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Make JSON from an Object (the opposite of JSON::Unmarshal)

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   use JSON::Marshal;

    class SomeClass {
      has Str $.string;
      has Int $.int;
      has Version $.version is marshalled-by('Str');

    my $object = SomeClass.new(string => "string", int => 42, version => Version.new("0.0.1"));

    my Str $json = marshal($object); # -> "{ "string" : "string", "int" : 42, "version" : "0.0.1" }'

Or with opt-in marshalling:

    use JSON::Marshal;
    use JSON::OptIn;

    class SomeClass {
      has Str $.string is json;
      has Int $.int    is json;
      has Str $.secret;
      has Version $.version is marshalled-by('Str');

    my $object = SomeClass.new(secret => "secret", string => "string", int => 42, version => Version.new("0.0.1"));

    my Str $json = marshal($object, :opt-in); # -> "{ "string" : "string", "int" : 42, "version" : "0.0.1" }'


This provides a single exported subroutine to create a JSON representation of an object. It should round trip back into an object of the same class using JSON::Unmarshal.

It only outputs the "public" attributes (that is those with accessors created by declaring them with the '.' twigil. Attributes without acccessors are ignored.

If you want to ignore any attributes without a value you can use the :skip-null adverb to marshal, which will supress the marshalling of any undefined attributes. Additionally if you want a finer-grained control over this behaviour there is a 'json-skip-null' attribute trait which will cause the specific attribute to be skipped if it isn't defined irrespective of the skip-null. If you want to always explicitly suppress the marshalling of an attribute then the the trait json-skip on an attribute will prevent it being output in the JSON.

By default all public attributes will be candidates to be marshalled to JSON, which may not be convenient for all applications (for example only a small number of attributes should be marshalled in a large class,) so the marshal provides an :opt-in adverb that inverts the behaviour so that only those attributes which have one of the traits that control marshalling (with the exception of json-skip,) will be candidates. The is json trait from JSON::OptIn can be supplied to an attribute to mark it for marshalling explicitly, (it is implicit in all the other traits bar json-skip.)

To allow a finer degree of control of how an attribute is marshalled an attribute trait is marshalled-by is provided, this can take either a Code object (an anonymous subroutine,) which should take as an argument the value to be marshalled and should return a value that can be completely represented as JSON, that is to say a string, number or boolean or a Hash or Array who's values are those things. Alternatively the name of a method that will be called on the value, the return value being constrained as above.

By default the JSON produced is pretty (that is newlines and indentation,) which is nice for humans to read but has a lot of superfluous characters in it, this can be controlled by passing :!pretty to marshal.


Assuming you have a working Rakudo installation, you can install this with zef :

# From the source directory

zef install .

# Remote installation

zef install JSON::Marshal


Suggestions/patches are welcomed via github at



Please see the LICENCE file in the distribution

© Jonathan Stowe 2015, 2016, 2017, 2018, 2019, 2020, 2021