switch.3

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.\" ========================================================================
.\"
.IX Title "Switch 3"
.TH Switch 3 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
Switch \- A switch statement for Perl
.SH "VERSION"
.IX Header "VERSION"
This document describes version 2.11 of Switch,
released Nov 22, 2006.
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    use Switch;
\&
\&    switch ($val) {
\&        case 1          { print "number 1" }
\&        case "a"        { print "string a" }
\&        case [1..10,42] { print "number in list" }
\&        case (@array)   { print "number in list" }
\&        case /\ew+/      { print "pattern" }
\&        case qr/\ew+/    { print "pattern" }
\&        case (%hash)    { print "entry in hash" }
\&        case (\e%hash)   { print "entry in hash" }
\&        case (\e&sub)    { print "arg to subroutine" }
\&        else            { print "previous case not true" }
\&    }
.Ve
.SH "BACKGROUND"
.IX Header "BACKGROUND"
[Skip ahead to \*(L"\s-1DESCRIPTION\s0\*(R" if you don't care about the whys
and wherefores of this control structure]
.PP
In seeking to devise a \*(L"Swiss Army\*(R" case mechanism suitable for Perl,
it is useful to generalize this notion of distributed conditional
testing as far as possible. Specifically, the concept of \*(L"matching\*(R"
between the switch value and the various case values need not be
restricted to numeric (or string or referential) equality, as it is in other 
languages. Indeed, as Table 1 illustrates, Perl
offers at least eighteen different ways in which two values could
generate a match.
.PP
.Vb 1
\&        Table 1: Matching a switch value ($s) with a case value ($c)
\&
\&        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);
.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).
.SH "DESCRIPTION"
.IX Header "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.
.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.