Erlang Simple Object Notation - dynamic name-value dictionary data type and syntax for Erlang
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Erlson - Erlang Simple Object Notation

Erlson is a dynamic name-value dictionary data type for Erlang.

Erlson dictionaries come with a convenient syntax and can be directly converted to and from JSON.


    % creating an empty dictionary
    X = #{},

    % associating field 'foo' with 1 and 'bar' with "abc"
    D = #{foo = 1, bar = "abc"},

    % accessing dictionary element
    1 =,

    % adding nested dictionaries to dictionary D
    D1 = D#{baz = #{fum = #{i = 0}}},

    % accessing elements of the nested dictionary
    0 = D1.baz.fum.i,

    % modifying elements of the nested dictionary
    D2 = D1#{baz.fum.i = 100, baz.fum.j = "new nested value"}.


    % converting Erlson dictionary to JSON iolist()

    % creating Erlson dictionary from JSON iolist()
    D = erlson:from_json(Json).

General properties

  • Erlson dictionaries contain zero or more Name->Value associations (fields), where each Name is atom() or binary() and Value can be of any() type.

  • Name->Value associations are unique. If a new association is created for the existing Name, the old value will be replaced by the new value.

  • Erlson dictionaries can be nested.

Runtime properties

  • Only fields with atom() names can be accessed using the Erlson dictionary syntax.

  • Unlike Erlang records, Erlson dictionaries can be dynamically constructed without any static type declarations.

  • At runtime, Erlson dictionaries are represented as a list of {Name, Value} tuples ordered by Name. This way, each Erlson dictionary is a valid proplist and orddict in terms of the correspondent stdlib modules.

  • Erlson dictionaries (dictionary syntax) can't be used as patterns and in guard expressions.

  • Erlson dicts can be used in both compiled modules and Erlang interactive shell.

Properties related to JSON

  • Each valid JSON object can be converted to correspondent Erlson dictionary.

  • An Erlson dictionary can be converted to JSON if it follows JSON data model.

  • JSON->Erlson->JSON conversion produces an equivalent JSON object (fields will be reordered).

Usage instructions

For compiled modules that use Erlson syntax, the Erlson library header must be included:


When rebar is used as a build tool, it should be configured to use "erlson_rebar_plugin". In order to do that, add the following line to the project's "rebar.config" file:

{rebar_plugins, [erlson_rebar_plugin]}.

In order to use Erlson syntax from Erlang shell, run the following command (e.g. include it in .erlang file):



Erlson relies on mochijson2 library for JSON encoding and decoding. It comes as a part of Mochiweb. Erlson doesn't automatically include it, but if you wish to do it with a rebar-enabled project, add it as dependency in your rebar.config. For example:

        % we need Mochiweb for mochijson2
        {mochiweb, "", {git, "", {branch, "master"}}}