| Switch - A switch statement for Perl |
Switch - A switch statement for Perl
This document describes version 2.09 of Switch, released June 12, 2002.
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 the perltodo manpage 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.
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';
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.
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 (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.
| Switch - A switch statement for Perl |