3 perlfaq7 - Perl Language Issues ($Revision: 1.3 $, $Date: 2001/10/19 14:39:24 $)
7 This section deals with general Perl language issues that don't
8 clearly fit into any of the other sections.
10 =head2 Can I get a BNF/yacc/RE for the Perl language?
12 There is no BNF, but you can paw your way through the yacc grammar in
13 perly.y in the source distribution if you're particularly brave. The
14 grammar relies on very smart tokenizing code, so be prepared to
15 venture into toke.c as well.
17 In the words of Chaim Frenkel: "Perl's grammar can not be reduced to BNF.
18 The work of parsing perl is distributed between yacc, the lexer, smoke
21 =head2 What are all these $@%&* punctuation signs, and how do I know when to use them?
23 They are type specifiers, as detailed in L<perldata>:
25 $ for scalar values (number, string or reference)
27 % for hashes (associative arrays)
28 & for subroutines (aka functions, procedures, methods)
29 * for all types of that symbol name. In version 4 you used them like
30 pointers, but in modern perls you can just use references.
32 There are couple of other symbols that you're likely to encounter that aren't
33 really type specifiers:
35 <> are used for inputting a record from a filehandle.
36 \ takes a reference to something.
38 Note that <FILE> is I<neither> the type specifier for files
39 nor the name of the handle. It is the C<< <> >> operator applied
40 to the handle FILE. It reads one line (well, record--see
41 L<perlvar/$/>) from the handle FILE in scalar context, or I<all> lines
42 in list context. When performing open, close, or any other operation
43 besides C<< <> >> on files, or even when talking about the handle, do
44 I<not> use the brackets. These are correct: C<eof(FH)>, C<seek(FH, 0,
45 2)> and "copying from STDIN to FILE".
47 =head2 Do I always/never have to quote my strings or use semicolons and commas?
49 Normally, a bareword doesn't need to be quoted, but in most cases
50 probably should be (and must be under C<use strict>). But a hash key
51 consisting of a simple word (that isn't the name of a defined
52 subroutine) and the left-hand operand to the C<< => >> operator both
53 count as though they were quoted:
56 ------------ ---------------
57 $foo{line} $foo{"line"}
58 bar => stuff "bar" => stuff
60 The final semicolon in a block is optional, as is the final comma in a
61 list. Good style (see L<perlstyle>) says to put them in except for
64 if ($whoops) { exit 1 }
71 "There Beren came from mountains cold",
72 "And lost he wandered under leaves",
75 =head2 How do I skip some return values?
77 One way is to treat the return values as a list and index into it:
79 $dir = (getpwnam($user))[7];
81 Another way is to use undef as an element on the left-hand-side:
83 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
85 =head2 How do I temporarily block warnings?
87 If you are running Perl 5.6.0 or better, the C<use warnings> pragma
88 allows fine control of what warning are produced.
89 See L<perllexwarn> for more details.
92 no warnings; # temporarily turn off warnings
93 $a = $b + $c; # I know these might be undef
96 If you have an older version of Perl, the C<$^W> variable (documented
97 in L<perlvar>) controls runtime warnings for a block:
100 local $^W = 0; # temporarily turn off warnings
101 $a = $b + $c; # I know these might be undef
104 Note that like all the punctuation variables, you cannot currently
105 use my() on C<$^W>, only local().
107 =head2 What's an extension?
109 An extension is a way of calling compiled C code from Perl. Reading
110 L<perlxstut> is a good place to learn more about extensions.
112 =head2 Why do Perl operators have different precedence than C operators?
114 Actually, they don't. All C operators that Perl copies have the same
115 precedence in Perl as they do in C. The problem is with operators that C
116 doesn't have, especially functions that give a list context to everything
117 on their right, eg. print, chmod, exec, and so on. Such functions are
118 called "list operators" and appear as such in the precedence table in
121 A common mistake is to write:
123 unlink $file || die "snafu";
125 This gets interpreted as:
127 unlink ($file || die "snafu");
129 To avoid this problem, either put in extra parentheses or use the
130 super low precedence C<or> operator:
132 (unlink $file) || die "snafu";
133 unlink $file or die "snafu";
135 The "English" operators (C<and>, C<or>, C<xor>, and C<not>)
136 deliberately have precedence lower than that of list operators for
137 just such situations as the one above.
139 Another operator with surprising precedence is exponentiation. It
140 binds more tightly even than unary minus, making C<-2**2> product a
141 negative not a positive four. It is also right-associating, meaning
142 that C<2**3**2> is two raised to the ninth power, not eight squared.
144 Although it has the same precedence as in C, Perl's C<?:> operator
145 produces an lvalue. This assigns $x to either $a or $b, depending
146 on the trueness of $maybe:
148 ($maybe ? $a : $b) = $x;
150 =head2 How do I declare/create a structure?
152 In general, you don't "declare" a structure. Just use a (probably
153 anonymous) hash reference. See L<perlref> and L<perldsc> for details.
156 $person = {}; # new anonymous hash
157 $person->{AGE} = 24; # set field AGE to 24
158 $person->{NAME} = "Nat"; # set field NAME to "Nat"
160 If you're looking for something a bit more rigorous, try L<perltoot>.
162 =head2 How do I create a module?
164 A module is a package that lives in a file of the same name. For
165 example, the Hello::There module would live in Hello/There.pm. For
166 details, read L<perlmod>. You'll also find L<Exporter> helpful. If
167 you're writing a C or mixed-language module with both C and Perl, then
168 you should study L<perlxstut>.
170 The C<h2xs> program will create stubs for all the important stuff for you:
172 % h2xs -XA -n My::Module
174 The C<-X> switch tells C<h2xs> that you are not using C<XS> extension
175 code. The C<-A> switch tells C<h2xs> that you are not using the
176 AutoLoader, and the C<-n> switch specifies the name of the module.
177 See L<h2xs> for more details.
179 =head2 How do I create a class?
181 See L<perltoot> for an introduction to classes and objects, as well as
182 L<perlobj> and L<perlbot>.
184 =head2 How can I tell if a variable is tainted?
186 See L<perlsec/"Laundering and Detecting Tainted Data">. Here's an
187 example (which doesn't use any system calls, because the kill()
188 is given no processes to signal):
191 return ! eval { join('',@_), kill 0; 1; };
194 This is not C<-w> clean, however. There is no C<-w> clean way to
195 detect taintedness--take this as a hint that you should untaint
196 all possibly-tainted data.
198 =head2 What's a closure?
200 Closures are documented in L<perlref>.
202 I<Closure> is a computer science term with a precise but
203 hard-to-explain meaning. Closures are implemented in Perl as anonymous
204 subroutines with lasting references to lexical variables outside their
205 own scopes. These lexicals magically refer to the variables that were
206 around when the subroutine was defined (deep binding).
208 Closures make sense in any programming language where you can have the
209 return value of a function be itself a function, as you can in Perl.
210 Note that some languages provide anonymous functions but are not
211 capable of providing proper closures: the Python language, for
212 example. For more information on closures, check out any textbook on
213 functional programming. Scheme is a language that not only supports
214 but encourages closures.
216 Here's a classic function-generating function:
218 sub add_function_generator {
219 return sub { shift + shift };
222 $add_sub = add_function_generator();
223 $sum = $add_sub->(4,5); # $sum is 9 now.
225 The closure works as a I<function template> with some customization
226 slots left out to be filled later. The anonymous subroutine returned
227 by add_function_generator() isn't technically a closure because it
228 refers to no lexicals outside its own scope.
230 Contrast this with the following make_adder() function, in which the
231 returned anonymous function contains a reference to a lexical variable
232 outside the scope of that function itself. Such a reference requires
233 that Perl return a proper closure, thus locking in for all time the
234 value that the lexical had when the function was created.
237 my $addpiece = shift;
238 return sub { shift + $addpiece };
241 $f1 = make_adder(20);
242 $f2 = make_adder(555);
244 Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas
245 C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece
246 in the closure sticks around.
248 Closures are often used for less esoteric purposes. For example, when
249 you want to pass in a bit of code into a function:
252 timeout( 30, sub { $line = <STDIN> } );
254 If the code to execute had been passed in as a string,
255 C<< '$line = <STDIN>' >>, there would have been no way for the
256 hypothetical timeout() function to access the lexical variable
257 $line back in its caller's scope.
259 =head2 What is variable suicide and how can I prevent it?
261 Variable suicide is when you (temporarily or permanently) lose the
262 value of a variable. It is caused by scoping through my() and local()
263 interacting with either closures or aliased foreach() iterator
264 variables and subroutine arguments. It used to be easy to
265 inadvertently lose a variable's value this way, but now it's much
266 harder. Take this code:
270 while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
273 print "Finally $f\n";
275 The $f that has "bar" added to it three times should be a new C<$f>
276 (C<my $f> should create a new local variable each time through the loop).
277 It isn't, however. This was a bug, now fixed in the latest releases
278 (tested against 5.004_05, 5.005_03, and 5.005_56).
280 =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
282 With the exception of regexes, you need to pass references to these
283 objects. See L<perlsub/"Pass by Reference"> for this particular
284 question, and L<perlref> for information on references.
286 See ``Passing Regexes'', below, for information on passing regular
291 =item Passing Variables and Functions
293 Regular variables and functions are quite easy to pass: just pass in a
294 reference to an existing or anonymous variable or function:
296 func( \$some_scalar );
298 func( \@some_array );
302 func( { this => 10, that => 20 } );
305 func( sub { $_[0] ** $_[1] } );
307 =item Passing Filehandles
309 To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations.
310 These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles">
311 and especially L<perlsub/"Pass by Reference"> for more information.
315 If you're passing around filehandles, you could usually just use the bare
316 typeglob, like *STDOUT, but typeglobs references would be better because
317 they'll still work properly under C<use strict 'refs'>. For example:
322 print $fh "her um well a hmmm\n";
325 $rec = get_rec(\*STDIN);
331 If you're planning on generating new filehandles, you could do this:
336 return open (FH, $path) ? *FH : undef;
338 $fh = openit('< /etc/motd');
341 =item Passing Regexes
343 To pass regexes around, you'll need to be using a release of Perl
344 sufficiently recent as to support the C<qr//> construct, pass around
345 strings and use an exception-trapping eval, or else be very, very clever.
347 Here's an example of how to pass in a string to be regex compared
351 my ($val1, $regex) = @_;
352 my $retval = $val1 =~ /$regex/;
355 $match = compare("old McDonald", qr/d.*D/i);
357 Notice how C<qr//> allows flags at the end. That pattern was compiled
358 at compile time, although it was executed later. The nifty C<qr//>
359 notation wasn't introduced until the 5.005 release. Before that, you
360 had to approach this problem much less intuitively. For example, here
361 it is again if you don't have C<qr//>:
364 my ($val1, $regex) = @_;
365 my $retval = eval { $val1 =~ /$regex/ };
370 $match = compare("old McDonald", q/($?i)d.*D/);
372 Make sure you never say something like this:
374 return eval "\$val =~ /$regex/"; # WRONG
376 or someone can sneak shell escapes into the regex due to the double
377 interpolation of the eval and the double-quoted string. For example:
379 $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
381 eval "\$string =~ /$pattern_of_evil/";
383 Those preferring to be very, very clever might see the O'Reilly book,
384 I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's
385 Build_MatchMany_Function() is particularly interesting. A complete
386 citation of this book is given in L<perlfaq2>.
388 =item Passing Methods
390 To pass an object method into a subroutine, you can do this:
392 call_a_lot(10, $some_obj, "methname")
394 my ($count, $widget, $trick) = @_;
395 for (my $i = 0; $i < $count; $i++) {
400 Or, you can use a closure to bundle up the object, its
401 method call, and arguments:
403 my $whatnot = sub { $some_obj->obfuscate(@args) };
410 You could also investigate the can() method in the UNIVERSAL class
411 (part of the standard perl distribution).
415 =head2 How do I create a static variable?
417 As with most things in Perl, TMTOWTDI. What is a "static variable" in
418 other languages could be either a function-private variable (visible
419 only within a single function, retaining its value between calls to
420 that function), or a file-private variable (visible only to functions
421 within the file it was declared in) in Perl.
423 Here's code to implement a function-private variable:
427 sub prev_counter { return --$counter }
428 sub next_counter { return $counter++ }
431 Now prev_counter() and next_counter() share a private variable $counter
432 that was initialized at compile time.
434 To declare a file-private variable, you'll still use a my(), putting
435 the declaration at the outer scope level at the top of the file.
436 Assume this is in file Pax.pm:
439 my $started = scalar(localtime(time()));
441 sub begun { return $started }
443 When C<use Pax> or C<require Pax> loads this module, the variable will
444 be initialized. It won't get garbage-collected the way most variables
445 going out of scope do, because the begun() function cares about it,
446 but no one else can get it. It is not called $Pax::started because
447 its scope is unrelated to the package. It's scoped to the file. You
448 could conceivably have several packages in that same file all
449 accessing the same private variable, but another file with the same
450 package couldn't get to it.
452 See L<perlsub/"Persistent Private Variables"> for details.
454 =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
456 C<local($x)> saves away the old value of the global variable C<$x>
457 and assigns a new value for the duration of the subroutine I<which is
458 visible in other functions called from that subroutine>. This is done
459 at run-time, so is called dynamic scoping. local() always affects global
460 variables, also called package variables or dynamic variables.
462 C<my($x)> creates a new variable that is only visible in the current
463 subroutine. This is done at compile-time, so it is called lexical or
464 static scoping. my() always affects private variables, also called
465 lexical variables or (improperly) static(ly scoped) variables.
470 print "var has value $var\n";
474 local $var = 'local'; # new temporary value for the still-global
475 visible(); # variable called $var
479 my $var = 'private'; # new private variable, $var
480 visible(); # (invisible outside of sub scope)
485 visible(); # prints global
486 dynamic(); # prints local
487 lexical(); # prints global
489 Notice how at no point does the value "private" get printed. That's
490 because $var only has that value within the block of the lexical()
491 function, and it is hidden from called subroutine.
493 In summary, local() doesn't make what you think of as private, local
494 variables. It gives a global variable a temporary value. my() is
495 what you're looking for if you want private variables.
497 See L<perlsub/"Private Variables via my()"> and
498 L<perlsub/"Temporary Values via local()"> for excruciating details.
500 =head2 How can I access a dynamic variable while a similarly named lexical is in scope?
502 You can do this via symbolic references, provided you haven't set
503 C<use strict "refs">. So instead of $var, use C<${'var'}>.
505 local $var = "global";
508 print "lexical is $var\n";
511 print "global is ${'var'}\n";
513 If you know your package, you can just mention it explicitly, as in
514 $Some_Pack::var. Note that the notation $::var is I<not> the dynamic
515 $var in the current package, but rather the one in the C<main>
516 package, as though you had written $main::var. Specifying the package
517 directly makes you hard-code its name, but it executes faster and
518 avoids running afoul of C<use strict "refs">.
520 =head2 What's the difference between deep and shallow binding?
522 In deep binding, lexical variables mentioned in anonymous subroutines
523 are the same ones that were in scope when the subroutine was created.
524 In shallow binding, they are whichever variables with the same names
525 happen to be in scope when the subroutine is called. Perl always uses
526 deep binding of lexical variables (i.e., those created with my()).
527 However, dynamic variables (aka global, local, or package variables)
528 are effectively shallowly bound. Consider this just one more reason
529 not to use them. See the answer to L<"What's a closure?">.
531 =head2 Why doesn't "my($foo) = <FILE>;" work right?
533 C<my()> and C<local()> give list context to the right hand side
534 of C<=>. The <FH> read operation, like so many of Perl's
535 functions and operators, can tell which context it was called in and
536 behaves appropriately. In general, the scalar() function can help.
537 This function does nothing to the data itself (contrary to popular myth)
538 but rather tells its argument to behave in whatever its scalar fashion is.
539 If that function doesn't have a defined scalar behavior, this of course
540 doesn't help you (such as with sort()).
542 To enforce scalar context in this particular case, however, you need
543 merely omit the parentheses:
545 local($foo) = <FILE>; # WRONG
546 local($foo) = scalar(<FILE>); # ok
547 local $foo = <FILE>; # right
549 You should probably be using lexical variables anyway, although the
550 issue is the same here:
552 my($foo) = <FILE>; # WRONG
553 my $foo = <FILE>; # right
555 =head2 How do I redefine a builtin function, operator, or method?
557 Why do you want to do that? :-)
559 If you want to override a predefined function, such as open(),
560 then you'll have to import the new definition from a different
561 module. See L<perlsub/"Overriding Built-in Functions">. There's
562 also an example in L<perltoot/"Class::Template">.
564 If you want to overload a Perl operator, such as C<+> or C<**>,
565 then you'll want to use the C<use overload> pragma, documented
568 If you're talking about obscuring method calls in parent classes,
569 see L<perltoot/"Overridden Methods">.
571 =head2 What's the difference between calling a function as &foo and foo()?
573 When you call a function as C<&foo>, you allow that function access to
574 your current @_ values, and you bypass prototypes.
575 The function doesn't get an empty @_--it gets yours! While not
576 strictly speaking a bug (it's documented that way in L<perlsub>), it
577 would be hard to consider this a feature in most cases.
579 When you call your function as C<&foo()>, then you I<do> get a new @_,
580 but prototyping is still circumvented.
582 Normally, you want to call a function using C<foo()>. You may only
583 omit the parentheses if the function is already known to the compiler
584 because it already saw the definition (C<use> but not C<require>),
585 or via a forward reference or C<use subs> declaration. Even in this
586 case, you get a clean @_ without any of the old values leaking through
587 where they don't belong.
589 =head2 How do I create a switch or case statement?
591 This is explained in more depth in the L<perlsyn>. Briefly, there's
592 no official case statement, because of the variety of tests possible
593 in Perl (numeric comparison, string comparison, glob comparison,
594 regex matching, overloaded comparisons, ...).
595 Larry couldn't decide how best to do this, so he left it out, even
596 though it's been on the wish list since perl1.
598 Starting from Perl 5.8 to get switch and case one can use the
599 Switch extension and say:
603 after which one has switch and case. It is not as fast as it could be
604 because it's not really part of the language (it's done using source
605 filters) but it is available, and it's very flexible.
607 But if one wants to use pure Perl, the general answer is to write a
610 for ($variable_to_test) {
611 if (/pat1/) { } # do something
612 elsif (/pat2/) { } # do something else
613 elsif (/pat3/) { } # do something else
617 Here's a simple example of a switch based on pattern matching, this
618 time lined up in a way to make it look more like a switch statement.
619 We'll do a multiway conditional based on the type of reference stored
622 SWITCH: for (ref $whatchamacallit) {
624 /^$/ && die "not a reference";
642 warn "can't print function ref";
648 warn "User defined type skipped";
652 See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other
653 examples in this style.
655 Sometimes you should change the positions of the constant and the variable.
656 For example, let's say you wanted to test which of many answers you were
657 given, but in a case-insensitive way that also allows abbreviations.
658 You can use the following technique if the strings all start with
659 different characters or if you want to arrange the matches so that
660 one takes precedence over another, as C<"SEND"> has precedence over
664 if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
665 elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
666 elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
667 elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
668 elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
670 A totally different approach is to create a hash of function references.
675 "done" => sub { die "See ya!" },
679 print "How are you? ";
680 chomp($string = <STDIN>);
681 if ($commands{$string}) {
682 $commands{$string}->();
684 print "No such command: $string\n";
687 =head2 How can I catch accesses to undefined variables/functions/methods?
689 The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and
690 L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to
691 undefined functions and methods.
693 When it comes to undefined variables that would trigger a warning
694 under C<-w>, you can use a handler to trap the pseudo-signal
695 C<__WARN__> like this:
697 $SIG{__WARN__} = sub {
699 for ( $_[0] ) { # voici un switch statement
701 /Use of uninitialized value/ && do {
702 # promote warning to a fatal
706 # other warning cases to catch could go here;
713 =head2 Why can't a method included in this same file be found?
715 Some possible reasons: your inheritance is getting confused, you've
716 misspelled the method name, or the object is of the wrong type. Check
717 out L<perltoot> for details about any of the above cases. You may
718 also use C<print ref($object)> to find out the class C<$object> was
721 Another possible reason for problems is because you've used the
722 indirect object syntax (eg, C<find Guru "Samy">) on a class name
723 before Perl has seen that such a package exists. It's wisest to make
724 sure your packages are all defined before you start using them, which
725 will be taken care of if you use the C<use> statement instead of
726 C<require>. If not, make sure to use arrow notation (eg.,
727 C<< Guru->find("Samy") >>) instead. Object notation is explained in
730 Make sure to read about creating modules in L<perlmod> and
731 the perils of indirect objects in L<perlobj/"Method Invocation">.
733 =head2 How can I find out my current package?
735 If you're just a random program, you can do this to find
736 out what the currently compiled package is:
738 my $packname = __PACKAGE__;
740 But, if you're a method and you want to print an error message
741 that includes the kind of object you were called on (which is
742 not necessarily the same as the one in which you were compiled):
746 my $class = ref($self) || $self;
747 warn "called me from a $class object";
750 =head2 How can I comment out a large block of perl code?
752 Use embedded POD to discard it:
757 This paragraph is commented out
772 This can't go just anywhere. You have to put a pod directive where
773 the parser is expecting a new statement, not just in the middle
774 of an expression or some other arbitrary yacc grammar production.
776 =head2 How do I clear a package?
778 Use this code, provided by Mark-Jason Dominus:
783 die "Shouldn't delete main package"
784 if $pack eq "" || $pack eq "main";
785 my $stash = *{$pack . '::'}{HASH};
787 foreach $name (keys %$stash) {
788 my $fullname = $pack . '::' . $name;
789 # Get rid of everything with that name.
798 Or, if you're using a recent release of Perl, you can
799 just use the Symbol::delete_package() function instead.
801 =head2 How can I use a variable as a variable name?
803 Beginners often think they want to have a variable contain the name
808 ++$$varname; # $fred now 24
810 This works I<sometimes>, but it is a very bad idea for two reasons.
812 The first reason is that this technique I<only works on global
813 variables>. That means that if $fred is a lexical variable created
814 with my() in the above example, the code wouldn't work at all: you'd
815 accidentally access the global and skip right over the private lexical
816 altogether. Global variables are bad because they can easily collide
817 accidentally and in general make for non-scalable and confusing code.
819 Symbolic references are forbidden under the C<use strict> pragma.
820 They are not true references and consequently are not reference counted
821 or garbage collected.
823 The other reason why using a variable to hold the name of another
824 variable is a bad idea is that the question often stems from a lack of
825 understanding of Perl data structures, particularly hashes. By using
826 symbolic references, you are just using the package's symbol-table hash
827 (like C<%main::>) instead of a user-defined hash. The solution is to
828 use your own hash or a real reference instead.
832 $USER_VARS{$varname}++; # not $$varname++
834 There we're using the %USER_VARS hash instead of symbolic references.
835 Sometimes this comes up in reading strings from the user with variable
836 references and wanting to expand them to the values of your perl
837 program's variables. This is also a bad idea because it conflates the
838 program-addressable namespace and the user-addressable one. Instead of
839 reading a string and expanding it to the actual contents of your program's
842 $str = 'this has a $fred and $barney in it';
843 $str =~ s/(\$\w+)/$1/eeg; # need double eval
845 it would be better to keep a hash around like %USER_VARS and have
846 variable references actually refer to entries in that hash:
848 $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
850 That's faster, cleaner, and safer than the previous approach. Of course,
851 you don't need to use a dollar sign. You could use your own scheme to
852 make it less confusing, like bracketed percent symbols, etc.
854 $str = 'this has a %fred% and %barney% in it';
855 $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
857 Another reason that folks sometimes think they want a variable to
858 contain the name of a variable is because they don't know how to build
859 proper data structures using hashes. For example, let's say they
860 wanted two hashes in their program: %fred and %barney, and that they
861 wanted to use another scalar variable to refer to those by name.
864 $$name{WIFE} = "wilma"; # set %fred
867 $$name{WIFE} = "betty"; # set %barney
869 This is still a symbolic reference, and is still saddled with the
870 problems enumerated above. It would be far better to write:
872 $folks{"fred"}{WIFE} = "wilma";
873 $folks{"barney"}{WIFE} = "betty";
875 And just use a multilevel hash to start with.
877 The only times that you absolutely I<must> use symbolic references are
878 when you really must refer to the symbol table. This may be because it's
879 something that can't take a real reference to, such as a format name.
880 Doing so may also be important for method calls, since these always go
881 through the symbol table for resolution.
883 In those cases, you would turn off C<strict 'refs'> temporarily so you
884 can play around with the symbol table. For example:
886 @colors = qw(red blue green yellow orange purple violet);
887 for my $name (@colors) {
888 no strict 'refs'; # renege for the block
889 *$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
892 All those functions (red(), blue(), green(), etc.) appear to be separate,
893 but the real code in the closure actually was compiled only once.
895 So, sometimes you might want to use symbolic references to directly
896 manipulate the symbol table. This doesn't matter for formats, handles, and
897 subroutines, because they are always global--you can't use my() on them.
898 For scalars, arrays, and hashes, though--and usually for subroutines--
899 you probably only want to use hard references.
901 =head1 AUTHOR AND COPYRIGHT
903 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
906 This documentation is free; you can redistribute it and/or modify it
907 under the same terms as Perl itself.
909 Irrespective of its distribution, all code examples in this file
910 are hereby placed into the public domain. You are permitted and
911 encouraged to use this code in your own programs for fun
912 or for profit as you see fit. A simple comment in the code giving
913 credit would be courteous but is not required.