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

       Switch - A switch statement for Perl

       This document describes version 2.09 of Switch, released June 12, 2002.

	       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" }

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

       In seeking to devise a "Swiss Army" case mechanism suitable for Perl, it is useful to gen-
       eralize 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
       values 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

	       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

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

	       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;

	       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 example, 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".

       The Switch.pm module implements a generalized case mechanism that covers the numerous pos-
       sible 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) con-
       trol variable.  The value is followed by a block which may contain one or more Perl state-
       ments (including the "case" statement described below).	The block is unconditionally exe-
       cuted once the switch value has been cached.

       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 deter-
       mined by the respective types of the switch value and the "case" argument, as specified in
       Table 1. If the match is successful, the mandatory block associated with the "case" state-
       ment is executed.

       In most other respects, the "case" statement is semantically identical to an "if" state-
       ment. 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 immediately enclosing "switch" block, rather than to the next state-
       ment 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 generalized 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 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 aggre-
       gating 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 "han-
       dle_..." 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(); }

       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

	       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 terminal "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

	       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

       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" state-
       ment 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';

	       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(); }

       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 exam-

	       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 expression in which it appears to a refer-
       ence 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:

	       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 overloading 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 overloaded. 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 sub-
       routines 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. Fortu-
       nately, the overloaded 'bool' operator catches this situation and flags it as a error.

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

       Damian Conway (damian@conway.org)

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

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

	   Copyright (c) 1997-2001, 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.

perl v5.8.0				    2002-06-01				      Switch(3pm)
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