switch(3)

NAME

Switch - A switch statement for Perl

VERSION

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

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 /624
        case qr/576
        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
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.

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
immediately enclosing "switch" block, rather than to the
next statement within the block. In other words, the suc
cess 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 "homer0; } #

if $special{$_}
case /a-z/i { print "alpha0; } #

if $_ =~ /a-z/i
case [1..9] { print "small num0; } #

if $_ in [1..9]

case { $_[0] >= 10 } {

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

case 20 { print "teens0; } #

if 20 < $age
case 30 { print "twenties0; } #

if 30 < $age
else { print "history0; }

}

}

print "must be punctuation0 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 { re

turn 'zero' }
case [2,4,6,8] { re

turn 'even' }
case [1,3,4,7,9] { re

turn 'odd' }
case /[A-F]/i { re

turn '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 // { 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

$val < 7; }
case // { handle_dig_any(); }; }
If an untargetted "last" statement is executed in a case block, this immediately transfers control out of the enclosing "switch" block (in other words, there is an implicit "last" at the end of each normal "case" block). Thus the previous example could also have been written:: switch ($val) {
case [0..9] { handle_num_any(); last if

$val >= 7; next; }
case // { handle_dig_any(); }; }
Automating fall-through
In situations where case fall-through should be the norm, rather than an exception, an endless succession of termi nal "next"s is tedious and ugly. Hence, it is possible to reverse the default behaviour by specifying the string "fallthrough" when importing the module. For example, the following code is equivalent to the first example in "Allowing fall-through":: use Switch 'fallthrough';; switch ($val) {
case 1 { handle_num_1(); }
case "1" { handle_str_1(); }
case [0..9] { handle_num_any(); last }
case // { handle_dig_any(); }
case /.*/ { handle_str_any(); }; }
Note the explicit use of a "last" to preserve the nonfall-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 // { 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 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:: 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 (damian@conway.org)

BUGS

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

LIMITATION

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.

COPYRIGHT

served. This module is free software. It may be used, redis
tributed: and/or modified under the same terms as Perl it; self.

docs.sk

comprehensive documentation repository

In section