switch.3

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

.PP
Together these two new statements provide a fully generalized case
mechanism:
.PP
.Vb 1
\&        use Switch;
\&
\&        # AND LATER...
\&
\&        %special = ( woohoo => 1,  d\*(Aqoh => 1 );
\&
\&        while (<>) {
\&            chomp;
\&            switch ($_) {
\&                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]
\&                case { $_[0] >= 10 } { print "big num\en"; } # if $_ >= 10
\&                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 \*(Aqzero\*(Aq }
\&                                 case [2,4,6,8]    { return \*(Aqeven\*(Aq }
\&                                 case [1,3,5,7,9]  { return \*(Aqodd\*(Aq }
\&                                 case /[A\-F]/i     { return \*(Aqhex\*(Aq }
\&                               }
\&        }
.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 untargeted \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 ($val) {
\&                case 1      { handle_num_1(); next }    # and try next case...
\&                case "1"    { handle_str_1(); next }    # and try next case...
\&                case [0..9] { handle_num_any(); }       # and we\*(Aqre done
\&                case /\ed/   { handle_dig_any(); next }  # and try next case...
\&                case /.*/   { handle_str_any(); next }  # and try next case...
\&        }
.Ve
.PP
If \f(CW$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 third \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 $val < 7; }
\&                case /\ed/   { handle_dig_any(); }
\&        }
.Ve
.PP
If an untargeted \f(CW\*(C`last\*(C'\fR statement is executed in a case block, this
immediately transfers control out of the enclosing \f(CW\*(C`switch\*(C'\fR block
(in other words, there is an implicit \f(CW\*(C`last\*(C'\fR at the end of each
normal \f(CW\*(C`case\*(C'\fR block). Thus the previous example could also have been
written:
.PP
.Vb 4
\&        switch ($val) {
\&                case [0..9] { handle_num_any(); last if $val >= 7; next; }
\&                case /\ed/   { handle_dig_any(); }
\&        }
.Ve
.Sh "Automating fall-through"
.IX Subsection "Automating fall-through"
In situations where case fall-through should be the norm, rather than an
exception, an endless succession of terminal \f(CW\*(C`next\*(C'\fRs is tedious and ugly.
Hence, it is possible to reverse the default behaviour by specifying
the string \*(L"fallthrough\*(R" when importing the module. For example, the 
following code is equivalent to the first example in \*(L"Allowing fall-through\*(R":
.PP
.Vb 1
\&        use Switch \*(Aqfallthrough\*(Aq;
\&
\&        switch ($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 \*(AqPerl6\*(Aq;
\&
\&        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(); }
\&                default      { handle anything else; }
\&        }
.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 \*(AqPerl5\*(Aq, \*(AqPerl6\*(Aq;
.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 11
\&        sub beverage {
\&            switch (shift) {
\&                case { $_[0] < 10 } { return \*(Aqmilk\*(Aq }
\&                case { $_[0] < 20 } { return \*(Aqcoke\*(Aq }
\&                case { $_[0] < 30 } { return \*(Aqbeer\*(Aq }
\&                case { $_[0] < 40 } { return \*(Aqwine\*(Aq }
\&                case { $_[0] < 50 } { return \*(Aqmalt\*(Aq }
\&                case { $_[0] < 60 } { return \*(AqMoet\*(Aq }
\&                else                { return \*(Aqmilk\*(Aq }
\&            }
\&        }
.Ve
.PP
(This is equivalent to writing \f(CW\*(C`case (sub { $_[0] < 10 })\*(C'\fR, etc.; \f(CW$_[0]\fR
is the argument to the anonymous subroutine.)
.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 \*(Aq_\|_\*(Aq;
\&
\&        _\|_ < 2
.Ve
.PP
is equivalent to:
.PP
.Vb 1
\&        sub { $_[0] < 2 }
.Ve
.PP
With \f(CW\*(C`_\|_\*(C'\fR, the previous ugly case statements can be rewritten:
.PP
.Vb 7
\&        case  _\|_ < 10  { return \*(Aqmilk\*(Aq }
\&        case  _\|_ < 20  { return \*(Aqcoke\*(Aq }
\&        case  _\|_ < 30  { return \*(Aqbeer\*(Aq }
\&        case  _\|_ < 40  { return \*(Aqwine\*(Aq }
\&        case  _\|_ < 50  { return \*(Aqmalt\*(Aq }
\&        case  _\|_ < 60  { return \*(AqMoet\*(Aq }
\&        else           { return \*(Aqmilk\*(Aq }
.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 \*(Aqdigit\*(Aq }
.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\*(Aqbool\*(Aq\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 ([e-mail]). The maintainer of this module is now Rafael
Garcia-Suarez ([e-mail]).
.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 "LIMITATIONS"
.IX Header "LIMITATIONS"
Due to the heuristic nature of Switch.pm's source parsing, the presence of
regexes with embedded newlines that are specified with raw \f(CW\*(C`/.../\*(C'\fR
delimiters and don't have a modifier \f(CW\*(C`//x\*(C'\fR are indistinguishable from
code chunks beginning with the division operator \f(CW\*(C`/\*(C'\fR. As a workaround
you must use \f(CW\*(C`m/.../\*(C'\fR or \f(CW\*(C`m?...?\*(C'\fR for such patterns. Also, 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.
.PP
Due to the way source filters work in Perl, you can't use Switch inside
an string \f(CW\*(C`eval\*(C'\fR.
.PP
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.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
.Vb 3
\&    Copyright (c) 1997\-2006, 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