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.


    % create an empty dictionary
    X = #{},

    % associate fields 'foo' with 1, 'bar' with "abc" and 'fum' with 'true'
    D = #{foo = 1, bar = "abc", fum},

    % access dictionary element
    1 =,

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

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

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


    % convert Erlson dictionary to JSON iolist()

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


    % create Erlson dictionary from a proplist
    D = erlson:from_proplist(L).

    % create nested Erlson dictionary from a nested proplist
    D = erlson:from_nested_proplist(L).

    % create nested Erlson dictionary from a nested proplist up to the maximum
    % depth of 2
    D = erlson:from_nested_proplist(L, 2).

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. An error message will be returned by the Erlang compiler Erlson syntax is used in pattern-matching or guard contexts.

  • Erlson dictionaries 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 may be reordered).

  • There is one-to-one mapping between JSON and Erlang/Erlson values:

    • JSON object <-> Erlson dictionary
    • JSON array <-> Erlang list
    • JSON number <-> Erlang number() (i.e. floats and integers)
    • JSON true | false <-> Erlang boolean()
    • JSON string value <-> Erlang binary()
    • JSON null <-> Erlang atom undefined
  • JSON field names are decoded using the following function:

    decode_json_field_name(N) ->
        try binary_to_existing_atom(N, utf8)
            error:badarg -> N

Erlson and property lists

Property list can be converted to Erlson dictionaries using the erlson:from_proplist function and its variations.

Erlson dictionaries can also be used for property lists construction. Using Erlson, proplists look much cleaner. For example, compare

{application, erlson,
 [{description, "Erlang Simple Object Notation"},
  {vsn, git},
  {modules, []},
  {applications, [kernel, stdlib]},
  {env, []}]}.


{application, erlson,
 #{description = "Erlang Simple Object Notation",
   vsn = git,
   modules = [],
   applications = [kernel, stdlib],
   env = []}}.

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 lines to the project's "rebar.config" file:

    {plugins, [erlson_rebar_plugin]}. % for newer rebar
    {rebar_plugins, [erlson_rebar_plugin]}. % for older rebar

            {erlson, "", {git, "", {branch, "master"}}}

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



Erlson relies on modified versions of Erlang parsers taken from correspondent Erlang/OTP releases. While Erlson is fully compatible with R13, R14 and R15 Erlang releases, compatibility between Erlson and future Erlang versions can not be guaranteed.

Compatibility can break in one of the following ways:

  • Erlang adopts Erlson syntax for natively implemented dynamic dictionaries which will make Erlson obsolete.

  • Erlang introduces new unrelated syntax elements conflicting with Erlson grammar which will make Erlson completely unusable in its current form. In response to that, Erlson may adjust its grammar to remain compatible.


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"}}}


Erlson is created by Anton Lavrik

The first version was written as a Spawnfest submission which took the The Erlang Enhancement Prize.


Erlson is distributed under the terms of a MIT license. See the LICENSE file for details.