[ViewVC] Contents 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.

    In the following, functions with zero or one arguments have a prototype
    of "()" and "($)", respectively, so act as constants and unary
    operators.

  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.

    Types::Serialiser::as_bool $value
        Converts a Perl scalar into a boolean, which is useful syntactic
        sugar. Strictly equivalent to:

           $value ? $Types::Serialiser::true : $Types::Serialiser::false

    $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:	1.6
Committed:	Tue Dec 1 01:47:20 2020 UTC (3 years, 7 months ago) by root
Branch:	MAIN
CVS Tags:	rel-1_01, HEAD
Changes since 1.5:	+10 -0 lines
Log Message:	1.01
#	Content
1	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	In the following, functions with zero or one arguments have a prototype
19	of "()" and "($)", respectively, so act as constants and unary
20	operators.
21
22	BOOLEANS (Types::Serialiser::Boolean class)
23	This type has only two instances, true and false. A natural
24	representation for these in Perl is 1 and 0, but serialisation formats
25	need to be able to differentiate between them and mere numbers.
26
27	$Types::Serialiser::true, Types::Serialiser::true
28	This value represents the "true" value. In most contexts is acts
29	like the number 1. It is up to you whether you use the variable form
30	($Types::Serialiser::true) or the constant form
31	("Types::Serialiser::true").
32
33	The constant is represented as a reference to a scalar containing 1
34	- implementations are allowed to directly test for this.
35
36	$Types::Serialiser::false, Types::Serialiser::false
37	This value represents the "false" value. In most contexts is acts
38	like the number 0. It is up to you whether you use the variable form
39	($Types::Serialiser::false) or the constant form
40	("Types::Serialiser::false").
41
42	The constant is represented as a reference to a scalar containing 0
43	- implementations are allowed to directly test for this.
44
45	Types::Serialiser::as_bool $value
46	Converts a Perl scalar into a boolean, which is useful syntactic
47	sugar. Strictly equivalent to:
48
49	$value ? $Types::Serialiser::true : $Types::Serialiser::false
50
51	$is_bool = Types::Serialiser::is_bool $value
52	Returns true iff the $value is either $Types::Serialiser::true or
53	$Types::Serialiser::false.
54
55	For example, you could differentiate between a perl true value and a
56	"Types::Serialiser::true" by using this:
57
58	$value && Types::Serialiser::is_bool $value
59
60	$is_true = Types::Serialiser::is_true $value
61	Returns true iff $value is $Types::Serialiser::true.
62
63	$is_false = Types::Serialiser::is_false $value
64	Returns false iff $value is $Types::Serialiser::false.
65
66	ERROR (Types::Serialiser::Error class)
67	This class has only a single instance, "error". It is used to signal an
68	encoding or decoding error. In CBOR for example, and object that
69	couldn't be encoded will be represented by a CBOR undefined value, which
70	is represented by the error value in Perl.
71
72	$Types::Serialiser::error, Types::Serialiser::error
73	This value represents the "error" value. Accessing values of this
74	type will throw an exception.
75
76	The constant is represented as a reference to a scalar containing
77	"undef" - implementations are allowed to directly test for this.
78
79	$is_error = Types::Serialiser::is_error $value
80	Returns false iff $value is $Types::Serialiser::error.
81
82	NOTES FOR XS USERS
83	The recommended way to detect whether a scalar is one of these objects
84	is to check whether the stash is the "Types::Serialiser::Boolean" or
85	"Types::Serialiser::Error" stash, and then follow the scalar reference
86	to see if it's 1 (true), 0 (false) or "undef" (error).
87
88	While it is possible to use an isa test, directly comparing stash
89	pointers is faster and guaranteed to work.
90
91	For historical reasons, the "Types::Serialiser::Boolean" stash is just
92	an alias for "JSON::PP::Boolean". When printed, the classname with
93	usually be "JSON::PP::Boolean", but isa tests and stash pointer
94	comparison will normally work correctly (i.e. Types::Serialiser::true
95	ISA JSON::PP::Boolean, but also ISA Types::Serialiser::Boolean).
96
97	A GENERIC OBJECT SERIALIATION PROTOCOL
98	This section explains the object serialisation protocol used by
99	CBOR::XS. It is meant to be generic enough to support any kind of
100	generic object serialiser.
101
102	This protocol is called "the Types::Serialiser object serialisation
103	protocol".
104
105	ENCODING
106	When the encoder encounters an object that it cannot otherwise encode
107	(for example, CBOR::XS can encode a few special types itself, and will
108	first attempt to use the special "TO_CBOR" serialisation protocol), it
109	will look up the "FREEZE" method on the object.
110
111	Note that the "FREEZE" method will normally be called during encoding,
112	and MUST NOT change the data structure that is being encoded in any
113	way, or it might cause memory corruption or worse.
114
115	If it exists, it will call it with two arguments: the object to
116	serialise, and a constant string that indicates the name of the data
117	model. For example CBOR::XS uses "CBOR", and the JSON and JSON::XS
118	modules (or any other JSON serialiser), would use "JSON" as second
119	argument.
120
121	The "FREEZE" method can then return zero or more values to identify the
122	object instance. The serialiser is then supposed to encode the class
123	name and all of these return values (which must be encodable in the
124	format) using the relevant form for Perl objects. In CBOR for example,
125	there is a registered tag number for encoded perl objects.
126
127	The values that "FREEZE" returns must be serialisable with the
128	serialiser that calls it. Therefore, it is recommended to use simple
129	types such as strings and numbers, and maybe array references and hashes
130	(basically, the JSON data model). You can always use a more complex
131	format for a specific data model by checking the second argument, the
132	data model.
133
134	The "data model" is not the same as the "data format" - the data model
135	indicates what types and kinds of return values can be returned from
136	"FREEZE". For example, in "CBOR" it is permissible to return tagged CBOR
137	values, while JSON does not support these at all, so "JSON" would be a
138	valid (but too limited) data model name for "CBOR::XS". similarly, a
139	serialising format that supports more or less the same data model as
140	JSON could use "JSON" as data model without losing anything.
141
142	DECODING
143	When the decoder then encounters such an encoded perl object, it should
144	look up the "THAW" method on the stored classname, and invoke it with
145	the classname, the constant string to identify the data model/data
146	format, and all the return values returned by "FREEZE".
147
148	EXAMPLES
149	See the "OBJECT SERIALISATION" section in the CBOR::XS manpage for more
150	details, an example implementation, and code examples.
151
152	Here is an example "FREEZE"/"THAW" method pair:
153
154	sub My::Object::FREEZE {
155	my ($self, $model) = @_;
156
157	($self->{type}, $self->{id}, $self->{variant})
158	}
159
160	sub My::Object::THAW {
161	my ($class, $model, $type, $id, $variant) = @_;
162
163	$class->new (type => $type, id => $id, variant => $variant)
164	}
165
166	BUGS
167	The use of overload makes this module much heavier than it should be (on
168	my system, this module: 4kB RSS, overload: 260kB RSS).
169
170	SEE ALSO
171	Currently, JSON::XS and CBOR::XS use these types.
172
173	AUTHOR
174	Marc Lehmann <schmorp@schmorp.de>
175	http://home.schmorp.de/
176