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Erlang Simple Object Notation - dynamic name-value dictionary data type and syntax for Erlang
Erlang Shell
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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()`

* 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

Properties related to JSON

* Each valid JSON object can be converted to correspondent Erlson

* An Erlson dictionary can be converted to JSON if it follows JSON data

* 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


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"}}}
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