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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 ($val) {
case 1 { print "number 1" }
case "a" { print "string a" }
case [1..10,42] { print "number in list" }
case (@array) { print "number in list" }
case /\w+/ { print "pattern" }
case qr/\w+/ { print "pattern" }
case (%hash) { print "entry in hash" }
case (\%hash) { print "entry in hash" }
case (<!>sub) { print "arg to subroutine" }
else { print "previous case not true" }
}
BACKGROUND
[Skip ahead to "DESCRIPTION" if you don't care about the whys and
wherefores of this control structure]
In seeking to devise a "Swiss Army" case mechanism suitable for Perl, it is
useful to generalize this notion of distributed conditional testing as far
as possible. Specifically, the concept of "matching" between the switch
value and the various case values need not be restricted to numeric (or
string or referential) equality, as it is in other languages. Indeed, as
Table 1 illustrates, Perl offers at least eighteen different ways in which
two values could generate a match.
Table 1: Matching a switch value ($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}").
As perltodo observes, a Perl case mechanism must support all these "ways to
do it".
DESCRIPTION
The Switch.pm module implements a generalized case mechanism 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 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 (<>) {
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 $_ ~= /\W/
}
Note that "switch"es can be nested within "case" (or any other) blocks, and
a series of "case" statements can try different types of matches -- hash
membership, pattern match, array intersection, simple equality, etc. --
against the same switch value.
The use of intersection tests against an array reference is particularly
useful for aggregating integral cases:
sub classify_digit
{
switch ($_[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 $val < 7; }
case /\d/ { handle_dig_any(); }
}
If an untargetted "last" statement is executed in a case block, this
immediately transfers control out of the enclosing "switch" block (in other
words, there is an implicit "last" at the end of each normal "case" block).
Thus the previous example could also have been written:
switch ($val) {
case [0..9] { handle_num_any(); last if $val >= 7; next; }
case /\d/ { handle_dig_any(); }
}
Automating fall-through
In situations where case fall-through should be the norm, rather than an
exception, an endless succession of terminal "next"s is tedious and ugly.
Hence, it is possible to reverse the default behaviour by specifying the
string "fallthrough" when importing the module. For example, the following
code is equivalent to the first example in "Allowing fall-through":
use Switch 'fallthrough';
switch ($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(); }
}
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 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 + $_[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 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 "?...?" delimiters may cause mysterious errors.
The workaround is to use "m?...?" instead.
COPYRIGHT
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.
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