package Switch;

use 5.005;

use strict;

use vars qw($VERSION);

use Carp;

use if $] >= 5.011, 'deprecate';

$VERSION = '2.17';

# LOAD FILTERING MODULE...

use Filter::Util::Call;

sub __();

# CATCH ATTEMPTS TO CALL case OUTSIDE THE SCOPE OF ANY switch

$::_S_W_I_T_C_H = sub { croak "case/when statement not in switch/given block" };

my $offset;

my $fallthrough;

my ($Perl5, $Perl6) = (0,0);

sub import

{

	my ($class) = @_;

	my $self = bless {}, $class;

	$fallthrough = grep /\bfallthrough\b/, @_;

	$offset = (caller)[2]+1;

	filter_add($self) unless @_>1 && $_[1] eq 'noimport';

	my $pkg = caller;

	no strict 'refs';

	for ( qw( on_defined on_exists ) )

	{

		*{"${pkg}::$_"} = \&$;;

	}

	*{"${pkg}::__"} = \&__ if grep /__/, @_;

	$Perl6 = 1 if grep(/Perl\s*6/i, @_);

	$Perl5 = 1 if grep(/Perl\s*5/i, @_) || !grep(/Perl\s*6/i, @_);

	1;

}

sub unimport

{	

	filter_del()

}

sub filter

{

	my($self) = @_ ;

	local $Switch::file = (caller)[1];

	my $status = 1;

	$status = filter_read(1_000_000);

	return $status if $status<0;

    	$_ = filter_blocks($_,$offset);

	$_ = "# line $offset\n" . $_ if $offset; undef $offset;

	return $status;

}

use Text::Balanced ':ALL';

sub line

{

	my ($pretext,$offset) = @_;

	($pretext=~tr/\n/\n/)+($offset||0);

}

sub is_block

{

	local $SIG{__WARN__}=sub{die$@};

	local $^W=1;

	my $ishash = defined  eval 'my $hr='.$_[0];

	undef $@;

	return !$ishash;

}

my $pod_or_DATA = qr/ ^=[A-Za-z] .*? ^=cut (?![A-Za-z]) .*? $

		    | ^__(DATA|END)__\n.*

		    /smx;

my $casecounter = 1;

sub filter_blocks

{

	my ($source, $line) = @_;

	return $source unless $Perl5 && $source =~ /case|switch/

			   || $Perl6 && $source =~ /when|given|default/;

	pos $source = 0;

	my $text = "";

	component: while (pos $source < length $source)

	{

		if ($source =~ m/(\G\s*use\s+Switch\b)/gc)

		{

			$text .= q{use Switch 'noimport'};

			next component;

		}

		my @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,0);

		if (defined $pos[0])

		{

			my $pre = substr($source,$pos[0],$pos[1]); # matched prefix

                        my $iEol;

                        if( substr($source,$pos[4],$pos[5]) eq '/' && # 1st delimiter

                            substr($source,$pos[2],$pos[3]) eq '' && # no op like 'm'

                            index( substr($source,$pos[16],$pos[17]), 'x' ) == -1 && # no //x

                            ($iEol = index( $source, "\n", $pos[4] )) > 0         &&

                            $iEol < $pos[8] ){ # embedded newlines

                            # If this is a pattern, it isn't compatible with Switch. Backup past 1st '/'.

                            pos( $source ) = $pos[6];

			    $text .= $pre . substr($source,$pos[2],$pos[6]-$pos[2]);

			} else {

			    $text .= $pre . substr($source,$pos[2],$pos[18]-$pos[2]);

			}

			next component;

		}

		if ($source =~ m/(\G\s*$pod_or_DATA)/gc) {

			$text .= $1;

			next component;

		}

		@pos = Text::Balanced::_match_variable(\$source,qr/\s*/);

		if (defined $pos[0])

		{

			$text .= " " if $pos[0] < $pos[2];

			$text .= substr($source,$pos[0],$pos[4]-$pos[0]);

			next component;

		}

		if ($Perl5 && $source =~ m/\G(\n*)(\s*)(switch)\b(?=\s*[(])/gc

		 || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(?=\s*[(])/gc

		 || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(.*)(?=\{)/gc)

		{

			my $keyword = $3;

			my $arg = $4;

			$text .= $1.$2.'S_W_I_T_C_H: while (1) ';

			unless ($arg) {

				@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\(/,qr/\)/,qr/[[{(<]/,qr/[]})>]/,undef) 

				or do {

					die "Bad $keyword statement (problem in the parentheses?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n";

				};

				$arg = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line));

			}

			$arg =~ s {^\s*[(]\s*%}   { ( \\\%}	||

			$arg =~ s {^\s*[(]\s*m\b} { ( qr}	||

			$arg =~ s {^\s*[(]\s*/}   { ( qr/}	||

			$arg =~ s {^\s*[(]\s*qw}  { ( \\qw};

			@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)

			or do {

				die "Bad $keyword statement (problem in the code block?) near $Switch::file line ", line(substr($source,0, pos $source), $line), "\n";

			};

			my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line));

			$code =~ s/{/{ local \$::_S_W_I_T_C_H; Switch::switch($arg);/;

			$text .= $code . 'continue {last}';

			next component;

		}

		elsif ($Perl5 && $source =~ m/\G(\s*)(case\b)(?!\s*=>)/gc

		    || $Perl6 && $source =~ m/\G(\s*)(when\b)(?!\s*=>)/gc

		    || $Perl6 && $source =~ m/\G(\s*)(default\b)(?=\s*\{)/gc)

		{

			my $keyword = $2;

			$text .= $1 . ($keyword eq "default"

					? "if (1)"

					: "if (Switch::case");

			if ($keyword eq "default") {

				# Nothing to do

			}

			elsif (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)) {

				my $code = substr($source,$pos[0],$pos[4]-$pos[0]);

				$text .= " " if $pos[0] < $pos[2];

				$text .= "sub " if is_block $code;

				$text .= filter_blocks($code,line(substr($source,0,$pos[0]),$line)) . ")";

			}

			elsif (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/[[(]/,qr/[])]/,qr/[[({]/,qr/[])}]/,undef)) {

				my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line));

				$code =~ s {^\s*[(]\s*%}   { ( \\\%}	||

				$code =~ s {^\s*[(]\s*m\b} { ( qr}	||

				$code =~ s {^\s*[(]\s*/}   { ( qr/}	||

				$code =~ s {^\s*[(]\s*qw}  { ( \\qw};

				$text .= " " if $pos[0] < $pos[2];

				$text .= "$code)";

			}

			elsif ($Perl6 && do{@pos = Text::Balanced::_match_variable(\$source,qr/\s*/)}) {

				my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line));

				$code =~ s {^\s*%}  { \%}	||

				$code =~ s {^\s*@}  { \@};

				$text .= " " if $pos[0] < $pos[2];

				$text .= "$code)";

			}

			elsif ( @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,0)) {

				my $code = substr($source,$pos[2],$pos[18]-$pos[2]);

				$code = filter_blocks($code,line(substr($source,0,$pos[2]),$line));

				$code =~ s {^\s*m}  { qr}	||

				$code =~ s {^\s*/}  { qr/}	||

				$code =~ s {^\s*qw} { \\qw};

				$text .= " " if $pos[0] < $pos[2];

				$text .= "$code)";

			}

			elsif ($Perl5 && $source =~ m/\G\s*(([^\$\@{])[^\$\@{]*)(?=\s*{)/gc

			   ||  $Perl6 && $source =~ m/\G\s*([^;{]*)()/gc) {

				my $code = filter_blocks($1,line(substr($source,0,pos $source),$line));

				$text .= ' \\' if $2 eq '%';

				$text .= " $code)";

			}

			else {

				die "Bad $keyword statement (invalid $keyword value?) near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n";

			}

		        die "Missing opening brace or semi-colon after 'when' value near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n"

				unless !$Perl6 || $source =~ m/\G(\s*)(?=;|\{)/gc;

			do{@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)}

			or do {

				if ($source =~ m/\G\s*(?=([};]|\Z))/gc) {

					$casecounter++;

					next component;

				}

				die "Bad $keyword statement (problem in the code block?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n";

			};

			my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line));

			$code =~ s/}(?=\s*\Z)/;last S_W_I_T_C_H }/

				unless $fallthrough;

			$text .= "{ while (1) $code continue { goto C_A_S_E_$casecounter } last S_W_I_T_C_H; C_A_S_E_$casecounter: }";

			$casecounter++;

			next component;

		}

		$source =~ m/\G(\s*(-[sm]\s+|\w+|#.*\n|\W))/gc;

		$text .= $1;

	}

	$text;

}

sub in

{

	my ($x,$y) = @_;

	my @numy;

	for my $nextx ( @$x )

	{

		my $numx = ref($nextx) || defined $nextx && (~$nextx&$nextx) eq 0;

		for my $j ( 0..$#$y )

		{

			my $nexty = $y->[$j];

			push @numy, ref($nexty) || defined $nexty && (~$nexty&$nexty) eq 0

				if @numy <= $j;

			return 1 if $numx && $numy[$j] && $nextx==$nexty

			         || $nextx eq $nexty;

		}

	}

	return "";

}

sub on_exists

{

	my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ };

	[ keys %$ref ]

}

sub on_defined

{

	my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ };

	[ grep { defined $ref->{$_} } keys %$ref ]

}

sub switch(;$)

{

	my ($s_val) = @_ ? $_[0] : $_;

	my $s_ref = ref $s_val;

	if ($s_ref eq 'CODE')

	{

		$::_S_W_I_T_C_H =

		      sub { my $c_val = $_[0];

			    return $s_val == $c_val  if ref $c_val eq 'CODE';

			    return $s_val->(@$c_val) if ref $c_val eq 'ARRAY';

			    return $s_val->($c_val);

			  };

	}

	elsif ($s_ref eq "" && defined $s_val && (~$s_val&$s_val) eq 0)	# NUMERIC SCALAR

	{

		$::_S_W_I_T_C_H =

		      sub { my $c_val = $_[0];

			    my $c_ref = ref $c_val;

			    return $s_val == $c_val 	if $c_ref eq ""

							&& defined $c_val

							&& (~$c_val&$c_val) eq 0;

			    return $s_val eq $c_val 	if $c_ref eq "";

			    return in([$s_val],$c_val)	if $c_ref eq 'ARRAY';

			    return $c_val->($s_val)	if $c_ref eq 'CODE';

			    return $c_val->call($s_val)	if $c_ref eq 'Switch';

			    return scalar $s_val=~/$c_val/

							if $c_ref eq 'Regexp';

			    return scalar $c_val->{$s_val}

							if $c_ref eq 'HASH';

		            return;	

			  };

	}

	elsif ($s_ref eq "")				# STRING SCALAR

	{

		$::_S_W_I_T_C_H =

		      sub { my $c_val = $_[0];

			    my $c_ref = ref $c_val;

			    return $s_val eq $c_val 	if $c_ref eq "";

			    return in([$s_val],$c_val)	if $c_ref eq 'ARRAY';

			    return $c_val->($s_val)	if $c_ref eq 'CODE';

			    return $c_val->call($s_val)	if $c_ref eq 'Switch';

			    return scalar $s_val=~/$c_val/

							if $c_ref eq 'Regexp';

			    return scalar $c_val->{$s_val}

							if $c_ref eq 'HASH';

		            return;	

			  };

	}

	elsif ($s_ref eq 'ARRAY')

	{

		$::_S_W_I_T_C_H =

		      sub { my $c_val = $_[0];

			    my $c_ref = ref $c_val;

			    return in($s_val,[$c_val]) 	if $c_ref eq "";

			    return in($s_val,$c_val)	if $c_ref eq 'ARRAY';

			    return $c_val->(@$s_val)	if $c_ref eq 'CODE';

			    return $c_val->call(@$s_val)

							if $c_ref eq 'Switch';

			    return scalar grep {$_=~/$c_val/} @$s_val

							if $c_ref eq 'Regexp';

			    return scalar grep {$c_val->{$_}} @$s_val

							if $c_ref eq 'HASH';

		            return;	

			  };

	}

	elsif ($s_ref eq 'Regexp')

	{

		$::_S_W_I_T_C_H =

		      sub { my $c_val = $_[0];

			    my $c_ref = ref $c_val;

			    return $c_val=~/s_val/ 	if $c_ref eq "";

			    return scalar grep {$_=~/s_val/} @$c_val

							if $c_ref eq 'ARRAY';

			    return $c_val->($s_val)	if $c_ref eq 'CODE';

			    return $c_val->call($s_val)	if $c_ref eq 'Switch';

			    return $s_val eq $c_val	if $c_ref eq 'Regexp';

			    return grep {$_=~/$s_val/ && $c_val->{$_}} keys %$c_val

							if $c_ref eq 'HASH';

		            return;	

			  };

	}

	elsif ($s_ref eq 'HASH')

	{

		$::_S_W_I_T_C_H =

		      sub { my $c_val = $_[0];

			    my $c_ref = ref $c_val;

			    return $s_val->{$c_val} 	if $c_ref eq "";

			    return scalar grep {$s_val->{$_}} @$c_val

							if $c_ref eq 'ARRAY';

			    return $c_val->($s_val)	if $c_ref eq 'CODE';

			    return $c_val->call($s_val)	if $c_ref eq 'Switch';

			    return grep {$_=~/$c_val/ && $s_val->{"$_"}} keys %$s_val

							if $c_ref eq 'Regexp';

			    return $s_val==$c_val	if $c_ref eq 'HASH';

		            return;	

			  };

	}

	elsif ($s_ref eq 'Switch')

	{

		$::_S_W_I_T_C_H =

		      sub { my $c_val = $_[0];

			    return $s_val == $c_val  if ref $c_val eq 'Switch';

			    return $s_val->call(@$c_val)

						     if ref $c_val eq 'ARRAY';

			    return $s_val->call($c_val);

			  };

	}

	else

	{

		croak "Cannot switch on $s_ref";

	}

	return 1;

}

sub case($) { local $SIG{__WARN__} = \&carp;

	      $::_S_W_I_T_C_H->(@_); }

# IMPLEMENT __

my $placeholder = bless { arity=>1, impl=>sub{$_[1+$_[0]]} };

sub __() { $placeholder }

sub __arg($)

{

	my $index = $_[0]+1;

	bless { arity=>0, impl=>sub{$_[$index]} };

}

sub hosub(&@)

{

	# WRITE THIS

}

sub call

{

	my ($self,@args) = @_;

	return $self->{impl}->(0,@args);

}

sub meta_bop(&)

{

	my ($op) = @_;

	sub

	{

		my ($left, $right, $reversed) = @_;

		($right,$left) = @_ if $reversed;

		my $rop = ref $right eq 'Switch'

			? $right

			: bless { arity=>0, impl=>sub{$right} };

		my $lop = ref $left eq 'Switch'

			? $left

			: bless { arity=>0, impl=>sub{$left} };

		my $arity = $lop->{arity} + $rop->{arity};

		return bless {

				arity => $arity,

				impl  => sub { my $start = shift;

					       return $op->($lop->{impl}->($start,@_),

						            $rop->{impl}->($start+$lop->{arity},@_));

					     }

			     };

	};

}

sub meta_uop(&)

{

	my ($op) = @_;

	sub

	{

		my ($left) = @_;

		my $lop = ref $left eq 'Switch'

			? $left

			: bless { arity=>0, impl=>sub{$left} };

		my $arity = $lop->{arity};

		return bless {

				arity => $arity,

				impl  => sub { $op->($lop->{impl}->(@_)) }

			     };

	};

}

use overload

	"+"	=> 	meta_bop {$_[0] + $_[1]},

	"-"	=> 	meta_bop {$_[0] - $_[1]},  

	"*"	=>  	meta_bop {$_[0] * $_[1]},

	"/"	=>  	meta_bop {$_[0] / $_[1]},

	"%"	=>  	meta_bop {$_[0] % $_[1]},

	"**"	=>  	meta_bop {$_[0] ** $_[1]},

	"<<"	=>  	meta_bop {$_[0] << $_[1]},

	">>"	=>  	meta_bop {$_[0] >> $_[1]},

	"x"	=>  	meta_bop {$_[0] x $_[1]},

	"."	=>  	meta_bop {$_[0] . $_[1]},

	"<"	=>  	meta_bop {$_[0] < $_[1]},

	"<="	=>  	meta_bop {$_[0] <= $_[1]},

	">"	=>  	meta_bop {$_[0] > $_[1]},

	">="	=>  	meta_bop {$_[0] >= $_[1]},

	"=="	=>  	meta_bop {$_[0] == $_[1]},

	"!="	=>  	meta_bop {$_[0] != $_[1]},

	"<=>"	=>  	meta_bop {$_[0] <=> $_[1]},

	"lt"	=>  	meta_bop {$_[0] lt $_[1]},

	"le"	=> 	meta_bop {$_[0] le $_[1]},

	"gt"	=> 	meta_bop {$_[0] gt $_[1]},

	"ge"	=> 	meta_bop {$_[0] ge $_[1]},

	"eq"	=> 	meta_bop {$_[0] eq $_[1]},

	"ne"	=> 	meta_bop {$_[0] ne $_[1]},

	"cmp"	=> 	meta_bop {$_[0] cmp $_[1]},

	"\&"	=> 	meta_bop {$_[0] & $_[1]},

	"^"	=> 	meta_bop {$_[0] ^ $_[1]},

	"|"	=>	meta_bop {$_[0] | $_[1]},

	"atan2"	=>	meta_bop {atan2 $_[0], $_[1]},

	"neg"	=>	meta_uop {-$_[0]},

	"!"	=>	meta_uop {!$_[0]},

	"~"	=>	meta_uop {~$_[0]},

	"cos"	=>	meta_uop {cos $_[0]},

	"sin"	=>	meta_uop {sin $_[0]},

	"exp"	=>	meta_uop {exp $_[0]},

	"abs"	=>	meta_uop {abs $_[0]},

	"log"	=>	meta_uop {log $_[0]},

	"sqrt"  =>	meta_uop {sqrt $_[0]},

	"bool"  =>	sub { croak "Can't use && or || in expression containing __" },

	#	"&()"	=>	sub { $_[0]->{impl} },

	#	"||"	=>	meta_bop {$_[0] || $_[1]},

	#	"&&"	=>	meta_bop {$_[0] && $_[1]},

	# fallback => 1,

	;

1;

__END__

=head1 NAME

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

=head1 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" }

    }

=head1 BACKGROUND

[Skip ahead to L<"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 C<$s> and C<$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 (C<match if exists $s-E<gt>{$c}>),

one could test for the existence of a single key in a series of hashes

(C<match if exists $c-E<gt>{$s}>).

=head1 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: C<switch> and C<case>. The C<switch> statement takes a

single scalar argument of any type, specified in parentheses.

C<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 C<case> statement described below).

The block is unconditionally executed once the switch value has

been cached.

A C<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 C<case> argument, as

specified in Table 1. If the match is successful, the mandatory

block associated with the C<case> statement is executed.

In most other respects, the C<case> statement is semantically identical

to an C<if> statement. For example, it can be followed by an C<else>

clause, and can be used as a postfix statement qualifier. 

However, when a C<case> block has been executed control is automatically

transferred to the statement after the immediately enclosing C<switch>

block, rather than to the next statement within the block. In other

words, the success of any C<case> statement prevents other cases in the

same scope from executing. But see L<"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 C<switch>es can be nested within C<case> (or any other) blocks,

and a series of C<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' }

                               }

        }

=head2 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 I<is> a way to do it, if you must.

If a C<case> block executes an untargeted C<next>, control is

immediately transferred to the statement I<after> the C<case> statement

(i.e. usually another case), rather than out of the surrounding

C<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 C<1>, the above C<switch> block would call the

first three C<handle_...> subroutines, jumping to the next case test

each time it encountered a C<next>. After the third C<case> block

was executed, control would jump to the end of the enclosing

C<switch> block.

On the other hand, if $val held C<10>, then only the last two C<handle_...>

subroutines would be called.

Note that this mechanism allows the notion of I<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 C<last> statement is executed in a case block, this

immediately transfers control out of the enclosing C<switch> block

(in other words, there is an implicit C<last> at the end of each

normal C<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(); }

        }

=head2 Automating fall-through

In situations where case fall-through should be the norm, rather than an

exception, an endless succession of terminal C<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 L<"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 C<last> to preserve the non-fall-through

behaviour of the third case.

=head2 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 C<switch> will be spelled C<given>, and

C<case> will be pronounced C<when>. In addition, the C<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 C<"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';

=head2 Higher-order Operations

One situation in which C<switch> and C<case> do not provide a good

substitute for a cascaded C<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 C<case (sub { $_[0] < 10 })>, etc.; C<$_[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 C<__> [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 C<__>, 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 C<__> 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 C<&&> and C<||> 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 C<&&>:

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

This boolean expression is inevitably true, since both references are

non-false. Fortunately, the overloaded C<'bool'> operator catches this

situation and flags it as an error. 

=head1 DEPENDENCIES

The module is implemented using Filter::Util::Call and Text::Balanced

and requires both these modules to be installed. 

=head1 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.

=head1 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.

=head1 LIMITATIONS

Due to the heuristic nature of Switch.pm's source parsing, the presence of

regexes with embedded newlines that are specified with raw C</.../>

delimiters and don't have a modifier C<//x> are indistinguishable from

code chunks beginning with the division operator C. As a workaround

you must use C<m/.../> or C<m?...?> for such patterns. Also, the presence

of regexes specified with raw C delimiters may cause mysterious

errors. The workaround is to use C<m?...?> instead.

Due to the way source filters work in Perl, you can't use Switch inside

an string C<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