Switch(3p) Perl Programmers Reference Guide Switch(3p)NAMESwitch - A switch statement for Perl
VERSION
This document describes version 2.10 of Switch, released Dec
29, 2003.
SYNOPSIS
use Switch;
switch ($val) {
case 1 { print "number 1" }
case "a" { print "string a" }
case [1..10,42] { print "number in list" }
case (@array) { print "number in list" }
case /\w+/ { print "pattern" }
case qr/\w+/ { print "pattern" }
case (%hash) { print "entry in hash" }
case (\%hash) { print "entry in hash" }
case (\&sub) { print "arg to subroutine" }
else { print "previous case not true" }
}
BACKGROUND
[Skip ahead to "DESCRIPTION" if you don't care about the
whys and wherefores of this control structure]
In seeking to devise a "Swiss Army" case mechanism suitable
for Perl, it is useful to generalize this notion of distri-
buted conditional testing as far as possible. Specifically,
the concept of "matching" between the switch value and the
various case values need not be restricted to numeric (or
string or referential) equality, as it is in other
languages. Indeed, as Table 1 illustrates, Perl offers at
least eighteen different ways in which two values could gen-
erate a match.
Table 1: Matching a switch value ($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
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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" state-
ment takes a single scalar argument of any type, specified
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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 manda-
tory 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 fol-
lowed 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 immedi-
ately 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 general-
ized 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 } ) {
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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 suc-
ceeded) is usually a Bad Idea in a switch statement. How-
ever, 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
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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 $val < 7; }
case /\d/ { handle_dig_any(); }
}
If an untargeted "last" statement is executed in a case
block, this immediately transfers control out of the enclos-
ing "switch" block (in other words, there is an implicit
"last" at the end of each normal "case" block). Thus the
previous example could also have been written:
switch ($val) {
case [0..9] { handle_num_any(); last if $val >= 7; next; }
case /\d/ { handle_dig_any(); }
}
Automating fall-through
In situations where case fall-through should be the norm,
rather than an exception, an endless succession of terminal
"next"s is tedious and ugly. Hence, it is possible to
reverse the default behaviour by specifying the string
"fallthrough" when importing the module. For example, the
following code is equivalent to the first example in "Allow-
ing fall-through":
use Switch 'fallthrough';
switch ($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 essen-
tially the same semantics as those offered by Switch.pm, but
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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 import-
ing 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 exam-
ple:
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 spe-
cial "placeholder" subroutine named "__" [sic] may also be
imported. This subroutine converts (almost) any expression
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in which it appears to a reference to a higher-order func-
tion. That is, the expression:
use Switch '__';
__ < 2 + __
is equivalent to:
sub { $_[0] < 2 + $_[1] }
With "__", the previous ugly case statements can be rewrit-
ten:
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 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 (rgarciasuarez@free.fr).
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There are undoubtedly serious bugs lurking somewhere in code
this funky :-) Bug reports and other feedback are most wel-
come.
LIMITATIONS
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.
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.
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