Erlang Simple Object Notation - dynamic name-value dictionary data type and syntax for Erlang
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Update (2014-11-16): Erlson syntax now uses brackets instead of curly braces.

Erlang 17 introduced a new built-in maps type. Maps syntax conflicts with the original Erlson syntax that relied on curly braces.

For this reason, we changed Erlson syntax to use brackets instead. Such change eliminates grammar conflicts and makes Erlson syntax look sufficiently different from maps.

This syntax change is backward compatible. The old syntax is still supported with Erlang R16 and earlier releases.

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).

Other API functions

Parsing and generating top-level JSON arrays.

ErlsonList = erlson:list_from_json_array(<JSON array represented as iolist()>)

JsonArray = erlson:list_to_json_array(ErlsonList)

Accessing fields dynamically (i.e. without using Erlson syntax). The main difference between Erlson field access syntax and get_value/1 is that get_value/1 returns undefined if the field is missing whereas Erlson will crash.

erlson:get_value(Path, Dict)  % alias for get_value(Path, 'undefined')

erlson:get_value(Path, Dict, Default)

% Path :: atom() | [atom()]

Macros similar to get_value/2,3 functions for gracefully handling missing fields but still using Erlson syntax for accessing them. Example:




?erlson_default(, Default)

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.

Erlson and 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). The only exception is {} (empty JSON object), because it can not be directly represented in Erlson when loaded from JSON.

  • 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
  • The erlson:to_json function supports quoted JSON values represented as {json, iodata()} Erlang terms. The function will write the iodata() part of this term directly into JSON output. For example, {json, "{}"} Erlson value will turn into empty JSON object.

  • Erlson uses Erlang built-in binary_to_existing_atom/1 function for converting JSON field names into Erlang atoms. This is to prevent the VM from running out of atom space on malicious JSON input.

    Generally, everything should just work out of the box in compiled projects. However, if you are using Erlson from Erlang shell, not all of your modules and correspondent atoms may be loaded. In this case, JSON field names for unknown atoms will be represented as binaries after parsing. To fix that, you can load all the necessary modules. This can be done, for example, by running this command:

    Modules = [application:get_key(A, modules) || A <- application:loaded_applications()],
    [ code:load(M) || lists:append(Modules) ].

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):


Erlang compatibility

Erlson relies on modified versions of Erlang parsers taken from correspondent Erlang/OTP releases. While Erlson is fully compatible with R13, R14, R15, R16, R17 Erlang releases, compatibility between Erlson and future Erlang versions can not be guaranteed.

Compatibility can break if future Erlang versions 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


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