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Switch(3p)       Perl Programmers Reference Guide      Switch(3p)


NAME
       Switch - A switch statement for Perl

VERSION
       This document describes version 2.10 of Switch, released
       Dec 29, 2003.

SYNOPSIS
               use Switch;

               switch ($val) {

                       case 1          { print "number 1" }
                       case "a"        { print "string a" }
                       case [1..10,42] { print "number in list" }
                       case (@array)   { print "number in list" }
                       case /\w+/      { print "pattern" }
                       case qr/\w+/    { print "pattern" }
                       case (%hash)    { print "entry in hash" }
                       case (\%hash)   { print "entry in hash" }
                       case (\&sub)    { print "arg to subroutine" }
                       else            { print "previous case not true" }
               }

BACKGROUND
       [Skip ahead to "DESCRIPTION" if you don't care about the
       whys and wherefores of this control structure]

       In seeking to devise a "Swiss Army" case mechanism suit-
       able for Perl, it is useful to generalize this notion of
       distributed conditional testing as far as possible.
       Specifically, the concept of "matching" between the switch
       value and the various case values need not be restricted
       to numeric (or string or referential) equality, as it is
       in other languages. Indeed, as Table 1 illustrates, Perl
       offers at least eighteen different ways in which two val-
       ues could generate a match.

               Table 1: Matching a switch value ($s) with a case value ($c)

               Switch  Case    Type of Match Implied   Matching Code
               Value   Value
               ======  =====   =====================   =============

               number  same    numeric or referential  match if $s == $c;
               or ref          equality

               object  method  result of method call   match if $s->$c();
               ref     name                            match if defined $s->$c();
                       or ref

               other   other   string equality         match if $s eq $c;
               non-ref non-ref
               scalar  scalar



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Switch(3p)       Perl Programmers Reference Guide      Switch(3p)


               string  regexp  pattern match           match if $s =~ /$c/;

               array   scalar  array entry existence   match if 0<=$c && $c<@$s;
               ref             array entry definition  match if defined $s->[$c];
                               array entry truth       match if $s->[$c];

               array   array   array intersection      match if intersects(@$s, @$c);
               ref     ref     (apply this table to
                                all pairs of elements
                                $s->[$i] and
                                $c->[$j])

               array   regexp  array grep              match if grep /$c/, @$s;
               ref

               hash    scalar  hash entry existence    match if exists $s->{$c};
               ref             hash entry definition   match if defined $s->{$c};
                               hash entry truth        match if $s->{$c};

               hash    regexp  hash grep               match if grep /$c/, keys %$s;
               ref

               sub     scalar  return value defn       match if defined $s->($c);
               ref             return value truth      match if $s->($c);

               sub     array   return value defn       match if defined $s->(@$c);
               ref     ref     return value truth      match if $s->(@$c);

       In reality, Table 1 covers 31 alternatives, because only
       the equality and intersection tests are commutative; in
       all other cases, the roles of the $s and $c variables
       could be reversed to produce a different test. For exam-
       ple, instead of testing a single hash for the existence of
       a series of keys ("match if exists $s->{$c}"), one could
       test for the existence of a single key in a series of
       hashes ("match if exists $c->{$s}").

       As perltodo observes, a Perl case mechanism must support
       all these "ways to do it".

DESCRIPTION
       The Switch.pm module implements a generalized case mecha-
       nism that covers the numerous possible combinations of
       switch and case values described above.

       The module augments the standard Perl syntax with two new
       control statements: "switch" and "case". The "switch"
       statement takes a single scalar argument of any type,
       specified in parentheses.  "switch" stores this value as
       the current switch value in a (localized) control vari-
       able.  The value is followed by a block which may contain
       one or more Perl statements (including the "case" state-
       ment described below).  The block is unconditionally exe-
       cuted once the switch value has been cached.



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Switch(3p)       Perl Programmers Reference Guide      Switch(3p)


       A "case" statement takes a single scalar argument (in
       mandatory parentheses if it's a variable; otherwise the
       parens are optional) and selects the appropriate type of
       matching between that argument and the current switch
       value. The type of matching used is determined by the
       respective types of the switch value and the "case" argu-
       ment, as specified in Table 1. If the match is successful,
       the mandatory block associated with the "case" statement
       is executed.

       In most other respects, the "case" statement is semanti-
       cally identical to an "if" statement. For example, it can
       be followed by an "else" clause, and can be used as a
       postfix statement qualifier.

       However, when a "case" block has been executed control is
       automatically transferred to the statement after the imme-
       diately enclosing "switch" block, rather than to the next
       statement within the block. In other words, the success of
       any "case" statement prevents other cases in the same
       scope from executing. But see "Allowing fall-through"
       below.

       Together these two new statements provide a fully general-
       ized case mechanism:

               use Switch;

               # AND LATER...

               %special = ( woohoo => 1,  d'oh => 1 );

               while (<>) {
                   switch ($_) {

                       case (%special) { print "homer\n"; }      # if $special{$_}
                       case /a-z/i     { print "alpha\n"; }      # if $_ =~ /a-z/i
                       case [1..9]     { print "small num\n"; }  # if $_ in [1..9]

                       case { $_[0] >= 10 } {                    # if $_ >= 10
                           my $age = <>;
                           switch (sub{ $_[0] < $age } ) {

                               case 20  { print "teens\n"; }     # if 20 < $age
                               case 30  { print "twenties\n"; }  # if 30 < $age
                               else     { print "history\n"; }
                           }
                       }

                       print "must be punctuation\n" case /\W/;  # if $_ ~= /\W/
               }

       Note that "switch"es can be nested within "case" (or any
       other) blocks, and a series of "case" statements can try



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Switch(3p)       Perl Programmers Reference Guide      Switch(3p)


       different types of matches -- hash membership, pattern
       match, array intersection, simple equality, etc. --
       against the same switch value.

       The use of intersection tests against an array reference
       is particularly useful for aggregating integral cases:

               sub classify_digit
               {
                       switch ($_[0]) { case 0            { return 'zero' }
                                        case [2,4,6,8]    { return 'even' }
                                        case [1,3,4,7,9]  { return 'odd' }
                                        case /[A-F]/i     { return 'hex' }
                                      }
               }

       Allowing fall-through

       Fall-though (trying another case after one has already
       succeeded) is usually a Bad Idea in a switch statement.
       However, this is Perl, not a police state, so there is a
       way to do it, if you must.

       If a "case" block executes an untargetted "next", control
       is immediately transferred to the statement after the
       "case" statement (i.e. usually another case), rather than
       out of the surrounding "switch" block.

       For example:

               switch ($val) {
                       case 1      { handle_num_1(); next }    # and try next case...
                       case "1"    { handle_str_1(); next }    # and try next case...
                       case [0..9] { handle_num_any(); }       # and we're done
                       case /\d/   { handle_dig_any(); next }  # and try next case...
                       case /.*/   { handle_str_any(); next }  # and try next case...
               }

       If $val held the number 1, the above "switch" block would
       call the first three "handle_..." subroutines, jumping to
       the next case test each time it encountered a "next".
       After the thrid "case" block was executed, control would
       jump to the end of the enclosing "switch" block.

       On the other hand, if $val held 10, then only the last two
       "handle_..."  subroutines would be called.

       Note that this mechanism allows the notion of conditional
       fall-through.  For example:

               switch ($val) {
                       case [0..9] { handle_num_any(); next if $val < 7; }
                       case /\d/   { handle_dig_any(); }
               }



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Switch(3p)       Perl Programmers Reference Guide      Switch(3p)


       If an untargetted "last" statement is executed in a case
       block, this immediately transfers control out of the
       enclosing "switch" block (in other words, there is an
       implicit "last" at the end of each normal "case" block).
       Thus the previous example could also have been written:

               switch ($val) {
                       case [0..9] { handle_num_any(); last if $val >= 7; next; }
                       case /\d/   { handle_dig_any(); }
               }

       Automating fall-through

       In situations where case fall-through should be the norm,
       rather than an exception, an endless succession of termi-
       nal "next"s is tedious and ugly.  Hence, it is possible to
       reverse the default behaviour by specifying the string
       "fallthrough" when importing the module. For example, the
       following code is equivalent to the first example in
       "Allowing fall-through":

               use Switch 'fallthrough';

               switch ($val) {
                       case 1      { handle_num_1(); }
                       case "1"    { handle_str_1(); }
                       case [0..9] { handle_num_any(); last }
                       case /\d/   { handle_dig_any(); }
                       case /.*/   { handle_str_any(); }
               }

       Note the explicit use of a "last" to preserve the non-
       fall-through behaviour of the third case.

       Alternative syntax

       Perl 6 will provide a built-in switch statement with
       essentially the same semantics as those offered by
       Switch.pm, but with a different pair of keywords. In Perl
       6 "switch" will be spelled "given", and "case" will be
       pronounced "when". In addition, the "when" statement will
       not require switch or case values to be parenthesized.

       This future syntax is also (largely) available via the
       Switch.pm module, by importing it with the argument
       "Perl6".  For example:

               use Switch 'Perl6';









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Switch(3p)       Perl Programmers Reference Guide      Switch(3p)


               given ($val) {
                       when 1       { handle_num_1(); }
                       when ($str1) { handle_str_1(); }
                       when [0..9]  { handle_num_any(); last }
                       when /\d/    { handle_dig_any(); }
                       when /.*/    { handle_str_any(); }
                       default      { handle anything else; }
               }

       Note that scalars still need to be parenthesized, since
       they would be ambiguous in Perl 5.

       Note too that you can mix and match both syntaxes by
       importing the module with:

               use Switch 'Perl5', 'Perl6';

       Higher-order Operations

       One situation in which "switch" and "case" do not provide
       a good substitute for a cascaded "if", is where a switch
       value needs to be tested against a series of conditions.
       For example:

               sub beverage {
                   switch (shift) {

                       case sub { $_[0] < 10 }  { return 'milk' }
                       case sub { $_[0] < 20 }  { return 'coke' }
                       case sub { $_[0] < 30 }  { return 'beer' }
                       case sub { $_[0] < 40 }  { return 'wine' }
                       case sub { $_[0] < 50 }  { return 'malt' }
                       case sub { $_[0] < 60 }  { return 'Moet' }
                       else                     { return 'milk' }
                   }
               }

       The need to specify each condition as a subroutine block
       is tiresome. To overcome this, when importing Switch.pm, a
       special "placeholder" subroutine named "__" [sic] may also
       be imported. This subroutine converts (almost) any expres-
       sion in which it appears to a reference to a higher-order
       function. That is, the expression:

               use Switch '__';

               __ < 2 + __

       is equivalent to:

               sub { $_[0] < 2 + $_[1] }

       With "__", the previous ugly case statements can be
       rewritten:



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Switch(3p)       Perl Programmers Reference Guide      Switch(3p)


               case  __ < 10  { return 'milk' }
               case  __ < 20  { return 'coke' }
               case  __ < 30  { return 'beer' }
               case  __ < 40  { return 'wine' }
               case  __ < 50  { return 'malt' }
               case  __ < 60  { return 'Moet' }
               else           { return 'milk' }

       The "__" subroutine makes extensive use of operator over-
       loading to perform its magic. All operations involving __
       are overloaded to produce an anonymous subroutine that
       implements a lazy version of the original operation.

       The only problem is that operator overloading does not
       allow the boolean operators "&&" and "||" to be over-
       loaded. So a case statement like this:

               case  0 <= __ && __ < 10  { return 'digit' }

       doesn't act as expected, because when it is executed, it
       constructs two higher order subroutines and then treats
       the two resulting references as arguments to "&&":

               sub { 0 <= $_[0] } && sub { $_[0] < 10 }

       This boolean expression is inevitably true, since both
       references are non-false. Fortunately, the overloaded
       'bool' operator catches this situation and flags it as a
       error.

DEPENDENCIES
       The module is implemented using Filter::Util::Call and
       Text::Balanced and requires both these modules to be
       installed.

AUTHOR
       Damian Conway (damianATconway.org). The maintainer of this
       module is now Rafael Garcia-Suarez (rgarcia-
       suarezATfree.fr).

BUGS
       There are undoubtedly serious bugs lurking somewhere in
       code this funky :-) Bug reports and other feedback are
       most welcome.

LIMITATIONS
       Due to the heuristic nature of Switch.pm's source parsing,
       the presence of regexes specified with raw "?...?" delim-
       iters may cause mysterious errors. The workaround is to
       use "m?...?" instead.

       Due to the way source filters work in Perl, you can't use
       Switch inside an string "eval".




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Switch(3p)       Perl Programmers Reference Guide      Switch(3p)


       If your source file is longer then 1 million characters
       and you have a switch statement that crosses the 1 million
       (or 2 million, etc.)  character boundary you will get mys-
       terious errors. The workaround is to use smaller source
       files.

COPYRIGHT
           Copyright (c) 1997-2003, Damian Conway. All Rights Reserved.
           This module is free software. It may be used, redistributed
               and/or modified under the same terms as Perl itself.















































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