Switch - A switch statement for Perl
This document describes version 2.01 of Switch, released January 9, 2001.
use Switch;
switch ($val) {
case 1 { print "number 1" }
case "a" { print "string a" }
case [1..10,42] { print "number in list" }
case (@array) { print "number in list" }
case /\w+/ { print "pattern" }
case qr/\w+/ { print "pattern" }
case (%hash) { print "entry in hash" }
case (\%hash) { print "entry in hash" }
case (\&sub) { print "arg to subroutine" }
else { print "previous case not true" }
}[Skip ahead to "DESCRIPTION" if you don't care about the whys and wherefores of this control structure]
In seeking to devise a "Swiss Army" case mechanism suitable for Perl, it is useful to generalize this notion of distributed conditional testing as far as possible. Specifically, the concept of "matching" between the switch value and the various case values need not be restricted to numeric (or string or referential) equality, as it is in other languages. Indeed, as Table 1 illustrates, Perl offers at least eighteen different ways in which two values could generate a match.
Table 1: Matching a switch value ($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".
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 $_ ~= /\W/
}Note that switches can be nested within case (or any other) blocks, and a series of case statements can try different types of matches -- hash membership, pattern match, array intersection, simple equality, etc. -- against the same switch value.
The use of intersection tests against an array reference is particularly useful for aggregating integral cases:
sub classify_digit
{
switch ($_[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' }
}
}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 $val >= 7; next; }
case /\d/ { handle_dig_any(); }
}In situations where case fall-through should be the norm, rather than an exception, an endless succession of terminal nexts is tedious and ugly. Hence, it is possible to reverse the default behaviour by specifying the string "fallthrough" when importing the module. For example, the following code is equivalent to the first example in "Allowing fall-through":
use Switch 'fallthrough';
switch ($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.
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.
The module is implemented using Filter::Util::Call and Text::Balanced and requires both these modules to be installed.
Damian Conway (damian@conway.org)
There are undoubtedly serious bugs lurking somewhere in code this funky :-) Bug reports and other feedback are most welcome.
Copyright (c) 1997-2000, Damian Conway. All Rights Reserved. This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.