NAME¶
Switch - A switch statement for Perl
SYNOPSIS¶
use Switch;
switch ($val) {
case 1 { print "number 1" }
case "a" { print "string a" }
case [1..10,42] { print "number in list" }
case (\@array) { print "number in list" }
case /\w+/ { print "pattern" }
case qr/\w+/ { print "pattern" }
case (\%hash) { print "entry in hash" }
case (\&sub) { print "arg to subroutine" }
else { print "previous case not true" }
}
BACKGROUND¶
[Skip ahead to "DESCRIPTION" if you don't care about the whys and
wherefores of this control structure]
In seeking to devise a "Swiss Army" case mechanism suitable for Perl,
it is useful to generalize this notion of distributed conditional testing as
far as possible. Specifically, the concept of "matching" between the
switch value and the various case values need not be restricted to numeric (or
string or referential) equality, as it is in other languages. Indeed, as Table
1 illustrates, Perl offers at least eighteen different ways in which two
values could generate a match.
Table 1: Matching a switch value ($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}").
DESCRIPTION¶
The Switch.pm module implements a generalized case mechanism that covers most
(but not all) of 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 (<>) {
chomp;
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 } { print "big num\n"; } # if $_ >= 10
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 [1,3,5,7,9] { 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_..." subroutines, jumping to the next case test
each time it encountered a "next". After the third "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 $val < 7; }
case /\d/ { handle_dig_any(); }
}
If an untargeted "last" statement is executed in a case block, this
immediately transfers control out of the enclosing "switch" block
(in other words, there is an implicit "last" at the end of each
normal "case" block). Thus the previous example could also have been
written:
switch ($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(); }
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 { $_[0] < 10 } { return 'milk' }
case { $_[0] < 20 } { return 'coke' }
case { $_[0] < 30 } { return 'beer' }
case { $_[0] < 40 } { return 'wine' }
case { $_[0] < 50 } { return 'malt' }
case { $_[0] < 60 } { return 'Moet' }
else { return 'milk' }
}
}
(This is equivalent to writing "case (sub { $_[0] < 10 })", etc.;
$_[0] is the argument to the anonymous subroutine.)
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 }
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 an 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). This module is now maintained by Rafael
Garcia-Suarez (rgarciasuarez@gmail.com) and more generally by the Perl 5
Porters (perl5-porters@perl.org), as part of the Perl core.
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 with embedded newlines that are specified with raw "/.../"
delimiters and don't have a modifier "//x" are indistinguishable
from code chunks beginning with the division operator "/". As a
workaround you must use "m/.../" or "m?...?" for such
patterns. Also, 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) 1997-2008, 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.
