=head1 NAME
-perlfaq7 - Perl Language Issues ($Revision: 1.18 $, $Date: 1997/04/24 22:44:14 $)
+perlfaq7 - General Perl Language Issues ($Revision: 1.23 $, $Date: 2005/04/07 21:39:34 $)
=head1 DESCRIPTION
=head2 Can I get a BNF/yacc/RE for the Perl language?
-No, 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."
+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.
-=head2 What are all these $@%* punctuation signs, and how do I know when to use them?
+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."
+
+=head2 What are all these $@%&* punctuation signs, and how do I know when to use them?
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.
-While there are a few places where you don't actually need these type
-specifiers, you should always use them.
-
-A couple of others that you're likely to encounter that aren't
-really type specifiers are:
+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 E<lt>FILEE<gt> is I<neither> the type specifier for files
-nor the name of the handle. It is the C<E<lt>E<gt>> 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
+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/$
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<E<lt>E<gt>> on files, or even talking about the handle, do
+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,
2)> and "copying from STDIN to FILE".
Normally, a bareword doesn't need to be quoted, but in most cases
probably should be (and must be under C<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 C<=E<gt>> operator both
+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
+ $foo{line} $foo{'line'}
+ bar => stuff 'bar' => stuff
The final semicolon in a block is optional, as is the final comma in a
list. Good style (see L<perlstyle>) says to put them in except for
($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?
-The C<$^W> variable (documented in L<perlvar>) controls
-runtime warnings for a block:
+If you are running Perl 5.6.0 or better, the C<use warnings> pragma
+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 $^W = 0; # temporarily turn off warnings
Note that like all the punctuation variables, you cannot currently
use my() on C<$^W>, only local().
-A new C<use warnings> pragma is in the works to provide finer control
-over all this. The curious should check the perl5-porters mailing list
-archives for details.
-
=head2 What's an extension?
-A way of calling compiled C code from Perl. Reading L<perlxstut>
-is a good place to learn more about extensions.
+An extension is a way of calling compiled C code from Perl. Reading
+L<perlxstut> is a good place to learn more about extensions.
=head2 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
+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
L<perlop>.
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.
+Although it has the same precedence as in C, Perl's C<?:> operator
+produces an lvalue. This assigns $x to either $a or $b, depending
+on the trueness of $maybe:
+
+ ($maybe ? $a : $b) = $x;
+
=head2 How do I declare/create a structure?
In general, you don't "declare" a structure. Just use a (probably
=head2 How do I create a module?
-A module is a package that lives in a file of the same name. For
-example, the Hello::There module would live in Hello/There.pm. For
-details, read L<perlmod>. You'll also find L<Exporter> helpful. If
-you're writing a C or mixed-language module with both C and Perl, then
-you should study L<perlxstut>.
-
-Here's a convenient template you might wish you use when starting your
-own module. Make sure to change the names appropriately.
-
- package Some::Module; # assumes Some/Module.pm
-
- use strict;
-
- BEGIN {
- use Exporter ();
- use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS);
-
- ## 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: 1.18 $=~/\d+/g;sprintf '%d.'.'%02d'x$#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);
- }
- use vars @EXPORT_OK;
-
- # non-exported package globals go here
- use vars qw( @more $stuff );
-
- # initialize package globals, first exported ones
- $Var1 = '';
- %Hashit = ();
-
- # then the others (which are still accessible as $Some::Module::stuff)
- $stuff = '';
- @more = ();
-
- # all file-scoped lexicals must be created before
- # the functions below that use them.
-
- # file-private lexicals go here
- my $priv_var = '';
- my %secret_hash = ();
+(contributed by brian d foy)
- # here's a file-private function as a closure,
- # callable as &$priv_func; it cannot be prototyped.
- my $priv_func = sub {
- # stuff goes here.
- };
+L<perlmod>, L<perlmodlib>, L<perlmodstyle> explain modules
+in all the gory details. L<perlnewmod> gives a a brief
+overview of the process along with a couple of suggestions
+about style.
- # 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
+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.
- # this one isn't exported, but could be called!
- sub func4(\%) {} # proto'd to 1 hash ref
+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.
- END { } # module clean-up code here (global destructor)
-
- 1; # modules must return true
+You may also want to see Sam Tregar's "Writing Perl Modules
+for CPAN" ( http://apress.com/book/bookDisplay.html?bID=14 )
+which is the best hands-on guide to creating module
+distributions.
=head2 How do I create a class?
=head2 How can I tell if a variable is tainted?
-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):
-
- sub is_tainted {
- return ! eval { join('',@_), kill 0; 1; };
- }
-
-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 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
+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:
sub add_function_generator {
- return sub { shift + shift };
+ return sub { shift() + shift() };
}
$add_sub = add_function_generator();
- $sum = &$add_sub(4,5); # $sum is 9 now.
+ $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
sub make_adder {
my $addpiece = shift;
- return sub { shift + $addpiece };
+ return sub { shift() + $addpiece };
}
$f1 = make_adder(20);
my $line;
timeout( 30, sub { $line = <STDIN> } );
-If the code to execute had been passed in as a string, C<'$line =
-E<lt>STDINE<gt>'>, there would have been no way for the hypothetical
-timeout() function to access the lexical variable $line back in its
-caller's scope.
+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.
=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() interator
+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:
print "Finally $f\n";
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 is a bug, and will be fixed.
+(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).
-=head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regexp}?
+=head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
-With the exception of regexps, you need to pass references to these
+With the exception of regexes, you need to pass references to these
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.
+
=over 4
=item Passing Variables and Functions
-Regular variables and functions are quite easy: just pass in a
+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( \@some_array );
func( [ 1 .. 10 ] );
func( \%some_hash );
=item Passing Filehandles
-To create filehandles you can pass to subroutines, you can use C<*FH>
-or C<\*FH> notation ("typeglobs" - see L<perldata> for more information),
-or create filehandles dynamically using the old FileHandle or the new
-IO::File modules, both part of the standard Perl distribution.
+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 = <$fh>;
+ }
- use Fcntl;
- use IO::File;
- my $fh = new IO::File $filename, O_WRONLY|O_APPEND;
- or die "Can't append to $filename: $!";
- func($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 Regexps
+=item Passing Regexes
-To pass regexps around, you'll need to either use one of the highly
-experimental regular expression modules from CPAN (Nick Ing-Simmons's
-Regexp or Ilya Zakharevich's Devel::Regexp), pass around strings
-and use an exception-trapping eval, or else be be very, very clever.
-Here's an example of how to pass in a string to be regexp compared:
+To pass regexes around, you'll need to be using a release of Perl
+sufficiently recent as to support the C<qr//> construct, pass around
+strings and use an exception-trapping eval, or else be very, very clever.
+
+Here's an example of how to pass in a string to be regex compared
+using C<qr//>:
sub compare($$) {
- my ($val1, $regexp) = @_;
- my $retval = eval { $val =~ /$regexp/ };
+ 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/d.*D/);
+ $match = compare("old McDonald", q/($?i)d.*D/);
Make sure you never say something like this:
- return eval "\$val =~ /$regexp/"; # WRONG
+ return eval "\$val =~ /$regex/"; # WRONG
-or someone can sneak shell escapes into the regexp due to the double
+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';
}
}
-or you can use a closure to bundle up the object and its method call
-and arguments:
+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);
=head2 How do I create a static variable?
-As with most things in Perl, TMTOWTDI. What is a "static variable" in
-other languages could be either a function-private variable (visible
-only within a single function, retaining its value between calls to
-that function), or a file-private variable (visible only to functions
-within the file it was declared in) in Perl.
-
-Here's code to implement a function-private variable:
+(contributed by brian d foy)
+
+Perl doesn't have "static" variables, which can only be accessed from
+the function in which they are declared. You can get the same effect
+with lexical variables, though.
+
+You can fake a static variable by using a lexical variable which goes
+of scope. In this example, you define the subroutine C<counter>, and
+it uses the lexical variable C<$count>. Since you wrap this in a BEGIN
+block, C<$count> is defined at compile-time, but also goes out of
+scope at the end of the BEGIN block. The BEGIN block also ensures that
+the subroutine and the value it uses is defined at compile-time so the
+subroutine is ready to use just like any other subroutine, and you can
+put this code in the same place as other subroutines in the program
+text (i.e. at the end of the code, typically). The subroutine
+C<counter> still has a reference to the data, and is the only way you
+can access the value (and each time you do, you increment the value).
+The data in chunk of memory defined by C<$count> is private to
+C<counter>.
BEGIN {
- my $counter = 42;
- sub prev_counter { return --$counter }
- sub next_counter { return $counter++ }
+ my $count = 1;
+ sub counter { $count++ }
}
-Now prev_counter() and next_counter() share a private variable $counter
-that was initialized at compile time.
+ my $start = count();
-To declare a file-private variable, you'll still use a my(), putting
-it at the outer scope level at the top of the file. Assume this is in
-file Pax.pm:
+ .... # code that calls count();
- package Pax;
- my $started = scalar(localtime(time()));
+ my $end = count();
- sub begun { return $started }
+In the previous example, you created a function-private variable
+because only one function remembered its reference. You could define
+multiple functions while the variable is in scope, and each function
+can share the "private" variable. It's not really "static" because you
+can access it outside the function while the lexical variable is in
+scope, and even create references to it. In this example,
+C<increment_count> and C<return_count> share the variable. One
+function adds to the value and the other simply returns the value.
+They can both access C<$count>, and since it has gone out of scope,
+there is no other way to access it.
-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.
+ 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 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()?
-C<local($x)> saves away the old value of the global variable C<$x>,
-and assigns a new value for the duration of the subroutine, I<which is
+C<local($x)> saves away the old value of the global variable C<$x>
+and assigns a new value for the duration of the subroutine I<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.
C<my($x)> creates a new variable that is only visible in the current
-subroutine. This is done at compile-time, so is called lexical or
+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 (improperly) static(ly scoped) variables.
variables. It gives a global variable a temporary value. my() is
what you're looking for if you want private variables.
-See also L<perlsub>, which explains this all in more detail.
+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 "local($foo) = <FILE>;" work right?
+=head2 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?
-C<local()> gives list context to the right hand side of C<=>. The
-E<lt>FHE<gt> 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()).
+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
+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:
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 L<perlsub/"Overriding Builtin Functions">. There's
+module. See L<perlsub/"Overriding Built-in Functions">. There's
also an example in L<perltoot/"Class::Template">.
If you want to overload a Perl operator, such as C<+> or C<**>,
=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 by-pass prototypes. That means that
-the function doesn't get an empty @_, it gets yours! While not
+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.
-When you call your function as C<&foo()>, then you do get a new @_,
+When you call your function as C<&foo()>, then you I<do> get a new @_,
but prototyping is still circumvented.
Normally, you want to call a function using C<foo()>. You may only
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,
-regexp 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.
+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.
+
+Starting from Perl 5.8 to get switch and case one can use the
+Switch extension and say:
+
+ use Switch;
+
+after which one has switch and case. It is not as fast as it could be
+because it's not really part of the language (it's done using source
+filters) but it is available, and it's very flexible.
+
+But if one wants to use pure Perl, the general answer is to write a
+construct like this:
-Here's a simple example of a switch based on pattern matching. We'll
-do a multi-way conditional based on the type of reference stored in
-$whatchamacallit:
+ 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 multiway conditional based on the type of reference stored
+in $whatchamacallit:
- SWITCH:
- for (ref $whatchamacallit) {
+ SWITCH: for (ref $whatchamacallit) {
/^$/ && die "not a reference";
}
-=head2 How can I catch accesses to undefined variables/functions/methods?
+See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many 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 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,
+ );
+
+ print "How are you? ";
+ chomp($string = <STDIN>);
+ if ($commands{$string}) {
+ $commands{$string}->();
+ } else {
+ print "No such command: $string\n";
+ }
+
+=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 {
-
- for ( $_[0] ) {
+under C<use warnings>, you can promote the warning to an error.
- /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?
Some possible reasons: your inheritance is getting confused, you've
misspelled the method name, or the object is of the wrong type. Check
-out L<perltoot> for details on these. You may also use C<print
-ref($object)> to find out the class C<$object> was blessed into.
+out L<perltoot> for details about any of the above cases. You may
+also use C<print ref($object)> to find out the class C<$object> was
+blessed into.
Another possible reason for problems is because you've used the
indirect object syntax (eg, C<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 C<use> statement instead of
-C<require>. If not, make sure to use arrow notation (eg,
-C<Guru-E<gt>find("Samy")>) instead. Object notation is explained in
+C<require>. If not, make sure to use arrow notation (eg.,
+C<< Guru->find("Samy") >>) instead. Object notation is explained in
L<perlobj>.
+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?
If you're just a random program, you can do this to find
out what the currently compiled package is:
- my $packname = ref bless [];
+ my $packname = __PACKAGE__;
-But if you're a method and you want to print an error message
+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):
sub amethod {
- my $self = shift;
+ my $self = shift;
my $class = ref($self) || $self;
warn "called me from a $class object";
}
=head2 How can I comment out a large block of perl code?
-Use embedded POD to discard it:
+You can use embedded POD to discard it. Enclose the blocks you want
+to comment out in POD markers. The <=begin> directive marks a section
+for a specific formatter. Use the C<comment> format, which no formatter
+should claim to understand (by policy). Mark the end of the block
+with <=end>.
# program is here
- =for nobody
- This paragraph is commented out
-
- # program continues
-
- =begin comment text
+ =begin comment
all of this stuff
here will be ignored
by everyone
- =end comment text
+ =end comment
=cut
+ # program continues
+
+The pod directives cannot go just anywhere. You must put a
+pod directive where the parser is expecting a new statement,
+not just in the middle of an expression or some other
+arbitrary grammar production.
+
+See L<perlpod> for more details.
+
+=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;
+ }
+ }
+
+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";
+ ++$$varname; # $fred now 24
+
+This works I<sometimes>, but it is a very bad idea for two reasons.
+
+The first reason is that this technique I<only works on global
+variables>. That means that if $fred is a lexical variable created
+with my() in the above example, the code wouldn't work at all: you'd
+accidentally access the global and skip right over the private lexical
+altogether. Global variables are bad because they can easily collide
+accidentally and in general make for non-scalable and confusing code.
+
+Symbolic references are forbidden under the C<use strict> pragma.
+They are not true references and consequently are not reference counted
+or garbage collected.
+
+The other reason why using a variable to hold the name of another
+variable is a bad idea is that the question often stems from a lack of
+understanding of Perl data structures, particularly hashes. By using
+symbolic references, you are just using the package's symbol-table hash
+(like C<%main::>) instead of a user-defined hash. The solution is to
+use your own hash or a real reference instead.
+
+ $USER_VARS{"fred"} = 23;
+ $varname = "fred";
+ $USER_VARS{$varname}++; # not $$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 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 I<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 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>" };
+ }
+
+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.
+
+=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 Tom Christiansen and Nathan Torkington.
-All rights reserved. See L<perlfaq> for distribution information.
+Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
+other authors as noted. All rights reserved.
+
+This documentation is free; you can redistribute it and/or modify it
+under the same terms as Perl itself.
+
+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 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.
+
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