1Serialiser(3) User Contributed Perl Documentation Serialiser(3)
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6 Types::Serialiser - simple data types for common serialisation formats
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10 This module provides some extra datatypes that are used by common
11 serialisation formats such as JSON or CBOR. The idea is to have a
12 repository of simple/small constants and containers that can be shared
13 by different implementations so they become interoperable between each
14 other.
15
17 Simple scalar constants are values that are overloaded to act like
18 simple Perl values, but have (class) type to differentiate them from
19 normal Perl scalars. This is necessary because these have different
20 representations in the serialisation formats.
21 In the following, functions with zero or one arguments have a prototype
23 of "()" and "($)", respectively, so act as constants and unary
24 operators.
25 BOOLEANS (Types::Serialiser::Boolean class)
27 This type has only two instances, true and false. A natural
28 representation for these in Perl is 1 and 0, but serialisation formats
29 need to be able to differentiate between them and mere numbers.
30 $Types::Serialiser::true, Types::Serialiser::true
32 This value represents the "true" value. In most contexts is acts
33 like the number 1. It is up to you whether you use the variable
34 form ($Types::Serialiser::true) or the constant form
35 ("Types::Serialiser::true").
36 The constant is represented as a reference to a scalar containing 1
38 - implementations are allowed to directly test for this.
39 $Types::Serialiser::false, Types::Serialiser::false
41 This value represents the "false" value. In most contexts is acts
42 like the number 0. It is up to you whether you use the variable
43 form ($Types::Serialiser::false) or the constant form
44 ("Types::Serialiser::false").
45 The constant is represented as a reference to a scalar containing 0
47 - implementations are allowed to directly test for this.
48 Types::Serialiser::as_bool $value
50 Converts a Perl scalar into a boolean, which is useful syntactic
51 sugar. Strictly equivalent to:
52 $value ? $Types::Serialiser::true : $Types::Serialiser::false
54 $is_bool = Types::Serialiser::is_bool $value
56 Returns true iff the $value is either $Types::Serialiser::true or
57 $Types::Serialiser::false.
58 For example, you could differentiate between a perl true value and
60 a "Types::Serialiser::true" by using this:
61 $value && Types::Serialiser::is_bool $value
63 $is_true = Types::Serialiser::is_true $value
65 Returns true iff $value is $Types::Serialiser::true.
66 $is_false = Types::Serialiser::is_false $value
68 Returns false iff $value is $Types::Serialiser::false.
69 ERROR (Types::Serialiser::Error class)
71 This class has only a single instance, "error". It is used to signal an
72 encoding or decoding error. In CBOR for example, and object that
73 couldn't be encoded will be represented by a CBOR undefined value,
74 which is represented by the error value in Perl.
75 $Types::Serialiser::error, Types::Serialiser::error
77 This value represents the "error" value. Accessing values of this
78 type will throw an exception.
79 The constant is represented as a reference to a scalar containing
81 "undef" - implementations are allowed to directly test for this.
82 $is_error = Types::Serialiser::is_error $value
84 Returns false iff $value is $Types::Serialiser::error.
85
87 The recommended way to detect whether a scalar is one of these objects
88 is to check whether the stash is the "Types::Serialiser::Boolean" or
89 "Types::Serialiser::Error" stash, and then follow the scalar reference
90 to see if it's 1 (true), 0 (false) or "undef" (error).
91 While it is possible to use an isa test, directly comparing stash
93 pointers is faster and guaranteed to work.
94 For historical reasons, the "Types::Serialiser::Boolean" stash is just
96 an alias for "JSON::PP::Boolean". When printed, the classname with
97 usually be "JSON::PP::Boolean", but isa tests and stash pointer
98 comparison will normally work correctly (i.e. Types::Serialiser::true
99 ISA JSON::PP::Boolean, but also ISA Types::Serialiser::Boolean).
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102 This section explains the object serialisation protocol used by
103 CBOR::XS. It is meant to be generic enough to support any kind of
104 generic object serialiser.
105 This protocol is called "the Types::Serialiser object serialisation
107 protocol".
108 ENCODING
110 When the encoder encounters an object that it cannot otherwise encode
111 (for example, CBOR::XS can encode a few special types itself, and will
112 first attempt to use the special "TO_CBOR" serialisation protocol), it
113 will look up the "FREEZE" method on the object.
114 Note that the "FREEZE" method will normally be called during encoding,
116 and MUST NOT change the data structure that is being encoded in any
117 way, or it might cause memory corruption or worse.
118 If it exists, it will call it with two arguments: the object to
120 serialise, and a constant string that indicates the name of the data
121 model. For example CBOR::XS uses "CBOR", and the JSON and JSON::XS
122 modules (or any other JSON serialiser), would use "JSON" as second
123 argument.
124 The "FREEZE" method can then return zero or more values to identify the
126 object instance. The serialiser is then supposed to encode the class
127 name and all of these return values (which must be encodable in the
128 format) using the relevant form for Perl objects. In CBOR for example,
129 there is a registered tag number for encoded perl objects.
130 The values that "FREEZE" returns must be serialisable with the
132 serialiser that calls it. Therefore, it is recommended to use simple
133 types such as strings and numbers, and maybe array references and
134 hashes (basically, the JSON data model). You can always use a more
135 complex format for a specific data model by checking the second
136 argument, the data model.
137 The "data model" is not the same as the "data format" - the data model
139 indicates what types and kinds of return values can be returned from
140 "FREEZE". For example, in "CBOR" it is permissible to return tagged
141 CBOR values, while JSON does not support these at all, so "JSON" would
142 be a valid (but too limited) data model name for "CBOR::XS". similarly,
143 a serialising format that supports more or less the same data model as
144 JSON could use "JSON" as data model without losing anything.
145 DECODING
147 When the decoder then encounters such an encoded perl object, it should
148 look up the "THAW" method on the stored classname, and invoke it with
149 the classname, the constant string to identify the data model/data
150 format, and all the return values returned by "FREEZE".
151 EXAMPLES
153 See the "OBJECT SERIALISATION" section in the CBOR::XS manpage for more
154 details, an example implementation, and code examples.
155 Here is an example "FREEZE"/"THAW" method pair:
157 sub My::Object::FREEZE {
159 my ($self, $model) = @_;
160 ($self->{type}, $self->{id}, $self->{variant})
162 }
163 sub My::Object::THAW {
165 my ($class, $model, $type, $id, $variant) = @_;
166 $class->new (type => $type, id => $id, variant => $variant)
168 }
169
171 The use of overload makes this module much heavier than it should be
172 (on my system, this module: 4kB RSS, overload: 260kB RSS).
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175 Currently, JSON::XS and CBOR::XS use these types.
176
178 Marc Lehmann <schmorp@schmorp.de>
179 http://home.schmorp.de/
180perl v5.34.0 2022-01-21 Serialiser(3)