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perlfaq7 ()
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    NAME

         perlfaq7 - Perl Language Issues ($Revision: 1.28 $, $Date:
         1999/05/23 20:36:18 $)
    
    
    

    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 the perldata
         manpage:
    
             $ 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.
    
         A couple of others that you're likely to encounter that
         aren't really type specifiers are:
    
             <> 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 the
         section on "$/" in the perlvar manpage) 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 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:
    
             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 the perlstyle manpage)
         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);
    
    
         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 the perllexwarn manpage for more details.
    
    
    
             {
                 no warnings;          # temporarily turn off warnings
                 $a = $b + $c;         # I know these might be undef
             }
    
         If you have an older version of Perl, the `$^W' variable
         (documented in the perlvar manpage) 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?
    
         A way of calling compiled C code from Perl.  Reading the
         perlxstut manpage 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 the perlop manpage.
    
         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
         minus, making `-2**2' product 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 the perlref
         manpage and the perldsc manpage 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 the
         perltoot manpage.
    
         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 the perlmod manpage.
         You'll also find the Exporter manpage helpful.  If you're
         writing a C or mixed-language module with both C and Perl,
         then you should study the perlxstut manpage.
    
         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;
             use warnings;
    
             BEGIN {
                 use Exporter   ();
                 our ($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: 1.28 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r};
    
                 @ISA         = qw(Exporter);
                 @EXPORT      = qw(&func1 &func2 &func3);
                 %EXPORT_TAGS = ( );     # eg: TAG => [ qw!name1 name2! ],
    
                 # your exported package globals go here,
                 # as well as any optionally exported functions
                 @EXPORT_OK   = qw($Var1 %Hashit);
             }
             our @EXPORT_OK;
    
             # non-exported package globals go here
             our @more;
             our $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 = ();
    
             # here's a file-private function as a closure,
             # callable as &$priv_func;  it cannot be prototyped.
             my $priv_func = sub {
                 # stuff goes here.
             };
    
             # 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
    
             # this one isn't exported, but could be called!
             sub func4(\%)  {}    # proto'd to 1 hash ref
    
             END { }       # module clean-up code here (global destructor)
    
             1;            # modules must return true
    
         The h2xs program will create stubs for all the important
         stuff for you:
    
           % h2xs -XA -n My::Module
    
    
         How do I create a class?
    
         See the perltoot manpage for an introduction to classes and
         objects, as well as the perlobj manpage and the perlbot
         manpage.
    
         How can I tell if a variable is tainted?
    
         See the Laundering and Detecting Tainted Data entry in the
         perlsec manpage.  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 `-w' clean, however.  There is no `-w' clean way
         to detect taintedness - take this as a hint that you should
         untaint all possibly-tainted data.
    
         What's a closure?
    
         Closures are documented in the perlref manpage.
    
         Closure is a computer science term with a precise but hard-
         to-explain meaning. Closures are implemented in Perl as
         anonymous subroutines with lasting references to lexical
         variables outside their own scopes.  These lexicals
         magically refer to the variables that were around when the
         subroutine was defined (deep binding).
    
         Closures make sense in any programming language where you
         can have the return value of a function be itself a
         function, as you can in Perl.  Note that some languages
         provide anonymous functions but are not capable of providing
         proper closures; the Python language, for 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 };
             }
             $add_sub = add_function_generator();
             $sum = $add_sub->(4,5);                # $sum is 9 now.
    
         The closure works as a function template with some
         customization slots left out to be filled later.  The
         anonymous subroutine returned by add_function_generator()
         isn't technically a closure because it refers to no lexicals
         outside its own scope.
    
         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.
    
         What is variable suicide and how can I prevent it?
    
         Variable suicide is when you (temporarily or permanently)
         lose the value of a variable.  It is caused by scoping
         through my() and local() interacting with either closures or
         aliased foreach() iterator variables and subroutine
         arguments.  It used to be easy to inadvertently lose a
         variable's value this way, but now it's much harder.  Take
         this code:
    
    
    
             my $f = "foo";
             sub T {
               while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
             }
             T;
             print "Finally $f\n";
    
         The $f that has "bar" added to it three times should be a
         new `$f' (`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).
    
         How can I pass/return a {Function, FileHandle, Array, Hash,
         Method, Regex}?
    
         With the exception of regexes, you need to pass references
         to these objects.  See the Pass by Reference entry in the
         perlsub manpage for this particular question, and the
         perlref manpage for information on references.
    
         Passing Variables and Functions
             Regular variables and functions are quite easy: 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
             To pass filehandles to subroutines, use the `*FH' or
             `\*FH' notations.  These are "typeglobs" - see the
             Typeglobs and Filehandles entry in the perldata manpage
             and especially the Pass by Reference entry in the
             perlsub manpage for more information.
    
             Here's an excerpt:
    
             If you're passing around filehandles, you could usually
             just use the bare typeglob, like *STDOUT, but typeglobs
             references would be better because they'll still work
             properly under `use strict 'refs''.  For example:
    
                 splutter(\*STDOUT);
                 sub splutter {
                     my $fh = shift;
                     print $fh "her um well a hmmm\n";
                 }
    
                 $rec = get_rec(\*STDIN);
                 sub get_rec {
                     my $fh = shift;
                     return scalar <$fh>;
                 }
    
             If you're planning on generating new filehandles, you
             could do this:
    
                 sub openit {
                     my $name = shift;
                     local *FH;
                     return open (FH, $path) ? *FH : undef;
                 }
                 $fh = openit('< /etc/motd');
                 print <$fh>;
    
    
         Passing Regexes
             To pass regexes around, you'll need to be using a
             release of Perl sufficiently recent as to support the
             `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 `qr//':
    
                 sub compare($$) {
                     my ($val1, $regex) = @_;
                     my $retval = $val1 =~ /$regex/;
                     return $retval;
                 }
                 $match = compare("old McDonald", qr/d.*D/i);
    
             Notice how `qr//' allows flags at the end.  That pattern
             was compiled at compile time, although it was executed
             later.  The nifty `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 `qr//':
    
    
    
                 sub compare($$) {
                     my ($val1, $regex) = @_;
                     my $retval = eval { $val1 =~ /$regex/ };
                     die if $@;
                     return $retval;
                 }
    
                 $match = compare("old McDonald", q/($?i)d.*D/);
    
             Make sure you never say something like this:
    
                 return eval "\$val =~ /$regex/";   # WRONG
    
             or someone can sneak shell escapes into the regex due to
             the double interpolation of the eval and the double-
             quoted string.  For example:
    
                 $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
    
                 eval "\$string =~ /$pattern_of_evil/";
    
             Those preferring to be very, very clever might see the
             O'Reilly book, Mastering Regular Expressions, by Jeffrey
             Friedl.  Page 273's Build_MatchMany_Function() is
             particularly interesting.  A complete citation of this
             book is given in the perlfaq2 manpage.
    
         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 and 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?
    
         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:
    
             BEGIN {
                 my $counter = 42;
                 sub prev_counter { return --$counter }
                 sub next_counter { return $counter++ }
             }
    
         Now prev_counter() and next_counter() share a private
         variable $counter that was initialized at compile time.
    
         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:
    
             package Pax;
             my $started = scalar(localtime(time()));
    
             sub begun { return $started }
    
         When `use Pax' or `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.
    
         See the Persistent Private Variables entry in the perlsub
         manpage 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 is
         called lexical or static scoping.  my() always affects
         private variables, also called lexical variables or
         (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 the Private Variables via my() entry in the perlsub
         manpage and the Temporary Values via local() entry in the
         perlsub manpage for excruciating details.
    
         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 `use strict "refs"'.  So instead of $var, use
         `${'var'}'.
    
             local $var = "global";
             my    $var = "lexical";
    
             print "lexical is $var\n";
    
             no strict 'refs';
             print "global  is ${'var'}\n";
    
         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.  Specifying the package directly makes
         you hard-code its name, but it executes faster and avoids
         running afoul of `use strict "refs"'.
    
         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 the section on "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
    
    
         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 the Overriding Built-in Functions
         entry in the perlsub manpage.  There's also an example in
         the Class::Template entry in the perltoot manpage.
    
         If you want to overload a Perl operator, such as `+' or
         `**', then you'll want to use the `use overload' pragma,
         documented in the overload manpage.
    
         If you're talking about obscuring method calls in parent
         classes, see the Overridden Methods entry in the perltoot
         manpage.
    
         What's the difference between calling a function as &foo and
         foo()?
    
         When you call a function as `&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 strictly speaking a bug
         (it's documented that way in the perlsub manpage), it would
         be hard to consider this a feature in most cases.
    
         When you call your function as `&foo()', then you do get a
         new @_, but prototyping is still circumvented.
    
         Normally, you want to call a function using `foo()'.  You
         may only omit the parentheses if the function is already
         known to the compiler because it already saw the definition
         (`use' but not `require'), or via a forward reference or
         `use subs' declaration.  Even in this case, you get a clean
         @_ without any of the old values leaking through where they
         don't belong.
    
         How do I create a switch or case statement?
    
         This is explained in more depth in the the perlsyn manpage.
         Briefly, there's no official case statement, because of the
         variety of tests possible in Perl (numeric comparison,
         string comparison, glob comparison, regex matching,
         overloaded comparisons, ...).  Larry couldn't decide how
         best to do this, so he left it out, even though it's been on
         the wish list since perl1.
    
         The general answer is to write a construct like this:
    
    
             for ($variable_to_test) {
                 if    (/pat1/)  { }     # do something
                 elsif (/pat2/)  { }     # do something else
                 elsif (/pat3/)  { }     # do something else
                 else            { }     # default
             }
    
         Here's a simple example of a switch based on pattern
         matching, this time lined up in a way to make it look more
         like a switch statement.  We'll do a multi-way conditional
         based on the type of reference stored in $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;
                                 };
    
                 /CODE/          && do {
                                         warn "can't print function ref";
                                         last SWITCH;
                                 };
    
                 # DEFAULT
    
                 warn "User defined type skipped";
    
             }
    
         See `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 `"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";
             }
    
    
         How can I catch accesses to undefined
         variables/functions/methods?
    
         The AUTOLOAD method, discussed in the Autoloading entry in
         the perlsub manpage and the AUTOLOAD: Proxy Methods entry in
         the perltoot manpage, lets you capture calls to undefined
         functions and methods.
    
         When it comes to undefined variables that would trigger a
         warning under `-w', you can use a handler to trap the
         pseudo-signal `__WARN__' like this:
    
             $SIG{__WARN__} = sub {
    
                 for ( $_[0] ) {         # voici un switch statement
    
                     /Use of uninitialized value/  && do {
                         # promote warning to a fatal
                         die $_;
                     };
    
                     # other warning cases to catch could go here;
    
    
                     warn $_;
                 }
    
             };
    
    
         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 the perltoot manpage for details on
         these.  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 the perlobj
         manpage.
    
         Make sure to read about creating modules in the perlmod
         manpage and the perils of indirect objects in the WARNING
         entry in the perlobj manpage.
    
         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 = __PACKAGE__;
    
         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 $class = ref($self) || $self;
                 warn "called me from a $class object";
             }
    
    
         How can I comment out a large block of perl code?
    
         Use embedded POD to discard it:
    
    
             # program is here
    
             =for nobody
             This paragraph is commented out
    
             # program continues
    
             =begin comment text
    
             all of this stuff
    
             here will be ignored
             by everyone
    
             =end comment text
    
             =cut
    
         This can't go just anywhere.  You have to put a pod
         directive where the parser is expecting a new statement, not
         just in the middle of an expression or some other arbitrary
         yacc grammar production.
    
         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.
    
         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 they only work on global variables.
         That means above that if $fred is a lexical variable created
         with my(), that the code won't work at all: you'll
         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 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.
    
             $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
    
         Instead, 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 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>" };
             }
    
         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.  But for
         scalars, arrays, and hashes -- and usually for subroutines
         -- you probably want to use hard references only.
    
    
    

    AUTHOR AND COPYRIGHT

         Copyright (c) 1997-1999 Tom Christiansen and Nathan
         Torkington.  All rights reserved.
    
         When included as part of the Standard Version of Perl, or as
         part of its complete documentation whether printed or
         otherwise, this work may be distributed only under the terms
         of Perl's Artistic License.  Any distribution of this file
         or derivatives thereof outside of that package require that
         special arrangements be made with copyright holder.
    
         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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