Types-Serialiser/Serialiser.pm

=head1 NAME

Types::Serialiser - simple data types for common serialisation formats

=encoding utf-8

=head1 SYNOPSIS

=head1 DESCRIPTION

This module provides some extra datatypes that are used by common
serialisation formats such as JSON or CBOR. The idea is to have a
repository of simple/small constants and containers that can be shared by
different implementations so they become interoperable between each other.

=cut

package Types::Serialiser;

use common::sense; # required to suppress annoying warnings

our $VERSION = 0.02;

=head1 SIMPLE SCALAR CONSTANTS

Simple scalar constants are values that are overloaded to act like simple
Perl values, but have (class) type to differentiate them from normal Perl
scalars. This is necessary because these have different representations in
the serialisation formats.

=head2 BOOLEANS (Types::Serialiser::Boolean class)

This type has only two instances, true and false. A natural representation
for these in Perl is C<1> and C<0>, but serialisation formats need to be
able to differentiate between them and mere numbers.

=over 4

=item $Types::Serialiser::true, Types::Serialiser::true

This value represents the "true" value. In most contexts is acts like
the number C<1>. It is up to you whether you use the variable form
(C<$Types::Serialiser::true>) or the constant form (C<Types::Serialiser::true>).

The constant is represented as a reference to a scalar containing C<1> -
implementations are allowed to directly test for this.

=item $Types::Serialiser::false, Types::Serialiser::false

This value represents the "false" value. In most contexts is acts like
the number C<0>. It is up to you whether you use the variable form
(C<$Types::Serialiser::false>) or the constant form (C<Types::Serialiser::false>).

The constant is represented as a reference to a scalar containing C<0> -
implementations are allowed to directly test for this.

=item $is_bool = Types::Serialiser::is_bool $value

Returns true iff the C<$value> is either C<$Types::Serialiser::true> or
C<$Types::Serialiser::false>.

For example, you could differentiate between a perl true value and a
C<Types::Serialiser::true> by using this:

   $value && Types::Serialiser::is_bool $value

=item $is_true = Types::Serialiser::is_true $value

Returns true iff C<$value> is C<$Types::Serialiser::true>.

=item $is_false = Types::Serialiser::is_false $value

Returns false iff C<$value> is C<$Types::Serialiser::false>.

=back

=head2 ERROR (Types::Serialiser::Error class)

This class has only a single instance, C<error>. It is used to signal
an encoding or decoding error. In CBOR for example, and object that
couldn't be encoded will be represented by a CBOR undefined value, which
is represented by the error value in Perl.

=over 4

=item $Types::Serialiser::error, Types::Serialiser::error

This value represents the "error" value. Accessing values of this type
will throw an exception.

The constant is represented as a reference to a scalar containing C<undef>
- implementations are allowed to directly test for this.

=item $is_error = Types::Serialiser::is_error $value

Returns false iff C<$value> is C<$Types::Serialiser::error>.

=back

=cut

BEGIN {
   # for historical reasons, and to avoid extra dependencies in JSON::PP,
   # we alias *Types::Serialiser::Boolean with JSON::PP::Boolean.
   package JSON::PP::Boolean;
   *Types::Serialiser::Boolean:: = *JSON::PP::Boolean::;
}

our $true  = do { bless \(my $dummy = 1), Types::Serialiser::Boolean:: };
our $false = do { bless \(my $dummy = 0), Types::Serialiser::Boolean:: };
our $error = do { bless \(my $dummy    ), Types::Serialiser::Error::   };

sub true  () { $true  }
sub false () { $false }
sub error () { $error }

sub is_bool  ($) {           UNIVERSAL::isa $_[0], Types::Serialiser::Boolean:: }
sub is_true  ($) {  $_[0] && UNIVERSAL::isa $_[0], Types::Serialiser::Boolean:: }
sub is_false ($) { !$_[0] && UNIVERSAL::isa $_[0], Types::Serialiser::Boolean:: }
sub is_error ($) {           UNIVERSAL::isa $_[0], Types::Serialiser::Error::   }

package Types::Serialiser::BooleanBase;

use overload
   "0+"     => sub { ${$_[0]} },
   "++"     => sub { $_[0] = ${$_[0]} + 1 },
   "--"     => sub { $_[0] = ${$_[0]} - 1 },
   fallback => 1;

@Types::Serialiser::Boolean::ISA = Types::Serialiser::BooleanBase::;

package Types::Serialiser::Error;

sub error {
   require Carp;
   Carp::croak ("caught attempt to use the Types::Serialiser::error value");
};

use overload
   "0+"     => \&error,
   "++"     => \&error,
   "--"     => \&error,
   fallback => 1;

=head1 NOTES FOR XS USERS

The recommended way to detect whether a scalar is one of these objects
is to check whether the stash is the C<Types::Serialiser::Boolean> or
C<Types::Serialiser::Error> stash, and then follow the scalar reference to
see if it's C<1> (true), C<0> (false) or C<undef> (error).

While it is possible to use an isa test, directly comparing stash pointers
is faster and guaranteed to work.

For historical reasons, the C<Types::Serialiser::Boolean> stash is
just an alias for C<JSON::PP::Boolean>. When printed, the classname
withh usually be C<JSON::PP::Boolean>, but isa tests and stash pointer
comparison will normally work correctly (i.e. Types::Serialiser::true ISA
JSON::PP::Boolean, but also ISA Types::Serialiser::Boolean).

=head1 A GENERIC OBJECT SERIALIATION PROTOCOL

This section explains the object serialisation protocol used by
L<CBOR::XS>. It is meant to be generic enough to support any kind of
generic object serialiser.

This protocol is called "the Types::Serialiser object serialisation
protocol".

=head2 ENCODING

When the encoder encounters an object that it cannot otherwise encode (for
example, L<CBOR::XS> can encode a few special types itself, and will first
attempt to use the special C<TO_CBOR> serialisation protocol), it will
look up the C<FREEZE> method on the object.

If it exists, it will call it with two arguments: the object to
serialise, and a constant string that indicates the name of the
serialisationformat. For example L<CBOR::XS> uses C<CBOR>, and L<JSON> and
L<JSON::XS> (or any other JSON serialiser), would use C<JSON> as second
argument.

The C<FREEZE> method can then return zero or more values to identify the
object instance. The serialiser is then supposed to encode the class name
and all of these return values (which must be encodable in the format)
using the relevant form for perl objects. In CBOR for example, there is a
registered tag number for encoded perl objects.

The values that C<FREEZE> returns must be serialisable with the serialiser
that calls it. Therefore, it is recommended to use simple types such as
strings and numbers, and maybe array references and hashes (basically, the
JSON data model). You can always use a more complex format for a specific
serialiser by checking the second argument.

=head2 DECODING

When the decoder then encounters such an encoded perl object, it should
look up the C<THAW> method on the stored classname, and invoke it with the
classname, the constant string to identify the format, and all the return
values returned by C<FREEZE>.

=head2 EXAMPLES

See the C<OBJECT SERIALISATION> section in the L<CBOR::XS> manpage for
more details, an example implementation, and code examples.

Here is an example C<FREEZE>/C<THAW> method pair:

   sub My::Object::FREEZE {
      my ($self, $serialiser) = @_;

      ($self->{type}, $self->{id}, $self->{variant})
   }

   sub My::Object::THAW {
      my ($class, $serialiser, $type, $id, $variant) = @_;

      $class-<new (type => $type, id => $id, variant => $variant)
   }

=head1 BUGS

The use of L<overload> makes this module much heavier than it should be
(on my system, this module: 4kB RSS, overload: 260kB RSS).

=head1 SEE ALSO

Currently, L<JSON::XS> and L<CBOR::XS> use these types.

=head1 AUTHOR

 Marc Lehmann <schmorp@schmorp.de>
 http://home.schmorp.de/

=cut

1

Revision:	1.5
Committed:	Mon Oct 28 21:52:22 2013 UTC (11 years, 1 month ago) by root
Branch:	MAIN
Changes since 1.4:	+6 -0 lines
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	=head1 NAME
2
3			Types::Serialiser - simple data types for common serialisation formats
4
5			=encoding utf-8
6
7			=head1 SYNOPSIS
8
9			=head1 DESCRIPTION
10
11			This module provides some extra datatypes that are used by common
12			serialisation formats such as JSON or CBOR. The idea is to have a
13			repository of simple/small constants and containers that can be shared by
14			different implementations so they become interoperable between each other.
15
16			=cut
17
18			package Types::Serialiser;
19
20	root	1.4	use common::sense; # required to suppress annoying warnings
21
22			our $VERSION = 0.02;
23	root	1.1
24			=head1 SIMPLE SCALAR CONSTANTS
25
26			Simple scalar constants are values that are overloaded to act like simple
27			Perl values, but have (class) type to differentiate them from normal Perl
28			scalars. This is necessary because these have different representations in
29			the serialisation formats.
30
31			=head2 BOOLEANS (Types::Serialiser::Boolean class)
32
33			This type has only two instances, true and false. A natural representation
34			for these in Perl is C<1> and C<0>, but serialisation formats need to be
35			able to differentiate between them and mere numbers.
36
37			=over 4
38
39			=item $Types::Serialiser::true, Types::Serialiser::true
40
41			This value represents the "true" value. In most contexts is acts like
42			the number C<1>. It is up to you whether you use the variable form
43			(C<$Types::Serialiser::true>) or the constant form (C<Types::Serialiser::true>).
44
45			The constant is represented as a reference to a scalar containing C<1> -
46			implementations are allowed to directly test for this.
47
48			=item $Types::Serialiser::false, Types::Serialiser::false
49
50			This value represents the "false" value. In most contexts is acts like
51			the number C<0>. It is up to you whether you use the variable form
52			(C<$Types::Serialiser::false>) or the constant form (C<Types::Serialiser::false>).
53
54			The constant is represented as a reference to a scalar containing C<0> -
55			implementations are allowed to directly test for this.
56
57			=item $is_bool = Types::Serialiser::is_bool $value
58
59			Returns true iff the C<$value> is either C<$Types::Serialiser::true> or
60			C<$Types::Serialiser::false>.
61
62			For example, you could differentiate between a perl true value and a
63			C<Types::Serialiser::true> by using this:
64
65			$value && Types::Serialiser::is_bool $value
66
67			=item $is_true = Types::Serialiser::is_true $value
68
69			Returns true iff C<$value> is C<$Types::Serialiser::true>.
70
71			=item $is_false = Types::Serialiser::is_false $value
72
73			Returns false iff C<$value> is C<$Types::Serialiser::false>.
74
75			=back
76
77			=head2 ERROR (Types::Serialiser::Error class)
78
79			This class has only a single instance, C<error>. It is used to signal
80			an encoding or decoding error. In CBOR for example, and object that
81			couldn't be encoded will be represented by a CBOR undefined value, which
82			is represented by the error value in Perl.
83
84			=over 4
85
86			=item $Types::Serialiser::error, Types::Serialiser::error
87
88			This value represents the "error" value. Accessing values of this type
89			will throw an exception.
90
91			The constant is represented as a reference to a scalar containing C<undef>
92			- implementations are allowed to directly test for this.
93
94			=item $is_error = Types::Serialiser::is_error $value
95
96			Returns false iff C<$value> is C<$Types::Serialiser::error>.
97
98			=back
99
100			=cut
101
102	root	1.4	BEGIN {
103			# for historical reasons, and to avoid extra dependencies in JSON::PP,
104			# we alias *Types::Serialiser::Boolean with JSON::PP::Boolean.
105			package JSON::PP::Boolean;
106			Types::Serialiser::Boolean:: = JSON::PP::Boolean::;
107			}
108
109	root	1.1	our $true = do { bless \(my $dummy = 1), Types::Serialiser::Boolean:: };
110			our $false = do { bless \(my $dummy = 0), Types::Serialiser::Boolean:: };
111			our $error = do { bless \(my $dummy ), Types::Serialiser::Error:: };
112
113			sub true () { $true }
114			sub false () { $false }
115			sub error () { $error }
116
117			sub is_bool ($) { UNIVERSAL::isa $_[0], Types::Serialiser::Boolean:: }
118			sub is_true ($) { $_[0] && UNIVERSAL::isa $_[0], Types::Serialiser::Boolean:: }
119			sub is_false ($) { !$_[0] && UNIVERSAL::isa $_[0], Types::Serialiser::Boolean:: }
120			sub is_error ($) { UNIVERSAL::isa $_[0], Types::Serialiser::Error:: }
121
122	root	1.4	package Types::Serialiser::BooleanBase;
123	root	1.1
124			use overload
125			"0+" => sub { ${$_[0]} },
126			"++" => sub { $_[0] = ${$_[0]} + 1 },
127			"--" => sub { $_[0] = ${$_[0]} - 1 },
128			fallback => 1;
129
130	root	1.4	@Types::Serialiser::Boolean::ISA = Types::Serialiser::BooleanBase::;
131
132	root	1.1	package Types::Serialiser::Error;
133
134			sub error {
135			require Carp;
136	root	1.2	Carp::croak ("caught attempt to use the Types::Serialiser::error value");
137	root	1.1	};
138
139			use overload
140			"0+" => \&error,
141			"++" => \&error,
142			"--" => \&error,
143			fallback => 1;
144
145	root	1.2	=head1 NOTES FOR XS USERS
146
147			The recommended way to detect whether a scalar is one of these objects
148			is to check whether the stash is the C<Types::Serialiser::Boolean> or
149			C<Types::Serialiser::Error> stash, and then follow the scalar reference to
150			see if it's C<1> (true), C<0> (false) or C<undef> (error).
151
152			While it is possible to use an isa test, directly comparing stash pointers
153			is faster and guaranteed to work.
154
155	root	1.4	For historical reasons, the C<Types::Serialiser::Boolean> stash is
156			just an alias for C<JSON::PP::Boolean>. When printed, the classname
157			withh usually be C<JSON::PP::Boolean>, but isa tests and stash pointer
158			comparison will normally work correctly (i.e. Types::Serialiser::true ISA
159			JSON::PP::Boolean, but also ISA Types::Serialiser::Boolean).
160
161	root	1.3	=head1 A GENERIC OBJECT SERIALIATION PROTOCOL
162
163			This section explains the object serialisation protocol used by
164			L<CBOR::XS>. It is meant to be generic enough to support any kind of
165			generic object serialiser.
166
167			This protocol is called "the Types::Serialiser object serialisation
168			protocol".
169
170			=head2 ENCODING
171
172			When the encoder encounters an object that it cannot otherwise encode (for
173			example, L<CBOR::XS> can encode a few special types itself, and will first
174			attempt to use the special C<TO_CBOR> serialisation protocol), it will
175			look up the C<FREEZE> method on the object.
176
177			If it exists, it will call it with two arguments: the object to
178			serialise, and a constant string that indicates the name of the
179			serialisationformat. For example L<CBOR::XS> uses C<CBOR>, and L<JSON> and
180			L<JSON::XS> (or any other JSON serialiser), would use C<JSON> as second
181			argument.
182
183			The C<FREEZE> method can then return zero or more values to identify the
184			object instance. The serialiser is then supposed to encode the class name
185			and all of these return values (which must be encodable in the format)
186			using the relevant form for perl objects. In CBOR for example, there is a
187			registered tag number for encoded perl objects.
188
189	root	1.5	The values that C<FREEZE> returns must be serialisable with the serialiser
190			that calls it. Therefore, it is recommended to use simple types such as
191			strings and numbers, and maybe array references and hashes (basically, the
192			JSON data model). You can always use a more complex format for a specific
193			serialiser by checking the second argument.
194
195	root	1.3	=head2 DECODING
196
197			When the decoder then encounters such an encoded perl object, it should
198			look up the C<THAW> method on the stored classname, and invoke it with the
199			classname, the constant string to identify the format, and all the return
200			values returned by C<FREEZE>.
201
202			=head2 EXAMPLES
203
204			See the C<OBJECT SERIALISATION> section in the L<CBOR::XS> manpage for
205			more details, an example implementation, and code examples.
206
207			Here is an example C<FREEZE>/C<THAW> method pair:
208
209			sub My::Object::FREEZE {
210			my ($self, $serialiser) = @_;
211
212			($self->{type}, $self->{id}, $self->{variant})
213			}
214
215			sub My::Object::THAW {
216			my ($class, $serialiser, $type, $id, $variant) = @_;
217
218			$class-<new (type => $type, id => $id, variant => $variant)
219			}
220
221	root	1.1	=head1 BUGS
222
223			The use of L<overload> makes this module much heavier than it should be
224			(on my system, this module: 4kB RSS, overload: 260kB RSS).
225
226			=head1 SEE ALSO
227
228			Currently, L<JSON::XS> and L<CBOR::XS> use these types.
229
230			=head1 AUTHOR
231
232			Marc Lehmann <schmorp@schmorp.de>
233			http://home.schmorp.de/
234
235			=cut
236
237			1
238