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perlfaq7




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 tok­
       enizing code, so be prepared to venture into toke.c as
       well.

       In the words of Chaim Frenkel: "Perl's grammar can not be
       reduced to BNF.  The work of parsing perl is distributed
       between yacc, the lexer, smoke and mirrors."

       What are all these $@%&* punctuation signs, and how do I
       know when to use them?

       They are type specifiers, as detailed in perldata:

           $ for scalar values (number, string or reference)
           @ for arrays
           % for hashes (associative arrays)
           & for subroutines (aka functions, procedures, methods)
           * for all types of that symbol name.  In version 4 you used them like
             pointers, but in modern perls you can just use references.

       There are couple of other symbols that you're likely to
       encounter that aren't really type specifiers:

           <> are used for inputting a record from a filehandle.
           \  takes a reference to something.

       Note that <FILE> is neither the type specifier for files
       nor the name of the handle.  It is the "<>" operator
       applied to the handle FILE.  It reads one line (well,
       record--see "$/" in perlvar) from the handle FILE in
       scalar context, or all lines in list context.  When per­
       forming open, close, or any other operation besides "<>"
       on files, or even when talking about the handle, do not
       use the brackets.  These are correct: "eof(FH)", "seek(FH,
       0, 2)" and "copying from STDIN to FILE".

       Do I always/never have to quote my strings or use semi­
       colons 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

           if ($whoops) {
               exit 1;
           }
           @lines = (
               "There Beren came from mountains cold",
               "And lost he wandered under leaves",
           );

       How do I skip some return values?

       One way is to treat the return values as a list and index
       into it:

               $dir = (getpwnam($user))[7];

       Another way is to use undef as an element on the
       left-hand-side:

           ($dev, $ino, undef, undef, $uid, $gid) = stat($file);

       You can also use a list slice to select only the elements
       that you need:

               ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];

       How do I temporarily block warnings?

       If you are running Perl 5.6.0 or better, the "use warn­
       ings" pragma allows fine control of what warning are pro­
       duced.  See perllexwarn 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 perlvar) controls runtime warnings for a
       block:

           {
               local $^W = 0;        # temporarily turn off warnings
               $a = $b + $c;         # I know these might be undef
           }

       Note that like all the punctuation variables, you cannot
       currently use my() on $^W, only local().

       What's an extension?

       An extension is a way of calling compiled C code from
       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 oper­
       ators for just such situations as the one above.

       Another operator with surprising precedence is exponentia­
       tion.  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 perlref and
       perldsc for details.  Here's an example:

           $person = {};                   # new anonymous hash
           $person->{AGE}  = 24;           # set field AGE to 24
           $person->{NAME} = "Nat";        # set field NAME to "Nat"

       If you're looking for something a bit more rigorous, try
       perltoot.

       How do I create a module?

       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 perlmod.  You'll also
       find Exporter helpful.  If you're writing a C or mixed-
       language module with both C and Perl, then you should
       study perlxstut.


       How can I tell if a variable is tainted?

       You can use the tainted() function of the Scalar::Util
       module, available from CPAN (or included with Perl since
       release 5.8.0).  See also "Laundering and Detecting
       Tainted Data" in perlsec.

       What's a closure?

       Closures are documented in perlref.

       Closure is a computer science term with a precise but
       hard-to-explain meaning. Closures are implemented in Perl
       as anonymous subroutines with lasting references to lexi­
       cal 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 func­
       tion, as you can in Perl.  Note that some languages pro­
       vide 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 cus­
       tomization slots left out to be filled later.  The anony­
       mous 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;

           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 "Pass by Reference" in perlsub for
       this particular question, and perlref for information on
       references.

       See ``Passing Regexes'', below, for information on passing
       regular expressions.

       Passing Variables and Functions
           Regular variables and functions are quite easy to
           pass: just pass in a reference to an existing or
           anonymous variable or function:

               func( \$some_scalar );

               func( \@some_array  );
               func( [ 1 .. 10 ]   );

                           my $passed_fh = shift;

                           my $line = <$fh>;
                           }

           Before Perl 5.6, you had to use the *FH or "\*FH"
           notations.  These are "typeglobs"--see "Typeglobs and
           Filehandles" in perldata and especially "Pass by Ref­
           erence" in perlsub for more information.

       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 pat­
           tern 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 dou­
           ble-quoted string.  For example:

               $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
               sub call_a_lot {
                   my ($count, $widget, $trick) = @_;
                   for (my $i = 0; $i < $count; $i++) {
                       $widget->$trick();
                   }
               }

           Or, you can use a closure to bundle up the object, its
           method call, and arguments:

               my $whatnot =  sub { $some_obj->obfuscate(@args) };
               func($whatnot);
               sub func {
                   my $code = shift;
                   &$code();
               }

           You could also investigate the can() method in the
           UNIVERSAL class (part of the standard perl distribu­
           tion).

       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 the declaration 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-col­
       able $x and assigns a new value for the duration of the
       subroutine which is visible in other functions called from
       that subroutine.  This is done at run-time, so is called
       dynamic scoping.  local() always affects global variables,
       also called package variables or dynamic variables.

       "my($x)" creates a new variable that is only visible in
       the current subroutine.  This is done at compile-time, so
       it is called lexical or static scoping.  my() always
       affects private variables, also called lexical variables
       or (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 pri­
       vate, local variables.  It gives a global variable a tem­
       porary value.  my() is what you're looking for if you want
       private variables.

       See "Private Variables via my()" in perlsub and "Temporary
       Values via local()" in perlsub for excruciating details.

       How can I access a dynamic variable while a similarly
       named lexical is in scope?

       If you know your package, you can just mention it explic­
       itly, as in $Some_Pack::var. Note that the notation $::var
               require 5.006; # our() did not exist before 5.6
               use vars '$var';

               local $var = "global";
               my $var    = "lexical";

               print "lexical is $var\n";

               {
                 our $var;
                 print "global  is $var\n";
               }

       What's the difference between deep and shallow binding?

       In deep binding, lexical variables mentioned in anonymous
       subroutines are the same ones that were in scope when the
       subroutine was created.  In shallow binding, they are
       whichever variables with the same names happen to be in
       scope when the subroutine is called.  Perl always uses
       deep binding of lexical variables (i.e., those created
       with my()).  However, dynamic variables (aka global,
       local, or package variables) are effectively shallowly
       bound.  Consider this just one more reason not to use
       them.  See the answer to "What's a closure?".

       Why doesn't "my($foo) = <FILE>;" work right?

       "my()" and "local()" give list context to the right hand
       side of "=".  The <FH> read operation, like so many of
       Perl's functions and operators, can tell which context it
       was called in and behaves appropriately.  In general, the
       scalar() function can help.  This function does nothing to
       the data itself (contrary to popular myth) but rather
       tells its argument to behave in whatever its scalar fash­
       ion 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, how­
       ever, 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

       If you're talking about obscuring method calls in parent
       classes, see "Overridden Methods" in perltoot.

       What's the difference between calling a function as &foo
       and foo()?

       When you call a function as &foo, you allow that function
       access to your current @_ values, and you bypass proto­
       types.  The function doesn't get an empty @_--it gets
       yours!  While not strictly speaking a bug (it's documented
       that way in perlsub), 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 defini­
       tion ("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 perlsyn.  Briefly,
       there's no official case statement, because of the variety
       of tests possible in Perl (numeric comparison, string com­
       parison, glob comparison, regex matching, overloaded com­
       parisons, ...).  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:

           for ($variable_to_test) {
               if    (/pat1/)  { }     # do something
               elsif (/pat2/)  { }     # do something else
               elsif (/pat3/)  { }     # do something else
               else            { }     # default
                               };

               /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 func­
       tion references.

           my %commands = (
               "happy" => \&joy,
               "sad",  => \&sullen,
               "done"  => sub { die "See ya!" },
               "mad"   => \&angry,
           );


       When it comes to undefined variables that would trigger a
       warning under "use warnings", you can promote the warning
       to an error.

               use warnings FATAL => qw(uninitialized);

       Why can't a method included in this same file be found?

       Some possible reasons: your inheritance is getting con­
       fused, you've misspelled the method name, or the object is
       of the wrong type.  Check out perltoot for details about
       any of the above cases.  You may also use "print
       ref($object)" to find out the class $object was blessed
       into.

       Another possible reason for problems is because you've
       used the indirect object syntax (eg, "find Guru "Samy"")
       on a class name before Perl has seen that such a package
       exists.  It's wisest to make sure your packages are all
       defined before you start using them, which will be taken
       care of if you use the "use" statement instead of
       "require".  If not, make sure to use arrow notation (eg.,
       "Guru->find("Samy")") instead.  Object notation is
       explained in perlobj.

       Make sure to read about creating modules in perlmod and
       the perils of indirect objects in "Method Invocation" in
       perlobj.

       How can I find out my current 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?

       You can use embedded POD to discard it.  The =for direc­
       tive lasts until the next paragraph (two consecutive new­
           all of this stuff

           here will be ignored
           by everyone

           =end comment text

       The pod directives cannot go just anywhere.  You must put
       a pod directive where the parser is expecting a new state­
       ment, not just in the middle of an expression or some
       other arbitrary s grammar production.

       See perlpod for more details.

       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 this technique only works on
       stems from a lack of understanding of Perl data struc­
       tures, particularly hashes.  By using symbolic references,
       you are just using the package's symbol-table hash (like
       %main::) instead of a user-defined hash.  The solution is
       to use your own hash or a real reference instead.

           $USER_VARS{"fred"} = 23;
           $varname = "fred";
           $USER_VARS{$varname}++;  # not $$varname++

       There we're using the %USER_VARS hash instead of symbolic
       references.  Sometimes this comes up in reading strings
       from the user with variable references and wanting to
       expand them to the values of your perl program's vari­
       ables.  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 bet­
       ter to write:
       porarily 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.

       What does "bad interpreter" mean?

       The "bad interpreter" message comes from the shell, not
       perl.  The actual message may vary depending on your plat­
       form, 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 pro­
       gram 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.

       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 loca­
       tion of perl is not where you expect it so you need to
       adjust your shebang line.


AUTHOR AND COPYRIGHT

       Copyright (c) 1997-2002 Tom Christiansen and Nathan Tork­
       ington.  All rights reserved.

       This documentation is free; you can redistribute it and/or

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