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Switch(3)	      User Contributed Perl Documentation	     Switch(3)

NAME
       Switch -	A switch statement for Perl, do	not use	if you can use
       given/when

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 (\&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 suitable 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 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
				$c->[$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 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}").

DESCRIPTION
       The Switch.pm module implements a generalized case mechanism that
       covers most (but	not all) of 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
       variable.  The value is followed	by a block which may contain one or
       more Perl statements (including the "case" statement described below).
       The block is unconditionally executed 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 determined 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" statement is executed.

       In most other respects, the "case" statement is semantically 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 immediately 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 generalized case
       mechanism:

	       use Switch;

	       # AND LATER...

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

	       while (<>) {
		   chomp;
		   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 } { print "big num\n"; } # if	$_ >= 10
		       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 aggregating integral cases:

	       sub classify_digit
	       {
		       switch ($_[0]) {	case 0		  { return 'zero' }
					case [2,4,6,8]	  { return 'even' }
					case [1,3,5,7,9]  { 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 untargeted	"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 third "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 untargeted	"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 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 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';

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

       (This is	equivalent to writing "case (sub { $_[0] < 10 })", etc.; $_[0]
       is the argument to the anonymous	subroutine.)

       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 reference to
       a higher-order function.	That is, the expression:

	       use Switch '__';

	       __ < 2

       is equivalent to:

	       sub { $_[0] < 2 }

       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 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 an error.

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

AUTHOR
       Damian Conway (damian@conway.org). This module is now maintained	by
       Alexandr	Ciornii	(alexchorny@gmail.com).	Previously was maintained by
       Rafael Garcia-Suarez and	perl5 porters.

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

       May create syntax errors	in other parts of code.

       On perl 5.10.x may cause	syntax error if	"case" is present inside
       heredoc.

       In general, use given/when instead. It were introduced in perl 5.10.0.
       Perl 5.10.0 was released	in 2007.

LIMITATIONS
       Due to the heuristic nature of Switch.pm's source parsing, the presence
       of regexes with embedded	newlines that are specified with raw "/.../"
       delimiters and don't have a modifier "//x" are indistinguishable	from
       code chunks beginning with the division operator	"/". As	a workaround
       you must	use "m/.../" or	"m?...?" for such patterns. Also, the presence
       of regexes specified with raw "?...?" delimiters	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".

       May not work if sub prototypes are used (RT#33988).

       Regex captures in when are not available	to code.

       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 mysterious errors. The workaround is to
       use smaller source files.

COPYRIGHT
	   Copyright (c) 1997-2008, 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 v5.32.1			  2021-02-28			     Switch(3)

NAME | SYNOPSIS | BACKGROUND | DESCRIPTION | DEPENDENCIES | AUTHOR | BUGS | LIMITATIONS | COPYRIGHT

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