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https://github.com/alavrik/erlson

Erlang Simple Object Notation - dynamic name-value dictionary data type and syntax for Erlang
https://github.com/alavrik/erlson

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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.



Examples:



```erlang

    % 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 = D.foo,



    % 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()

    erlson:to_json(D2).



    % 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()



    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:



    -include_lib("erlson/include/erlson.hrl").



    ...



    ?erlson_default(X.foo.bar)



    ?erlson_default(X.foo.bar, 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:



    ```erlang

    decode_json_field_name(N) ->

        try binary_to_existing_atom(N, utf8)

        catch

            error:badarg -> N

        end.

    ```



- 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



```erlang

{application, erlson,

 [{description, "Erlang Simple Object Notation"},

  {vsn, git},

  {modules, []},

  {applications, [kernel, stdlib]},

  {env, []}]}.

```



and



```erlang

{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:



```erlang

    -include_lib("erlson/include/erlson.hrl").

```



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:



```erlang

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

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



    {deps,

        [

            {erlson, "", {git, "https://github.com/alavrik/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

    erlson:init().

```



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.



Dependencies

------------



Erlson relies on `mochijson2` library for JSON encoding and decoding. It comes

as a part of [Mochiweb](https://github.com/mochi/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:



```erlang

    {deps,

        [

            % we need Mochiweb for mochijson2

            {mochiweb, "", {git, "https://github.com/mochi/mochiweb.git", {branch, "master"}}}

        ]}.

```



Authors

-------



Erlson is created by Anton Lavrik .



License

-------



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

details.