[ViewVC] Annotation of: cvs/Types-Serialiser/README

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

SYNOPSIS
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.

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.

  BOOLEANS (Types::Serialiser::Boolean class)
    This type has only two instances, true and false. A natural
    representation for these in Perl is 1 and 0, but serialisation formats
    need to be able to differentiate between them and mere numbers.

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

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

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

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

    $is_bool = Types::Serialiser::is_bool $value
        Returns true iff the $value is either $Types::Serialiser::true or
        $Types::Serialiser::false.

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

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

    $is_true = Types::Serialiser::is_true $value
        Returns true iff $value is $Types::Serialiser::true.

    $is_false = Types::Serialiser::is_false $value
        Returns false iff $value is $Types::Serialiser::false.

  ERROR (Types::Serialiser::Error class)
    This class has only a single instance, "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.

    $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
        "undef" - implementations are allowed to directly test for this.

    $is_error = Types::Serialiser::is_error $value
        Returns false iff $value is $Types::Serialiser::error.

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 "Types::Serialiser::Boolean" or
    "Types::Serialiser::Error" stash, and then follow the scalar reference
    to see if it's 1 (true), 0 (false) or "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 "Types::Serialiser::Boolean" stash is just
    an alias for "JSON::PP::Boolean". When printed, the classname with
    usually be "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).

A GENERIC OBJECT SERIALIATION PROTOCOL
    This section explains the object serialisation protocol used by
    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".

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

    Note that the "FREEZE" method will normally be called *during* encoding,
    and *MUST NOT* change the data structure that is being encoded in any
    way, or it might cause memory corruption or worse.

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

    The "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 "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 data model by checking the second argument, the
    data model.

    The "data model" is not the same as the "data format" - the data model
    indicates what types and kinds of return values can be returned from
    "FREEZE". For example, in "CBOR" it is permissible to return tagged CBOR
    values, while JSON does not support these at all, so "JSON" would be a
    valid (but too limited) data model name for "CBOR::XS". similarly, a
    serialising format that supports more or less the same data model as
    JSON could use "JSON" as data model without losing anything.

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

  EXAMPLES
    See the "OBJECT SERIALISATION" section in the CBOR::XS manpage for more
    details, an example implementation, and code examples.

    Here is an example "FREEZE"/"THAW" method pair:

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

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

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

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

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

SEE ALSO
    Currently, JSON::XS and CBOR::XS use these types.

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

Revision:	1.5
Committed:	Sat Nov 30 18:33:51 2013 UTC (10 years, 7 months ago) by root
Branch:	MAIN
CVS Tags:	rel-1_0
Changes since 1.4:	+24 -11 lines
Log Message:	1.0
#	User	Rev	Content
1	root	1.2	NAME
2			Types::Serialiser - simple data types for common serialisation formats
3
4			SYNOPSIS
5			DESCRIPTION
6			This module provides some extra datatypes that are used by common
7			serialisation formats such as JSON or CBOR. The idea is to have a
8			repository of simple/small constants and containers that can be shared
9			by different implementations so they become interoperable between each
10			other.
11
12			SIMPLE SCALAR CONSTANTS
13			Simple scalar constants are values that are overloaded to act like
14			simple Perl values, but have (class) type to differentiate them from
15			normal Perl scalars. This is necessary because these have different
16			representations in the serialisation formats.
17
18			BOOLEANS (Types::Serialiser::Boolean class)
19			This type has only two instances, true and false. A natural
20			representation for these in Perl is 1 and 0, but serialisation formats
21			need to be able to differentiate between them and mere numbers.
22
23			$Types::Serialiser::true, Types::Serialiser::true
24			This value represents the "true" value. In most contexts is acts
25			like the number 1. It is up to you whether you use the variable form
26			($Types::Serialiser::true) or the constant form
27			("Types::Serialiser::true").
28
29			The constant is represented as a reference to a scalar containing 1
30			- implementations are allowed to directly test for this.
31
32			$Types::Serialiser::false, Types::Serialiser::false
33			This value represents the "false" value. In most contexts is acts
34			like the number 0. It is up to you whether you use the variable form
35			($Types::Serialiser::false) or the constant form
36			("Types::Serialiser::false").
37
38			The constant is represented as a reference to a scalar containing 0
39			- implementations are allowed to directly test for this.
40
41			$is_bool = Types::Serialiser::is_bool $value
42			Returns true iff the $value is either $Types::Serialiser::true or
43			$Types::Serialiser::false.
44
45			For example, you could differentiate between a perl true value and a
46			"Types::Serialiser::true" by using this:
47
48			$value && Types::Serialiser::is_bool $value
49
50			$is_true = Types::Serialiser::is_true $value
51			Returns true iff $value is $Types::Serialiser::true.
52
53			$is_false = Types::Serialiser::is_false $value
54			Returns false iff $value is $Types::Serialiser::false.
55
56			ERROR (Types::Serialiser::Error class)
57			This class has only a single instance, "error". It is used to signal an
58			encoding or decoding error. In CBOR for example, and object that
59			couldn't be encoded will be represented by a CBOR undefined value, which
60			is represented by the error value in Perl.
61
62			$Types::Serialiser::error, Types::Serialiser::error
63			This value represents the "error" value. Accessing values of this
64			type will throw an exception.
65
66			The constant is represented as a reference to a scalar containing
67			"undef" - implementations are allowed to directly test for this.
68
69			$is_error = Types::Serialiser::is_error $value
70			Returns false iff $value is $Types::Serialiser::error.
71
72			NOTES FOR XS USERS
73			The recommended way to detect whether a scalar is one of these objects
74			is to check whether the stash is the "Types::Serialiser::Boolean" or
75			"Types::Serialiser::Error" stash, and then follow the scalar reference
76			to see if it's 1 (true), 0 (false) or "undef" (error).
77
78			While it is possible to use an isa test, directly comparing stash
79			pointers is faster and guaranteed to work.
80
81	root	1.3	For historical reasons, the "Types::Serialiser::Boolean" stash is just
82	root	1.5	an alias for "JSON::PP::Boolean". When printed, the classname with
83	root	1.3	usually be "JSON::PP::Boolean", but isa tests and stash pointer
84			comparison will normally work correctly (i.e. Types::Serialiser::true
85			ISA JSON::PP::Boolean, but also ISA Types::Serialiser::Boolean).
86
87			A GENERIC OBJECT SERIALIATION PROTOCOL
88			This section explains the object serialisation protocol used by
89			CBOR::XS. It is meant to be generic enough to support any kind of
90			generic object serialiser.
91
92			This protocol is called "the Types::Serialiser object serialisation
93			protocol".
94
95			ENCODING
96			When the encoder encounters an object that it cannot otherwise encode
97			(for example, CBOR::XS can encode a few special types itself, and will
98			first attempt to use the special "TO_CBOR" serialisation protocol), it
99			will look up the "FREEZE" method on the object.
100
101	root	1.5	Note that the "FREEZE" method will normally be called during encoding,
102			and MUST NOT change the data structure that is being encoded in any
103			way, or it might cause memory corruption or worse.
104
105	root	1.3	If it exists, it will call it with two arguments: the object to
106	root	1.5	serialise, and a constant string that indicates the name of the data
107			model. For example CBOR::XS uses "CBOR", and the JSON and JSON::XS
108			modules (or any other JSON serialiser), would use "JSON" as second
109	root	1.3	argument.
110
111			The "FREEZE" method can then return zero or more values to identify the
112			object instance. The serialiser is then supposed to encode the class
113			name and all of these return values (which must be encodable in the
114	root	1.5	format) using the relevant form for Perl objects. In CBOR for example,
115	root	1.3	there is a registered tag number for encoded perl objects.
116
117	root	1.4	The values that "FREEZE" returns must be serialisable with the
118			serialiser that calls it. Therefore, it is recommended to use simple
119			types such as strings and numbers, and maybe array references and hashes
120			(basically, the JSON data model). You can always use a more complex
121	root	1.5	format for a specific data model by checking the second argument, the
122			data model.
123
124			The "data model" is not the same as the "data format" - the data model
125			indicates what types and kinds of return values can be returned from
126			"FREEZE". For example, in "CBOR" it is permissible to return tagged CBOR
127			values, while JSON does not support these at all, so "JSON" would be a
128			valid (but too limited) data model name for "CBOR::XS". similarly, a
129			serialising format that supports more or less the same data model as
130			JSON could use "JSON" as data model without losing anything.
131	root	1.4
132	root	1.3	DECODING
133			When the decoder then encounters such an encoded perl object, it should
134			look up the "THAW" method on the stored classname, and invoke it with
135	root	1.5	the classname, the constant string to identify the data model/data
136			format, and all the return values returned by "FREEZE".
137	root	1.3
138			EXAMPLES
139			See the "OBJECT SERIALISATION" section in the CBOR::XS manpage for more
140			details, an example implementation, and code examples.
141
142			Here is an example "FREEZE"/"THAW" method pair:
143
144			sub My::Object::FREEZE {
145	root	1.5	my ($self, $model) = @_;
146	root	1.3
147			($self->{type}, $self->{id}, $self->{variant})
148			}
149
150			sub My::Object::THAW {
151	root	1.5	my ($class, $model, $type, $id, $variant) = @_;
152	root	1.3
153	root	1.5	$class->new (type => $type, id => $id, variant => $variant)
154	root	1.3	}
155
156	root	1.2	BUGS
157			The use of overload makes this module much heavier than it should be (on
158			my system, this module: 4kB RSS, overload: 260kB RSS).
159
160			SEE ALSO
161			Currently, JSON::XS and CBOR::XS use these types.
162
163			AUTHOR
164			Marc Lehmann <schmorp@schmorp.de>
165			http://home.schmorp.de/
166