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Revision: 1.7
Committed: Tue Oct 29 18:33:11 2013 UTC (11 years, 1 month ago) by root
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# Content
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 use common::sense; # required to suppress annoying warnings
21
22 our $VERSION = 0.03;
23
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 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
107 *Types::Serialiser::Boolean:: = *JSON::PP::Boolean::;
108 }
109
110 {
111 # this must done before blessing to work around bugs
112 # in perl < 5.18 (it seems to be fixed in 5.18).
113 package Types::Serialiser::BooleanBase;
114
115 use overload
116 "0+" => sub { ${$_[0]} },
117 "++" => sub { $_[0] = ${$_[0]} + 1 },
118 "--" => sub { $_[0] = ${$_[0]} - 1 },
119 fallback => 1;
120
121 @Types::Serialiser::Boolean::ISA = Types::Serialiser::BooleanBase::;
122 }
123
124 our $true = do { bless \(my $dummy = 1), Types::Serialiser::Boolean:: };
125 our $false = do { bless \(my $dummy = 0), Types::Serialiser::Boolean:: };
126 our $error = do { bless \(my $dummy ), Types::Serialiser::Error:: };
127
128 sub true () { $true }
129 sub false () { $false }
130 sub error () { $error }
131
132 sub is_bool ($) { UNIVERSAL::isa $_[0], Types::Serialiser::Boolean:: }
133 sub is_true ($) { $_[0] && UNIVERSAL::isa $_[0], Types::Serialiser::Boolean:: }
134 sub is_false ($) { !$_[0] && UNIVERSAL::isa $_[0], Types::Serialiser::Boolean:: }
135 sub is_error ($) { UNIVERSAL::isa $_[0], Types::Serialiser::Error:: }
136
137 package Types::Serialiser::Error;
138
139 sub error {
140 require Carp;
141 Carp::croak ("caught attempt to use the Types::Serialiser::error value");
142 };
143
144 use overload
145 "0+" => \&error,
146 "++" => \&error,
147 "--" => \&error,
148 fallback => 1;
149
150 =head1 NOTES FOR XS USERS
151
152 The recommended way to detect whether a scalar is one of these objects
153 is to check whether the stash is the C<Types::Serialiser::Boolean> or
154 C<Types::Serialiser::Error> stash, and then follow the scalar reference to
155 see if it's C<1> (true), C<0> (false) or C<undef> (error).
156
157 While it is possible to use an isa test, directly comparing stash pointers
158 is faster and guaranteed to work.
159
160 For historical reasons, the C<Types::Serialiser::Boolean> stash is
161 just an alias for C<JSON::PP::Boolean>. When printed, the classname
162 withh usually be C<JSON::PP::Boolean>, but isa tests and stash pointer
163 comparison will normally work correctly (i.e. Types::Serialiser::true ISA
164 JSON::PP::Boolean, but also ISA Types::Serialiser::Boolean).
165
166 =head1 A GENERIC OBJECT SERIALIATION PROTOCOL
167
168 This section explains the object serialisation protocol used by
169 L<CBOR::XS>. It is meant to be generic enough to support any kind of
170 generic object serialiser.
171
172 This protocol is called "the Types::Serialiser object serialisation
173 protocol".
174
175 =head2 ENCODING
176
177 When the encoder encounters an object that it cannot otherwise encode (for
178 example, L<CBOR::XS> can encode a few special types itself, and will first
179 attempt to use the special C<TO_CBOR> serialisation protocol), it will
180 look up the C<FREEZE> method on the object.
181
182 If it exists, it will call it with two arguments: the object to
183 serialise, and a constant string that indicates the name of the
184 serialisationformat. For example L<CBOR::XS> uses C<CBOR>, and L<JSON> and
185 L<JSON::XS> (or any other JSON serialiser), would use C<JSON> as second
186 argument.
187
188 The C<FREEZE> method can then return zero or more values to identify the
189 object instance. The serialiser is then supposed to encode the class name
190 and all of these return values (which must be encodable in the format)
191 using the relevant form for perl objects. In CBOR for example, there is a
192 registered tag number for encoded perl objects.
193
194 The values that C<FREEZE> returns must be serialisable with the serialiser
195 that calls it. Therefore, it is recommended to use simple types such as
196 strings and numbers, and maybe array references and hashes (basically, the
197 JSON data model). You can always use a more complex format for a specific
198 serialiser by checking the second argument.
199
200 =head2 DECODING
201
202 When the decoder then encounters such an encoded perl object, it should
203 look up the C<THAW> method on the stored classname, and invoke it with the
204 classname, the constant string to identify the format, and all the return
205 values returned by C<FREEZE>.
206
207 =head2 EXAMPLES
208
209 See the C<OBJECT SERIALISATION> section in the L<CBOR::XS> manpage for
210 more details, an example implementation, and code examples.
211
212 Here is an example C<FREEZE>/C<THAW> method pair:
213
214 sub My::Object::FREEZE {
215 my ($self, $serialiser) = @_;
216
217 ($self->{type}, $self->{id}, $self->{variant})
218 }
219
220 sub My::Object::THAW {
221 my ($class, $serialiser, $type, $id, $variant) = @_;
222
223 $class->new (type => $type, id => $id, variant => $variant)
224 }
225
226 =head1 BUGS
227
228 The use of L<overload> makes this module much heavier than it should be
229 (on my system, this module: 4kB RSS, overload: 260kB RSS).
230
231 =head1 SEE ALSO
232
233 Currently, L<JSON::XS> and L<CBOR::XS> use these types.
234
235 =head1 AUTHOR
236
237 Marc Lehmann <schmorp@schmorp.de>
238 http://home.schmorp.de/
239
240 =cut
241
242 1
243