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.\" ========================================================================
.\"
.IX Title "Switch 3"
.TH Switch 3 "2002-11-24" "perl v5.8.0" "Perl Programmers Reference Guide"
.SH "NAME"
Switch \- A switch statement for Perl
.SH "VERSION"
.IX Header "VERSION"
This document describes version 2.09 of Switch,
released June 12, 2002.
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& use Switch;
.Ve
.PP
.Vb 1
\& switch ($val) {
.Ve
.PP
.Vb 11
\& case 1 { print "number 1" }
\& case "a" { print "string a" }
\& case [1..10,42] { print "number in list" }
\& case (@array) { print "number in list" }
\& case /\ew+/ { print "pattern" }
\& case qr/\ew+/ { print "pattern" }
\& case (%hash) { print "entry in hash" }
\& case (\e%hash) { print "entry in hash" }
\& case (\e&sub) { print "arg to subroutine" }
\& else { print "previous case not true" }
\& }
.Ve
.SH "BACKGROUND"
.IX Header "BACKGROUND"
[Skip ahead to \*(L"\s-1DESCRIPTION\s0\*(R" if you don't care about the whys
and wherefores of this control structure]
.PP
In seeking to devise a \*(L"Swiss Army\*(R" case mechanism suitable for Perl,
it is useful to generalize this notion of distributed conditional
testing as far as possible. Specifically, the concept of \*(L"matching\*(R"
between the switch value and the various case values need not be
restricted to numeric (or string or referential) equality, as it is in other
languages. Indeed, as Table 1 illustrates, Perl
offers at least eighteen different ways in which two values could
generate a match.
.PP
.Vb 1
\& Table 1: Matching a switch value ($s) with a case value ($c)
.Ve
.PP
.Vb 3
\& Switch Case Type of Match Implied Matching Code
\& Value Value
\& ====== ===== ===================== =============
.Ve
.PP
.Vb 2
\& number same numeric or referential match if $s == $c;
\& or ref equality
.Ve
.PP
.Vb 3
\& object method result of method call match if $s->$c();
\& ref name match if defined $s->$c();
\& or ref
.Ve
.PP
.Vb 3
\& other other string equality match if $s eq $c;
\& non-ref non-ref
\& scalar scalar
.Ve
.PP
.Vb 1
\& string regexp pattern match match if $s =~ /$c/;
.Ve
.PP
.Vb 3
\& 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];
.Ve
.PP
.Vb 5
\& array array array intersection match if intersects(@$s, @$c);
\& ref ref (apply this table to
\& all pairs of elements
\& $s->[$i] and
\& $c->[$j])
.Ve
.PP
.Vb 2
\& array regexp array grep match if grep /$c/, @$s;
\& ref
.Ve
.PP
.Vb 3
\& 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};
.Ve
.PP
.Vb 2
\& hash regexp hash grep match if grep /$c/, keys %$s;
\& ref
.Ve
.PP
.Vb 2
\& sub scalar return value defn match if defined $s->($c);
\& ref return value truth match if $s->($c);
.Ve
.PP
.Vb 2
\& sub array return value defn match if defined $s->(@$c);
\& ref ref return value truth match if $s->(@$c);
.Ve
.PP
In reality, Table 1 covers 31 alternatives, because only the equality and
intersection tests are commutative; in all other cases, the roles of
the \f(CW$s\fR and \f(CW$c\fR 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 (\f(CW\*(C`match if exists $s\->{$c}\*(C'\fR),
one could test for the existence of a single key in a series of hashes
(\f(CW\*(C`match if exists $c\->{$s}\*(C'\fR).
.PP
As perltodo observes, a Perl case mechanism must support all these
\&\*(L"ways to do it\*(R".
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The Switch.pm module implements a generalized case mechanism that covers
the numerous possible combinations of switch and case values described above.
.PP
The module augments the standard Perl syntax with two new control
statements: \f(CW\*(C`switch\*(C'\fR and \f(CW\*(C`case\*(C'\fR. The \f(CW\*(C`switch\*(C'\fR statement takes a
single scalar argument of any type, specified in parentheses.
\&\f(CW\*(C`switch\*(C'\fR 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 \f(CW\*(C`case\*(C'\fR statement described below).
The block is unconditionally executed once the switch value has
been cached.
.PP
A \f(CW\*(C`case\*(C'\fR 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 \f(CW\*(C`case\*(C'\fR argument, as
specified in Table 1. If the match is successful, the mandatory
block associated with the \f(CW\*(C`case\*(C'\fR statement is executed.
.PP
In most other respects, the \f(CW\*(C`case\*(C'\fR statement is semantically identical
to an \f(CW\*(C`if\*(C'\fR statement. For example, it can be followed by an \f(CW\*(C`else\*(C'\fR
clause, and can be used as a postfix statement qualifier.
.PP
However, when a \f(CW\*(C`case\*(C'\fR block has been executed control is automatically
transferred to the statement after the immediately enclosing \f(CW\*(C`switch\*(C'\fR
block, rather than to the next statement within the block. In other
words, the success of any \f(CW\*(C`case\*(C'\fR statement prevents other cases in the
same scope from executing. But see \*(L"Allowing fall\-through\*(R" below.
.PP
Together these two new statements provide a fully generalized case
mechanism:
.PP
.Vb 1
\& use Switch;
.Ve
.PP
.Vb 1
\& # AND LATER...
.Ve
.PP
.Vb 1
\& %special = ( woohoo => 1, d'oh => 1 );
.Ve
.PP
.Vb 2
\& while (<>) {
\& switch ($_) {
.Ve
.PP
.Vb 3
\& case (%special) { print "homer\en"; } # if $special{$_}
\& case /a-z/i { print "alpha\en"; } # if $_ =~ /a-z/i
\& case [1..9] { print "small num\en"; } # if $_ in [1..9]
.Ve
.PP
.Vb 3
\& case { $_[0] >= 10 } { # if $_ >= 10
\& my $age = <>;
\& switch (sub{ $_[0] < $age } ) {
.Ve
.PP
.Vb 5
\& case 20 { print "teens\en"; } # if 20 < $age
\& case 30 { print "twenties\en"; } # if 30 < $age
\& else { print "history\en"; }
\& }
\& }
.Ve
.PP
.Vb 2
\& print "must be punctuation\en" case /\eW/; # if $_ ~= /\eW/
\& }
.Ve
.PP
Note that \f(CW\*(C`switch\*(C'\fRes can be nested within \f(CW\*(C`case\*(C'\fR (or any other) blocks,
and a series of \f(CW\*(C`case\*(C'\fR statements can try different types of matches
\&\*(-- hash membership, pattern match, array intersection, simple equality,
etc. \*(-- against the same switch value.
.PP
The use of intersection tests against an array reference is particularly
useful for aggregating integral cases:
.PP
.Vb 8
\& sub classify_digit
\& {
\& switch ($_[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' }
\& }
\& }
.Ve
.Sh "Allowing fall-through"
.IX Subsection "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 \fIis\fR a way to do it, if you must.
.PP
If a \f(CW\*(C`case\*(C'\fR block executes an untargetted \f(CW\*(C`next\*(C'\fR, control is
immediately transferred to the statement \fIafter\fR the \f(CW\*(C`case\*(C'\fR statement
(i.e. usually another case), rather than out of the surrounding
\&\f(CW\*(C`switch\*(C'\fR block.
.PP
For example:
.PP
.Vb 7
\& switch ($val) {
\& case 1 { handle_num_1(); next } # and try next case...
\& case "1" { handle_str_1(); next } # and try next case...
\& case [0..9] { handle_num_any(); } # and we're done
\& case /\ed/ { handle_dig_any(); next } # and try next case...
\& case /.*/ { handle_str_any(); next } # and try next case...
\& }
.Ve
.PP
If \f(CW$val\fR held the number \f(CW1\fR, the above \f(CW\*(C`switch\*(C'\fR block would call the
first three \f(CW\*(C`handle_...\*(C'\fR subroutines, jumping to the next case test
each time it encountered a \f(CW\*(C`next\*(C'\fR. After the thrid \f(CW\*(C`case\*(C'\fR block
was executed, control would jump to the end of the enclosing
\&\f(CW\*(C`switch\*(C'\fR block.
.PP
On the other hand, if \f(CW$val\fR held \f(CW10\fR, then only the last two \f(CW\*(C`handle_...\*(C'\fR
subroutines would be called.
.PP
Note that this mechanism allows the notion of \fIconditional fall-through\fR.
For example:
.PP
.Vb 4
\& switch ($val) {
\& case [0..9] { handle_num_any(); next if $val < 7; }
\& case /\ed/ { handle_dig_any(); }
\& }
.Ve
.PP
If an untargetted \f(CW\*(C`last\*(C'\fR statement is executed in a case block, this
immediately transfers control out of the enclosing \f(CW\*(C`switch\*(C'\fR block
(in other words, there is an implicit \f(CW\*(C`last\*(C'\fR at the end of each
normal \f(CW\*(C`case\*(C'\fR block). Thus the previous example could also have been
written:
.PP
.Vb 4
\& switch ($val) {
\& case [0..9] { handle_num_any(); last if $val >= 7; next; }
\& case /\ed/ { handle_dig_any(); }
\& }
.Ve
.Sh "Automating fall-through"
.IX Subsection "Automating fall-through"
In situations where case fall-through should be the norm, rather than an
exception, an endless succession of terminal \f(CW\*(C`next\*(C'\fRs is tedious and ugly.
Hence, it is possible to reverse the default behaviour by specifying
the string \*(L"fallthrough\*(R" when importing the module. For example, the
following code is equivalent to the first example in \*(L"Allowing fall\-through\*(R":
.PP
.Vb 1
\& use Switch 'fallthrough';
.Ve
.PP
.Vb 7
\& switch ($val) {
\& case 1 { handle_num_1(); }
\& case "1" { handle_str_1(); }
\& case [0..9] { handle_num_any(); last }
\& case /\ed/ { handle_dig_any(); }
\& case /.*/ { handle_str_any(); }
\& }
.Ve
.PP
Note the explicit use of a \f(CW\*(C`last\*(C'\fR to preserve the non-fall-through
behaviour of the third case.
.Sh "Alternative syntax"
.IX Subsection "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 \f(CW\*(C`switch\*(C'\fR will be spelled \f(CW\*(C`given\*(C'\fR, and
\&\f(CW\*(C`case\*(C'\fR will be pronounced \f(CW\*(C`when\*(C'\fR. In addition, the \f(CW\*(C`when\*(C'\fR statement
will not require switch or case values to be parenthesized.
.PP
This future syntax is also (largely) available via the Switch.pm module, by
importing it with the argument \f(CW"Perl6"\fR. For example:
.PP
.Vb 1
\& use Switch 'Perl6';
.Ve
.PP
.Vb 7
\& given ($val) {
\& when 1 { handle_num_1(); }
\& when ($str1) { handle_str_1(); }
\& when [0..9] { handle_num_any(); last }
\& when /\ed/ { handle_dig_any(); }
\& when /.*/ { handle_str_any(); }
\& }
.Ve
.PP
Note that scalars still need to be parenthesized, since they would be
ambiguous in Perl 5.
.PP
Note too that you can mix and match both syntaxes by importing the module
with:
.PP
.Vb 1
\& use Switch 'Perl5', 'Perl6';
.Ve
.Sh "Higher-order Operations"
.IX Subsection "Higher-order Operations"
One situation in which \f(CW\*(C`switch\*(C'\fR and \f(CW\*(C`case\*(C'\fR do not provide a good
substitute for a cascaded \f(CW\*(C`if\*(C'\fR, is where a switch value needs to
be tested against a series of conditions. For example:
.PP
.Vb 2
\& sub beverage {
\& switch (shift) {
.Ve
.PP
.Vb 9
\& 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' }
\& }
\& }
.Ve
.PP
The need to specify each condition as a subroutine block is tiresome. To
overcome this, when importing Switch.pm, a special \*(L"placeholder\*(R"
subroutine named \f(CW\*(C`_\|_\*(C'\fR [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:
.PP
.Vb 1
\& use Switch '__';
.Ve
.PP
.Vb 1
\& __ < 2 + __
.Ve
.PP
is equivalent to:
.PP
.Vb 1
\& sub { $_[0] < 2 + $_[1] }
.Ve
.PP
With \f(CW\*(C`_\|_\*(C'\fR, the previous ugly case statements can be rewritten:
.PP
.Vb 7
\& 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' }
.Ve
.PP
The \f(CW\*(C`_\|_\*(C'\fR 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.
.PP
The only problem is that operator overloading does not allow the
boolean operators \f(CW\*(C`&&\*(C'\fR and \f(CW\*(C`||\*(C'\fR to be overloaded. So a case statement
like this:
.PP
.Vb 1
\& case 0 <= __ && __ < 10 { return 'digit' }
.Ve
.PP
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 \f(CW\*(C`&&\*(C'\fR:
.PP
.Vb 1
\& sub { 0 <= $_[0] } && sub { $_[0] < 10 }
.Ve
.PP
This boolean expression is inevitably true, since both references are
non\-false. Fortunately, the overloaded \f(CW'bool'\fR operator catches this
situation and flags it as a error.
.SH "DEPENDENCIES"
.IX Header "DEPENDENCIES"
The module is implemented using Filter::Util::Call and Text::Balanced
and requires both these modules to be installed.
.SH "AUTHOR"
.IX Header "AUTHOR"
Damian Conway (damian@conway.org)
.SH "BUGS"
.IX Header "BUGS"
There are undoubtedly serious bugs lurking somewhere in code this funky :\-)
Bug reports and other feedback are most welcome.
.SH "LIMITATION"
.IX Header "LIMITATION"
Due to the heuristic nature of Switch.pm's source parsing, the presence
of regexes specified with raw \f(CW\*(C`?...?\*(C'\fR delimiters may cause mysterious
errors. The workaround is to use \f(CW\*(C`m?...?\*(C'\fR instead.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
.Vb 3
\& 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.
.Ve
|
