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General Perl Language Issues

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NAME

perlfaq7 - General Perl Language Issues

DESCRIPTION

This section deals with general Perl language issues that don'tclearly 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 inperly.y in the source distribution if you're particularly brave. Thegrammar relies on very smart tokenizing code, so be prepared toventure 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, smokeand mirrors."

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

They are type specifiers, as detailed in perldata:

  1.  $ for scalar values (number, string or reference)
    2.  
  2.  @ for arrays
    3.  
  3.  % for hashes (associative arrays)
    4.  
  4.  & for subroutines (aka functions, procedures, methods)
    5.  
  5.  * for all types of that symbol name. In version 4 you used them like
    6.  
  6.   pointers, but in modern perls you can just use references.
    7.

There are a couple of other symbols thatyou're likely to encounter that aren'treally type specifiers:

  1.  <> are used for inputting a record from a filehandle.
    2.  
  2.  \  takes a reference to something.
    3.

Note that <FILE> is neither the type specifier for filesnor the name of the handle. It is the <> operator appliedto the handle FILE. It reads one line (well, record--see$/ in perlvar) from the handle FILE in scalar context, or all linesin list context. When performing open, close, or any other operationbesides <> on files, or even when talking about the handle, donot 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 casesprobably should be (and must be under use strict). But a hash keyconsisting of a simple word and the left-handoperand to the => operator bothcount as though they were quoted:

  1.  This is like this
    2.  
  2.  ------------ ---------------
    3.  
  3.  $foo{line}  $foo{'line'}
    4.  
  4.  bar => stuff 'bar' => stuff
    5.

The final semicolon in a block is optional, as is the final comma in alist. Good style (see perlstyle) says to put them in except forone-liners:

  1.  if ($whoops) { exit 1 }
    2.  
  2.  my @nums = (1, 2, 3);
    3.  
  3.  
  4.  if ($whoops) {
    5.  
  5.  exit 1;
    6.  
  6.  }
    7.  
  7.  
  8.  my @lines = (
    9.  
  9.  "There Beren came from mountains cold",
    10.  
  10.  "And lost he wandered under leaves",
    11.  
  11.  );
    12.

How do I skip some return values?

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

  1.  $dir = (getpwnam($user))[7];
    2.

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

  1.  ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
    2.

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

  1.  ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];
    2.

How do I temporarily block warnings?

If you are running Perl 5.6.0 or better, the use warnings pragmaallows fine control of what warnings are produced.See perllexwarn for more details.

  1.  {
    2.  
  2.  no warnings;  # temporarily turn off warnings
    3.  
  3.  $x = $y + $z; # I know these might be undef
    4.  
  4.  }
    5.

Additionally, you can enable and disable categories of warnings.You turn off the categories you want to ignore and you can stillget other categories of warnings. See perllexwarn for thecomplete details, including the category names and hierarchy.

  1.  {
    2.  
  2.  no warnings 'uninitialized';
    3.  
  3.  $x = $y + $z;
    4.  
  4.  }
    5.

If you have an older version of Perl, the $^W variable (documentedin perlvar) controls runtime warnings for a block:

  1.  {
    2.  
  2.  local $^W = 0; # temporarily turn off warnings
    3.  
  3.  $x = $y + $z; # I know these might be undef
    4.  
  4.  }
    5.

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

What's an extension?

An extension is a way of calling compiled C code from Perl. Readingperlxstut 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 sameprecedence in Perl as they do in C. The problem is with operators that Cdoesn't have, especially functions that give a list context to everythingon their right, eg. print, chmod, exec, and so on. Such functions arecalled "list operators" and appear as such in the precedence table inperlop.

A common mistake is to write:

  1.  unlink $file || die "snafu";
    2.

This gets interpreted as:

  1.  unlink ($file || die "snafu");
    2.

To avoid this problem, either put in extra parentheses or use thesuper low precedence or operator:

  1.  (unlink $file) || die "snafu";
    2.  
  2.  unlink $file or die "snafu";
    3.

The "English" operators (and, or, xor, and not)deliberately have precedence lower than that of list operators forjust such situations as the one above.

Another operator with surprising precedence is exponentiation. Itbinds more tightly even than unary minus, making -2**2 produce anegative four and not a positive one. It is also right-associating, meaningthat 2**3**2 is two raised to the ninth power, not eight squared.

Although it has the same precedence as in C, Perl's ?: operatorproduces an lvalue. This assigns $x to either $if_true or $if_false, dependingon the trueness of $maybe:

  1.  ($maybe ? $if_true : $if_false) = $x;
    2.

How do I declare/create a structure?

In general, you don't "declare" a structure. Just use a (probablyanonymous) hash reference. See perlref and perldsc for details.Here's an example:

  1.  $person = {};   # new anonymous hash
    2.  
  2.  $person->{AGE}  = 24;   # set field AGE to 24
    3.  
  3.  $person->{NAME} = "Nat"; # set field NAME to "Nat"
    4.

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

How do I create a module?

perlnewmod is a good place to start, ignore the bitsabout uploading to CPAN if you don't want to make yourmodule publicly available.

ExtUtils::ModuleMaker and Module::Starter are alsogood places to start. Many CPAN authors now use Dist::Zillato automate as much as possible.

Detailed documentation about modules can be found at:perlmod, perlmodlib, perlmodstyle.

If you need to include C code or C library interfaces use h2xs. h2xs will create the module distribution structure and the initial interface files.perlxs and perlxstut explain the details.

How do I adopt or take over a module already on CPAN?

Ask the current maintainer to make you a co-maintainer ortransfer the module to you.

If you can not reach the author for some reason contactthe PAUSE admins at [email protected] who may be able to help,but each case it treated seperatly.

  • Get a login for the Perl Authors Upload Server (PAUSE) if you don'talready have one: http://pause.perl.org

  • Write to [email protected] explaining what you did to contact thecurrent maintainer. The PAUSE admins will also try to reach themaintainer.

  • Post a public message in a heavily trafficked site announcing yourintention to take over the module.

  • Wait a bit. The PAUSE admins don't want to act too quickly in casethe current maintainer is on holiday. If there's no response toprivate communication or the public post, a PAUSE admin can transferit to you.

How do I create a class?

(contributed by brian d foy)

In Perl, a class is just a package, and methods are just subroutines.Perl doesn't get more formal than that and lets you set up the packagejust the way that you like it (that is, it doesn't set up anything foryou).

The Perl documentation has several tutorials that cover classcreation, including perlboot (Barnyard Object Oriented Tutorial),perltoot (Tom's Object Oriented Tutorial), perlbot (Bag o'Object Tricks), and perlobj.

How can I tell if a variable is tainted?

You can use the tainted() function of the Scalar::Util module, availablefrom 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 buthard-to-explain meaning. Usually, closures are implemented in Perl asanonymous subroutines with lasting references to lexical variablesoutside their own scopes. These lexicals magically refer to thevariables that were around when the subroutine was defined (deepbinding).

Closures are most often used in programming languages where you canhave the return value of a function be itself a function, as you canin Perl. Note that some languages provide anonymous functions but arenot capable of providing proper closures: the Python language, forexample. For more information on closures, check out any textbook onfunctional programming. Scheme is a language that not only supportsbut encourages closures.

Here's a classic non-closure function-generating function:

  1.  sub add_function_generator {
    2.  
  2.  return sub { shift() + shift() };
    3.  
  3.  }
    4.  
  4.  
  5.  my $add_sub = add_function_generator();
    6.  
  6.  my $sum = $add_sub->(4,5); # $sum is 9 now.
    7.

The anonymous subroutine returned by add_function_generator() isn'ttechnically a closure because it refers to no lexicals outside its ownscope. Using a closure gives you a function template with somecustomization slots left out to be filled later.

Contrast this with the following make_adder() function, in which thereturned anonymous function contains a reference to a lexical variableoutside the scope of that function itself. Such a reference requiresthat Perl return a proper closure, thus locking in for all time thevalue that the lexical had when the function was created.

  1.  sub make_adder {
    2.  
  2.  my $addpiece = shift;
    3.  
  3.  return sub { shift() + $addpiece };
    4.  
  4.  }
    5.  
  5.  
  6.  my $f1 = make_adder(20);
    7.  
  7.  my $f2 = make_adder(555);
    8.

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 $addpiecein the closure sticks around.

Closures are often used for less esoteric purposes. For example, whenyou want to pass in a bit of code into a function:

  1.  my $line;
    2.  
  2.  timeout( 30, sub { $line = <STDIN> } );
    3.

If the code to execute had been passed in as a string,'$line = <STDIN>', there would have been no way for thehypothetical timeout() function to access the lexical variable$line back in its caller's scope.

Another use for a closure is to make a variable private to anamed subroutine, e.g. a counter that gets initialized at creationtime of the sub and can only be modified from within the sub.This is sometimes used with a BEGIN block in package files to makesure a variable doesn't get meddled with during the lifetime of thepackage:

  1.  BEGIN {
    2.  
  2.  my $id = 0;
    3.  
  3.  sub next_id { ++$id }
    4.  
  4.  }
    5.

This is discussed in more detail in perlsub; see the entry onPersistent Private Variables.

What is variable suicide and how can I prevent it?

This problem was fixed in perl 5.004_05, so preventing it means upgradingyour version of perl. ;)

Variable suicide is when you (temporarily or permanently) lose the valueof a variable. It is caused by scoping through my() and local()interacting with either closures or aliased foreach() iterator variablesand subroutine arguments. It used to be easy to inadvertently lose avariable's value this way, but now it's much harder. Take this code:

  1.  my $f = 'foo';
    2.  
  2.  sub T {
    3.  
  3.  while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
    4.  
  4.  }
    5.  
  5.  
  6.  T;
    7.  
  7.  print "Finally $f\n";
    8.

If you are experiencing variable suicide, that my $f in the subroutinedoesn't pick up a fresh copy of the $f whose value is 'foo'. Theoutput shows that inside the subroutine the value of $f leaks throughwhen it shouldn't, as in this output:

  1.  foobar
    2.  
  2.  foobarbar
    3.  
  3.  foobarbarbar
    4.  
  4.  Finally foo
    5.

The $f that has "bar" added to it three times should be a new $fmy $f should create a new lexical variable each time through the loop.The expected output is:

  1.  foobar
    2.  
  2.  foobar
    3.  
  3.  foobar
    4.  
  4.  Finally foo
    5.

How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?

You need to pass references to these objects. See Pass by Reference in perlsub for this particular question, and perlref forinformation on references.

  • Passing Variables and Functions

    Regular variables and functions are quite easy to pass: just pass in areference to an existing or anonymous variable or function:

    1.  func( \$some_scalar );
      2.  
    2.  
    3.  func( \@some_array  );
      4.  
    4.  func( [ 1 .. 10 ]   );
      5.  
    5.  
    6.  func( \%some_hash   );
      7.  
    7.  func( { this => 10, that => 20 }   );
      8.  
    8.  
    9.  func( \&some_func   );
      10.  
    10.  func( sub { $_[0] ** $_[1] }   );
      11.
  • Passing Filehandles

    As of Perl 5.6, you can represent filehandles with scalar variableswhich you treat as any other scalar.

    1.  open my $fh, $filename or die "Cannot open $filename! $!";
      2.  
    2.  func( $fh );
      3.  
    3.  
    4.  sub func {
      5.  
    5.  my $passed_fh = shift;
      6.  
    6.  
    7.  my $line = <$passed_fh>;
      8.  
    8.  }
      9.

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

  • Passing Regexes

    Here's an example of how to pass in a string and a regular expressionfor it to match against. You construct the pattern with the qr//operator:

    1.  sub compare($$) {
      2.  
    2.  my ($val1, $regex) = @_;
      3.  
    3.  my $retval = $val1 =~ /$regex/;
      4.  
    4.  return $retval;
      5.  
    5.  }
      6.  
    6.  $match = compare("old McDonald", qr/d.*D/i);
      7.
  • Passing Methods

    To pass an object method into a subroutine, you can do this:

    1.  call_a_lot(10, $some_obj, "methname")
      2.  
    2.  sub call_a_lot {
      3.  
    3.  my ($count, $widget, $trick) = @_;
      4.  
    4.  for (my $i = 0; $i < $count; $i++) {
      5.  
    5.  $widget->$trick();
      6.  
    6.  }
      7.  
    7.  }
      8.

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

    1.  my $whatnot = sub { $some_obj->obfuscate(@args) };
      2.  
    2.  func($whatnot);
      3.  
    3.  sub func {
      4.  
    4.  my $code = shift;
      5.  
    5.  &$code();
      6.  
    6.  }
      7.

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

How do I create a static variable?

(contributed by brian d foy)

In Perl 5.10, declare the variable with state. The statedeclaration creates the lexical variable that persists between callsto the subroutine:

  1.  sub counter { state $count = 1; $count++ }
    2.

You can fake a static variable by using a lexical variable which goesout of scope. In this example, you define the subroutine counter, andit uses the lexical variable $count. Since you wrap this in a BEGINblock, $count is defined at compile-time, but also goes out ofscope at the end of the BEGIN block. The BEGIN block also ensures thatthe subroutine and the value it uses is defined at compile-time so thesubroutine is ready to use just like any other subroutine, and you canput this code in the same place as other subroutines in the programtext (i.e. at the end of the code, typically). The subroutinecounter still has a reference to the data, and is the only way youcan access the value (and each time you do, you increment the value).The data in chunk of memory defined by $count is private tocounter.

  1.  BEGIN {
    2.  
  2.  my $count = 1;
    3.  
  3.  sub counter { $count++ }
    4.  
  4.  }
    5.  
  5.  
  6.  my $start = counter();
    7.  
  7.  
  8.  .... # code that calls counter();
    9.  
  9.  
  10.  my $end = counter();
    11.

In the previous example, you created a function-private variablebecause only one function remembered its reference. You could definemultiple functions while the variable is in scope, and each functioncan share the "private" variable. It's not really "static" because youcan access it outside the function while the lexical variable is inscope, and even create references to it. In this example,increment_count and return_count share the variable. Onefunction adds to the value and the other simply returns the value.They can both access $count, and since it has gone out of scope,there is no other way to access it.

  1.  BEGIN {
    2.  
  2.  my $count = 1;
    3.  
  3.  sub increment_count { $count++ }
    4.  
  4.  sub return_count { $count }
    5.  
  5.  }
    6.

To declare a file-private variable, you still use a lexical variable.A file is also a scope, so a lexical variable defined in the filecannot be seen from any other file.

See Persistent Private Variables in perlsub for more information.The discussion of closures in perlref may help you even though wedid not use anonymous subroutines in this answer. SeePersistent Private Variables in perlsub for details.

What's the difference between dynamic and lexical (static) scoping? Between local() and my()?

local($x) saves away the old value of the global variable $xand assigns a new value for the duration of the subroutine which isvisible in other functions called from that subroutine. This is doneat run-time, so is called dynamic scoping. local() always affects globalvariables, also called package variables or dynamic variables.

my($x) creates a new variable that is only visible in the currentsubroutine. This is done at compile-time, so it is called lexical orstatic scoping. my() always affects private variables, also calledlexical variables or (improperly) static(ly scoped) variables.

For instance:

  1.  sub visible {
    2.  
  2.  print "var has value $var\n";
    3.  
  3.  }
    4.  
  4.  
  5.  sub dynamic {
    6.  
  6.  local $var = 'local'; # new temporary value for the still-global
    7.  
  7.  visible();  #   variable called $var
    8.  
  8.  }
    9.  
  9.  
  10.  sub lexical {
    11.  
  11.  my $var = 'private'; # new private variable, $var
    12.  
  12.  visible();  # (invisible outside of sub scope)
    13.  
  13.  }
    14.  
  14.  
  15.  $var = 'global';
    16.  
  16.  
  17.  visible();  # prints global
    18.  
  18.  dynamic();  # prints local
    19.  
  19.  lexical();  # prints global
    20.

Notice how at no point does the value "private" get printed. That'sbecause $var only has that value within the block of the lexical()function, and it is hidden from the called subroutine.

In summary, local() doesn't make what you think of as private, localvariables. It gives a global variable a temporary value. my() iswhat you're looking for if you want private variables.

See Private Variables via my() in perlsub andTemporary Values via local() in perlsub for excruciating details.

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

If you know your package, you can just mention it explicitly, as in$Some_Pack::var. Note that the notation $::var is not the dynamic $varin the current package, but rather the one in the "main" package, asthough you had written $main::var.

  1.  use vars '$var';
    2.  
  2.  local $var = "global";
    3.  
  3.  my $var = "lexical";
    4.  
  4.  
  5.  print "lexical is $var\n";
    6.  
  6.  print "global  is $main::var\n";
    7.

Alternatively you can use the compiler directive our() to bring adynamic variable into the current lexical scope.

  1.  require 5.006; # our() did not exist before 5.6
    2.  
  2.  use vars '$var';
    3.  
  3.  
  4.  local $var = "global";
    5.  
  5.  my $var = "lexical";
    6.  
  6.  
  7.  print "lexical is $var\n";
    8.  
  8.  
  9.  {
    10.  
  10.  our $var;
    11.  
  11.  print "global  is $var\n";
    12.  
  12.  }
    13.

What's the difference between deep and shallow binding?

In deep binding, lexical variables mentioned in anonymous subroutinesare the same ones that were in scope when the subroutine was created.In shallow binding, they are whichever variables with the same nameshappen to be in scope when the subroutine is called. Perl always usesdeep 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 reasonnot to use them. See the answer to What's a closure?.

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

my() and local() give list context to the right hand sideof =. The <$fh> read operation, like so many of Perl'sfunctions and operators, can tell which context it was called in andbehaves 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 coursedoesn't help you (such as with sort()).

To enforce scalar context in this particular case, however, you needmerely omit the parentheses:

  1.  local($foo) = <$fh>; # WRONG
    2.  
  2.  local($foo) = scalar(<$fh>);   # ok
    3.  
  3.  local $foo  = <$fh>; # right
    4.

You should probably be using lexical variables anyway, although theissue is the same here:

  1.  my($foo) = <$fh>; # WRONG
    2.  
  2.  my $foo  = <$fh>; # right
    3.

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 differentmodule. See Overriding Built-in Functions in perlsub.

If you want to overload a Perl operator, such as + or **,then you'll want to use the use overload pragma, documentedin overload.

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

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

(contributed by brian d foy)

Calling a subroutine as &foo with no trailing parentheses ignoresthe prototype of foo and passes it the current value of the argumentlist, @_. Here's an example; the bar subroutine calls &foo,which prints its arguments list:

  1.  sub bar { &foo }
    2.  
  2.  
  3.  sub foo { print "Args in foo are: @_\n" }
    4.  
  4.  
  5.  bar( qw( a b c ) );
    6.

When you call bar with arguments, you see that foo got the same @_:

  1.  Args in foo are: a b c
    2.

Calling the subroutine with trailing parentheses, with or without arguments,does not use the current @_ and respects the subroutine prototype. Changingthe example to put parentheses after the call to foo changes the program:

  1.  sub bar { &foo() }
    2.  
  2.  
  3.  sub foo { print "Args in foo are: @_\n" }
    4.  
  4.  
  5.  bar( qw( a b c ) );
    6.

Now the output shows that foo doesn't get the @_ from its caller.

  1.  Args in foo are:
    2.

The main use of the @_ pass-through feature is to write subroutineswhose main job it is to call other subroutines for you. For furtherdetails, see perlsub.

How do I create a switch or case statement?

In Perl 5.10, use the given-when construct described in perlsyn:

  1.  use 5.010;
    2.  
  2.  
  3.  given ( $string ) {
    4.  
  4.  when( 'Fred' ) { say "I found Fred!" }
    5.  
  5.  when( 'Barney' )  { say "I found Barney!" }
    6.  
  6.  when( /Bamm-?Bamm/ )  { say "I found Bamm-Bamm!" }
    7.  
  7.  default   { say "I don't recognize the name!" }
    8.  
  8.  };
    9.

If one wants to use pure Perl and to be compatible with Perl versionsprior to 5.10, the general answer is to use if-elsif-else:

  1.  for ($variable_to_test) {
    2.  
  2.  if (/pat1/)  { } # do something
    3.  
  3.  elsif (/pat2/)  { } # do something else
    4.  
  4.  elsif (/pat3/)  { } # do something else
    5.  
  5.  else { } # default
    6.  
  6.  }
    7.

Here's a simple example of a switch based on pattern matching,lined up in a way to make it look more like a switch statement.We'll do a multiway conditional based on the type of reference storedin $whatchamacallit:

  1.  SWITCH: for (ref $whatchamacallit) {
    2.  
  2.  
  3.  /^$/   && die "not a reference";
    4.  
  4.  
  5.  /SCALAR/   && do {
    6.  
  6.  print_scalar($$ref);
    7.  
  7.  last SWITCH;
    8.  
  8.   };
    9.  
  9.  
  10.  /ARRAY/ && do {
    11.  
  11.  print_array(@$ref);
    12.  
  12.  last SWITCH;
    13.  
  13.   };
    14.  
  14.  
  15.  /HASH/ && do {
    16.  
  16.  print_hash(%$ref);
    17.  
  17.  last SWITCH;
    18.  
  18.   };
    19.  
  19.  
  20.  /CODE/ && do {
    21.  
  21.  warn "can't print function ref";
    22.  
  22.  last SWITCH;
    23.  
  23.   };
    24.  
  24.  
  25.  # DEFAULT
    26.  
  26.  
  27.  warn "User defined type skipped";
    28.  
  28.  
  29.  }
    30.

See perlsyn for other examples in this style.

Sometimes you should change the positions of the constant and the variable.For example, let's say you wanted to test which of many answers you weregiven, but in a case-insensitive way that also allows abbreviations.You can use the following technique if the strings all start withdifferent characters or if you want to arrange the matches so thatone takes precedence over another, as "SEND" has precedence over"STOP" here:

  1.  chomp($answer = <>);
    2.  
  2.  if ("SEND"  =~ /^\Q$answer/i) { print "Action is send\n"  }
    3.  
  3.  elsif ("STOP"  =~ /^\Q$answer/i) { print "Action is stop\n"  }
    4.  
  4.  elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
    5.  
  5.  elsif ("LIST"  =~ /^\Q$answer/i) { print "Action is list\n"  }
    6.  
  6.  elsif ("EDIT"  =~ /^\Q$answer/i) { print "Action is edit\n"  }
    7.

A totally different approach is to create a hash of function references.

  1.  my %commands = (
    2.  
  2.  "happy" => \&joy,
    3.  
  3.  "sad",  => \&sullen,
    4.  
  4.  "done"  => sub { die "See ya!" },
    5.  
  5.  "mad"   => \&angry,
    6.  
  6.  );
    7.  
  7.  
  8.  print "How are you? ";
    9.  
  9.  chomp($string = <STDIN>);
    10.  
  10.  if ($commands{$string}) {
    11.  
  11.  $commands{$string}->();
    12.  
  12.  } else {
    13.  
  13.  print "No such command: $string\n";
    14.  
  14.  }
    15.

Starting from Perl 5.8, a source filter module, Switch, can also beused to get switch and case. Its use is now discouraged, because it'snot fully compatible with the native switch of Perl 5.10, and because,as it's implemented as a source filter, it doesn't always work as intendedwhen complex syntax is involved.

How can I catch accesses to undefined variables, functions, or methods?

The AUTOLOAD method, discussed in Autoloading in perlsub andAUTOLOAD: Proxy Methods in perltoot, lets you capture calls toundefined functions and methods.

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

  1.  use warnings FATAL => qw(uninitialized);
    2.

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

Some possible reasons: your inheritance is getting confused, you'vemisspelled the method name, or the object is of the wrong type. Checkout perltoot for details about any of the above cases. You mayalso use print ref($object) to find out the class $object wasblessed into.

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

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

How can I find out my current or calling package?

(contributed by brian d foy)

To find the package you are currently in, use the special literal__PACKAGE__, as documented in perldata. You can only use thespecial literals as separate tokens, so you can't interpolate theminto strings like you can with variables:

  1.  my $current_package = __PACKAGE__;
    2.  
  2.  print "I am in package $current_package\n";
    3.

If you want to find the package calling your code, perhaps to give betterdiagnostics as Carp does, use the caller built-in:

  1.  sub foo {
    2.  
  2.  my @args = ...;
    3.  
  3.  my( $package, $filename, $line ) = caller;
    4.  
  4.  
  5.  print "I was called from package $package\n";
    6.  
  6.  );
    7.

By default, your program starts in package main, so you willalways be in some package.

This is different from finding out the package an object is blessedinto, which might not be the current package. For that, use blessedfrom Scalar::Util, part of the Standard Library since Perl 5.8:

  1.  use Scalar::Util qw(blessed);
    2.  
  2.  my $object_package = blessed( $object );
    3.

Most of the time, you shouldn't care what package an object is blessedinto, however, as long as it claims to inherit from that class:

  1.  my $is_right_class = eval { $object->isa( $package ) }; # true or false
    2.

And, with Perl 5.10 and later, you don't have to check for aninheritance to see if the object can handle a role. For that, you canuse DOES, which comes from UNIVERSAL:

  1.  my $class_does_it = eval { $object->DOES( $role ) }; # true or false
    2.

You can safely replace isa with DOES (although the converse is not true).

How can I comment out a large block of Perl code?

(contributed by brian d foy)

The quick-and-dirty way to comment out more than one line of Perl isto surround those lines with Pod directives. You have to put thesedirectives at the beginning of the line and somewhere where Perlexpects a new statement (so not in the middle of statements like the #comments). You end the comment with =cut, ending the Pod section:

  1.  =pod
    2.  
  2.  
  3.  my $object = NotGonnaHappen->new();
    4.  
  4.  
  5.  ignored_sub();
    6.  
  6.  
  7.  $wont_be_assigned = 37;
    8.  
  8.  
  9.  =cut
    10.

The quick-and-dirty method only works well when you don't plan toleave the commented code in the source. If a Pod parser comes along,you're multiline comment is going to show up in the Pod translation.A better way hides it from Pod parsers as well.

The =begin directive can mark a section for a particular purpose.If the Pod parser doesn't want to handle it, it just ignores it. Labelthe comments with comment. End the comment using =end with thesame label. You still need the =cut to go back to Perl code fromthe Pod comment:

  1.  =begin comment
    2.  
  2.  
  3.  my $object = NotGonnaHappen->new();
    4.  
  4.  
  5.  ignored_sub();
    6.  
  6.  
  7.  $wont_be_assigned = 37;
    8.  
  8.  
  9.  =end comment
    10.  
  10.  
  11.  =cut
    12.

For more information on Pod, check out perlpod and perlpodspec.

How do I clear a package?

Use this code, provided by Mark-Jason Dominus:

  1.  sub scrub_package {
    2.  
  2.  no strict 'refs';
    3.  
  3.  my $pack = shift;
    4.  
  4.  die "Shouldn't delete main package"
    5.  
  5.  if $pack eq "" || $pack eq "main";
    6.  
  6.  my $stash = *{$pack . '::'}{HASH};
    7.  
  7.  my $name;
    8.  
  8.  foreach $name (keys %$stash) {
    9.  
  9.  my $fullname = $pack . '::' . $name;
    10.  
  10.  # Get rid of everything with that name.
    11.  
  11.  undef $$fullname;
    12.  
  12.  undef @$fullname;
    13.  
  13.  undef %$fullname;
    14.  
  14.  undef &$fullname;
    15.  
  15.  undef *$fullname;
    16.  
  16.  }
    17.  
  17.  }
    18.

Or, if you're using a recent release of Perl, you canjust 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 nameof a variable.

  1.  $fred = 23;
    2.  
  2.  $varname = "fred";
    3.  
  3.  ++$$varname; # $fred now 24
    4.

This works sometimes, but it is a very bad idea for two reasons.

The first reason is that this technique only works on globalvariables. That means that if $fred is a lexical variable createdwith my() in the above example, the code wouldn't work at all: you'daccidentally access the global and skip right over the private lexicalaltogether. Global variables are bad because they can easily collideaccidentally 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-countedor garbage-collected.

The other reason why using a variable to hold the name of anothervariable is a bad idea is that the question often stems from a lack ofunderstanding of Perl data structures, particularly hashes. By usingsymbolic references, you are just using the package's symbol-table hash(like %main::) instead of a user-defined hash. The solution is touse your own hash or a real reference instead.

  1.  $USER_VARS{"fred"} = 23;
    2.  
  2.  my $varname = "fred";
    3.  
  3.  $USER_VARS{$varname}++;  # not $$varname++
    4.

There we're using the %USER_VARS hash instead of symbolic references.Sometimes this comes up in reading strings from the user with variablereferences and wanting to expand them to the values of your perlprogram's variables. This is also a bad idea because it conflates theprogram-addressable namespace and the user-addressable one. Instead ofreading a string and expanding it to the actual contents of your program'sown variables:

  1.  $str = 'this has a $fred and $barney in it';
    2.  
  2.  $str =~ s/(\$\w+)/$1/eeg;  # need double eval
    3.

it would be better to keep a hash around like %USER_VARS and havevariable references actually refer to entries in that hash:

  1.  $str =~ s/\$(\w+)/$USER_VARS{$1}/g;   # no /e here at all
    2.

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 tomake it less confusing, like bracketed percent symbols, etc.

  1.  $str = 'this has a %fred% and %barney% in it';
    2.  
  2.  $str =~ s/%(\w+)%/$USER_VARS{$1}/g;   # no /e here at all
    3.

Another reason that folks sometimes think they want a variable tocontain the name of a variable is that they don't know how to buildproper data structures using hashes. For example, let's say theywanted two hashes in their program: %fred and %barney, and that theywanted to use another scalar variable to refer to those by name.

  1.  $name = "fred";
    2.  
  2.  $$name{WIFE} = "wilma"; # set %fred
    3.  
  3.  
  4.  $name = "barney";
    5.  
  5.  $$name{WIFE} = "betty"; # set %barney
    6.

This is still a symbolic reference, and is still saddled with theproblems enumerated above. It would be far better to write:

  1.  $folks{"fred"}{WIFE}   = "wilma";
    2.  
  2.  $folks{"barney"}{WIFE} = "betty";
    3.

And just use a multilevel hash to start with.

The only times that you absolutely must use symbolic references arewhen you really must refer to the symbol table. This may be because it'ssomething that one can't take a real reference to, such as a format name.Doing so may also be important for method calls, since these always gothrough the symbol table for resolution.

In those cases, you would turn off strict 'refs' temporarily so youcan play around with the symbol table. For example:

  1.  @colors = qw(red blue green yellow orange purple violet);
    2.  
  2.  for my $name (@colors) {
    3.  
  3.  no strict 'refs';  # renege for the block
    4.  
  4.  *$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
    5.  
  5.  }
    6.

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 manipulatethe symbol table directly. This doesn't matter for formats, handles, andsubroutines, because they are always global--you can't use my() on them.For scalars, arrays, and hashes, though--and usually for subroutines--you probably only want to use hard references.

What does "bad interpreter" mean?

(contributed by brian d foy)

The "bad interpreter" message comes from the shell, not perl. Theactual message may vary depending on your platform, shell, and localesettings.

If you see "bad interpreter - no such file or directory", the firstline in your perl script (the "shebang" line) does not contain theright path to perl (or any other program capable of running scripts).Sometimes this happens when you move the script from one machine toanother and each machine has a different path to perl--/usr/bin/perlversus /usr/local/bin/perl for instance. It may also indicatethat the source machine has CRLF line terminators and thedestination machine has LF only: the shell tries to find/usr/bin/perl<CR>, but can't.

If you see "bad interpreter: Permission denied", you need to make yourscript executable.

In either case, you should still be able to run the scripts with perlexplicitly:

  1.  % perl script.pl
    2.

If you get a message like "perl: command not found", perl is not inyour PATH, which might also mean that the location of perl is notwhere you expect it so you need to adjust your shebang line.

AUTHOR AND COPYRIGHT

Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, andother authors as noted. All rights reserved.

This documentation is free; you can redistribute it and/or modify itunder the same terms as Perl itself.

Irrespective of its distribution, all code examples in this fileare hereby placed into the public domain. You are permitted andencouraged to use this code in your own programs for funor for profit as you see fit. A simple comment in the code givingcredit would be courteous but is not required.

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