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=head1 NAME |
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Types::Serialiser - simple data types for common serialisation formats |
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=encoding utf-8 |
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=head1 SYNOPSIS |
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=head1 DESCRIPTION |
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This module provides some extra datatypes that are used by common |
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serialisation formats such as JSON or CBOR. The idea is to have a |
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repository of simple/small constants and containers that can be shared by |
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different implementations so they become interoperable between each other. |
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=cut |
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package Types::Serialiser; |
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use common::sense; # required to suppress annoying warnings |
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our $VERSION = '1.0'; |
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=head1 SIMPLE SCALAR CONSTANTS |
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Simple scalar constants are values that are overloaded to act like simple |
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Perl values, but have (class) type to differentiate them from normal Perl |
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scalars. This is necessary because these have different representations in |
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the serialisation formats. |
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=head2 BOOLEANS (Types::Serialiser::Boolean class) |
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This type has only two instances, true and false. A natural representation |
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for these in Perl is C<1> and C<0>, but serialisation formats need to be |
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able to differentiate between them and mere numbers. |
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=over 4 |
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=item $Types::Serialiser::true, Types::Serialiser::true |
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This value represents the "true" value. In most contexts is acts like |
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the number C<1>. It is up to you whether you use the variable form |
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(C<$Types::Serialiser::true>) or the constant form (C<Types::Serialiser::true>). |
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The constant is represented as a reference to a scalar containing C<1> - |
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implementations are allowed to directly test for this. |
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=item $Types::Serialiser::false, Types::Serialiser::false |
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This value represents the "false" value. In most contexts is acts like |
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the number C<0>. It is up to you whether you use the variable form |
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(C<$Types::Serialiser::false>) or the constant form (C<Types::Serialiser::false>). |
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The constant is represented as a reference to a scalar containing C<0> - |
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implementations are allowed to directly test for this. |
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=item $is_bool = Types::Serialiser::is_bool $value |
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Returns true iff the C<$value> is either C<$Types::Serialiser::true> or |
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C<$Types::Serialiser::false>. |
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For example, you could differentiate between a perl true value and a |
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C<Types::Serialiser::true> by using this: |
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$value && Types::Serialiser::is_bool $value |
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=item $is_true = Types::Serialiser::is_true $value |
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Returns true iff C<$value> is C<$Types::Serialiser::true>. |
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=item $is_false = Types::Serialiser::is_false $value |
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Returns false iff C<$value> is C<$Types::Serialiser::false>. |
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=back |
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=head2 ERROR (Types::Serialiser::Error class) |
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This class has only a single instance, C<error>. It is used to signal |
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an encoding or decoding error. In CBOR for example, and object that |
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couldn't be encoded will be represented by a CBOR undefined value, which |
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is represented by the error value in Perl. |
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=over 4 |
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=item $Types::Serialiser::error, Types::Serialiser::error |
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This value represents the "error" value. Accessing values of this type |
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will throw an exception. |
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The constant is represented as a reference to a scalar containing C<undef> |
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- implementations are allowed to directly test for this. |
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=item $is_error = Types::Serialiser::is_error $value |
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Returns false iff C<$value> is C<$Types::Serialiser::error>. |
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=back |
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=cut |
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BEGIN { |
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# for historical reasons, and to avoid extra dependencies in JSON::PP, |
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# we alias *Types::Serialiser::Boolean with JSON::PP::Boolean. |
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package JSON::PP::Boolean; |
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*Types::Serialiser::Boolean:: = *JSON::PP::Boolean::; |
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} |
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{ |
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# this must done before blessing to work around bugs |
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# in perl < 5.18 (it seems to be fixed in 5.18). |
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package Types::Serialiser::BooleanBase; |
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use overload |
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"0+" => sub { ${$_[0]} }, |
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"++" => sub { $_[0] = ${$_[0]} + 1 }, |
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"--" => sub { $_[0] = ${$_[0]} - 1 }, |
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fallback => 1; |
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@Types::Serialiser::Boolean::ISA = Types::Serialiser::BooleanBase::; |
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} |
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our $true = do { bless \(my $dummy = 1), Types::Serialiser::Boolean:: }; |
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our $false = do { bless \(my $dummy = 0), Types::Serialiser::Boolean:: }; |
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our $error = do { bless \(my $dummy ), Types::Serialiser::Error:: }; |
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sub true () { $true } |
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sub false () { $false } |
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sub error () { $error } |
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sub is_bool ($) { UNIVERSAL::isa $_[0], Types::Serialiser::Boolean:: } |
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sub is_true ($) { $_[0] && UNIVERSAL::isa $_[0], Types::Serialiser::Boolean:: } |
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sub is_false ($) { !$_[0] && UNIVERSAL::isa $_[0], Types::Serialiser::Boolean:: } |
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sub is_error ($) { UNIVERSAL::isa $_[0], Types::Serialiser::Error:: } |
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package Types::Serialiser::Error; |
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sub error { |
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require Carp; |
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Carp::croak ("caught attempt to use the Types::Serialiser::error value"); |
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}; |
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use overload |
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"0+" => \&error, |
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"++" => \&error, |
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"--" => \&error, |
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fallback => 1; |
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=head1 NOTES FOR XS USERS |
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The recommended way to detect whether a scalar is one of these objects |
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is to check whether the stash is the C<Types::Serialiser::Boolean> or |
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C<Types::Serialiser::Error> stash, and then follow the scalar reference to |
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see if it's C<1> (true), C<0> (false) or C<undef> (error). |
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While it is possible to use an isa test, directly comparing stash pointers |
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is faster and guaranteed to work. |
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For historical reasons, the C<Types::Serialiser::Boolean> stash is |
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just an alias for C<JSON::PP::Boolean>. When printed, the classname |
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with usually be C<JSON::PP::Boolean>, but isa tests and stash pointer |
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comparison will normally work correctly (i.e. Types::Serialiser::true ISA |
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JSON::PP::Boolean, but also ISA Types::Serialiser::Boolean). |
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=head1 A GENERIC OBJECT SERIALIATION PROTOCOL |
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This section explains the object serialisation protocol used by |
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L<CBOR::XS>. It is meant to be generic enough to support any kind of |
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generic object serialiser. |
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This protocol is called "the Types::Serialiser object serialisation |
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protocol". |
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=head2 ENCODING |
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When the encoder encounters an object that it cannot otherwise encode (for |
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example, L<CBOR::XS> can encode a few special types itself, and will first |
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attempt to use the special C<TO_CBOR> serialisation protocol), it will |
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look up the C<FREEZE> method on the object. |
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Note that the C<FREEZE> method will normally be called I<during> encoding, |
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and I<MUST NOT> change the data structure that is being encoded in any |
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way, or it might cause memory corruption or worse. |
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If it exists, it will call it with two arguments: the object to serialise, |
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and a constant string that indicates the name of the data model. For |
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example L<CBOR::XS> uses C<CBOR>, and the L<JSON> and L<JSON::XS> modules |
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(or any other JSON serialiser), would use C<JSON> as second argument. |
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The C<FREEZE> method can then return zero or more values to identify the |
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object instance. The serialiser is then supposed to encode the class name |
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and all of these return values (which must be encodable in the format) |
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using the relevant form for Perl objects. In CBOR for example, there is a |
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registered tag number for encoded perl objects. |
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The values that C<FREEZE> returns must be serialisable with the serialiser |
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that calls it. Therefore, it is recommended to use simple types such as |
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strings and numbers, and maybe array references and hashes (basically, the |
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JSON data model). You can always use a more complex format for a specific |
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data model by checking the second argument, the data model. |
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The "data model" is not the same as the "data format" - the data model |
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indicates what types and kinds of return values can be returned from |
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C<FREEZE>. For example, in C<CBOR> it is permissible to return tagged CBOR |
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values, while JSON does not support these at all, so C<JSON> would be a |
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valid (but too limited) data model name for C<CBOR::XS>. similarly, a |
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serialising format that supports more or less the same data model as JSON |
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could use C<JSON> as data model without losing anything. |
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=head2 DECODING |
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When the decoder then encounters such an encoded perl object, it should |
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look up the C<THAW> method on the stored classname, and invoke it with the |
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classname, the constant string to identify the data model/data format, and |
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all the return values returned by C<FREEZE>. |
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=head2 EXAMPLES |
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See the C<OBJECT SERIALISATION> section in the L<CBOR::XS> manpage for |
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more details, an example implementation, and code examples. |
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Here is an example C<FREEZE>/C<THAW> method pair: |
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sub My::Object::FREEZE { |
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my ($self, $model) = @_; |
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($self->{type}, $self->{id}, $self->{variant}) |
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} |
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sub My::Object::THAW { |
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my ($class, $model, $type, $id, $variant) = @_; |
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$class->new (type => $type, id => $id, variant => $variant) |
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} |
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=head1 BUGS |
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The use of L<overload> makes this module much heavier than it should be |
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(on my system, this module: 4kB RSS, overload: 260kB RSS). |
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=head1 SEE ALSO |
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Currently, L<JSON::XS> and L<CBOR::XS> use these types. |
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=head1 AUTHOR |
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Marc Lehmann <schmorp@schmorp.de> |
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http://home.schmorp.de/ |
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=cut |
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1 |
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