Switch(3pm) Perl Programmers Reference Guide Switch(3pm)
NAME
Switch - A switch statement for Perl
VERSION
This document describes version 2.10 of Switch, released Dec 29, 2003.
SYNOPSIS
use Switch;
switch ($val) {
case 1 { print "number 1" }
case "a" { print "string a" }
case [1..10,42] { print "number in list" }
case (@array) { print "number in list" }
case /\w+/ { print "pattern" }
case qr/\w+/ { print "pattern" }
case (%hash) { print "entry in hash" }
case (\%hash) { print "entry in hash" }
case (\&sub) { print "arg to subroutine" }
else { print "previous case not true" }
}
BACKGROUND
[Skip ahead to "DESCRIPTION" if you don't care about the whys and wherefores of this control struc-ture] structure]
ture]
In seeking to devise a "Swiss Army" case mechanism suitable for Perl, it is useful to generalize this
notion of distributed conditional testing as far as possible. Specifically, the concept of "matching"
between the switch value and the various case values need not be restricted to numeric (or string or
referential) equality, as it is in other languages. Indeed, as Table 1 illustrates, Perl offers at
least eighteen different ways in which two values could generate a match.
Table 1: Matching a switch value ($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 com-mutative; commutative;
mutative; 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 combi-nations combinations
nations 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; oth-erwise otherwise
erwise 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 manda-tory mandatory
tory 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 exe-cuting. executing.
cuting. 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 $_ ~= /\W/
}
Note that "switch"es can be nested within "case" (or any other) blocks, and a series of "case" state-
ments can try different types of matches -- hash membership, pattern match, array intersection, sim-ple simple
ple equality, etc. -- against the same switch value.
The use of intersection tests against an array reference is particularly useful for aggregating inte-gral integral
gral cases:
sub classify_digit
{
switch ($_[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 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_..." subrou-tines, subroutines,
tines, 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 $val < 7; }
case /\d/ { handle_dig_any(); }
}
If an untargeted "last" statement is executed in a case block, this immediately transfers control out
of the enclosing "switch" block (in other words, there is an implicit "last" at the end of each nor-
mal "case" block). Thus the previous example could also have been written:
switch ($val) {
case [0..9] { handle_num_any(); last if $val >= 7; next; }
case /\d/ { handle_dig_any(); }
}
Automating fall-through
In situations where case fall-through should be the norm, rather than an exception, an endless suc-cession succession
cession of terminal "next"s is tedious and ugly. Hence, it is possible to reverse the default behav-iour behaviour
iour by specifying the string "fallthrough" when importing the module. For example, the following
code is equivalent to the first example in "Allowing fall-through":
use Switch 'fallthrough';
switch ($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 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 import-
ing Switch.pm, a special "placeholder" subroutine named "__" [sic] may also be imported. This subrou-tine subroutine
tine 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). The maintainer of this module is now Rafael Garcia-Suarez (rgarci-
asuarez@free.fr).
BUGS
There are undoubtedly serious bugs lurking somewhere in code this funky :-) Bug reports and other
feedback are most welcome.
LIMITATIONS
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.
Due to the way source filters work in Perl, you can't use Switch inside an string "eval".
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) 1997-2003, 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 v5.8.8 2001-09-21 Switch(3pm)
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