reading
a string and expanding it to the actual contents of your
program's own variables:
$str = 'this has a $fred and $barney in it';
$str =~ s/(\$\w+)/$1/eeg; # need double eval
it would be better to keep a hash around like %USER_VARS and
have variable references actually refer to entries in that
perl v5.12.5 Last change: 2012-11-03 19
Perl Programmers Reference Guide PERLFAQ7(1)
hash:
$str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
That's faster, cleaner, and safer than the previous
approach. Of course, you don't need to use a dollar sign.
You could use your own scheme to make it less confusing,
like bracketed percent symbols, etc.
$str = 'this has a %fred% and %barney% in it';
$str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
Another reason that folks sometimes think they want a
variable to contain the name of a variable is because they
don't know how to build proper data structures using hashes.
For example, let's say they wanted two hashes in their
program: %fred and %barney, and that they wanted to use
another scalar variable to refer to those by name.
$name = "fred";
$$name{WIFE} = "wilma"; # set %fred
$name = "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";
And just use a multilevel hash to start with.
The only times that you absolutely must use symbolic
references are when you really must refer to the symbol
table. This may be because it's something that can't take a
real reference to, such as a format name. Doing so may also
be important for method calls, since these always go through
the symbol table for resolution.
In those cases, you would turn off "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>" };
}
perl v5.12.5 Last change: 2012-11-03 20
Perl Programmers Reference Guide PERLFAQ7(1)
All those functions (red(), blue(), green(), etc.) appear to
be separate, but the real code in the closure actually was
compiled only once.
So, sometimes you might want to use symbolic references to
directly manipulate the symbol table. This doesn't matter
for formats, handles, and subroutines, because they are
always global--you can't use my() on them. For scalars,
arrays, and hashes, though--and usually for subroutines--
you probably only want to use hard references.
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.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2010 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.
Irrespective of its distribution, all code examples in this
file are hereby placed into the public domain. You are
permitted and encouraged to use this code in your own
perl v5.12.5 Last change: 2012-11-03 21
Perl Programmers Reference Guide PERLFAQ7(1)
programs for fun or for profit as you see fit. A simple
comment in the code giving credit would be courteous but is
not required.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
perl v5.12.5 Last change: 2012-11-03 22