* ej helps you work with Erlang terms representing JSON

The ej module makes it easier to work with Erlang terms representing
[[http://json.org][JSON]] in the format returned by [[https://github.com/davisp/jiffy][jiffy]], [[https://github.com/mochi/mochiweb][mochijson2]], or [[https://github.com/benoitc/ejson][ejson]].  You can use
=ej:get/2= to walk an object and return a particular value, =ej:set/3=
to update a value within an object, or =ej:delete/2= to remove a value
from an object.

In ej, paths into JSON objects are expressed using a tuple of keys
like so:

| Javascript              | ej                                         |
| =Obj.author.name.first= | =ej:get({"author", "name", "first"}, Obj)= |

To get started using ej, see [[ej by example]] below.

ej also provides a means of validating a JSON object according to a
specification you provide. This feature is useful if you need to
process JSON request bodies. As a brief example, here's a
specification for a JSON object describing a person and their favorite
foods:

#+BEGIN_SRC erlang
{[
  {<<"name">>, {string_match, regex_for(name)}},
  {{opt, <<"nick_name">>}, {string_match, regex_for(name)}},
  {<<"foods">>, {array_map, string}}
 ]}
#+END_SRC


ej is independent of the library used for JSON serialization and
has no dependencies.

** ej by example

ej is best explained by example.  Consider the following JSON data
(borrowed from http://www.json.org/example.html):

#+BEGIN_SRC js
  {"menu": {
    "id": "file",
    "value": "File",
    "popup": {
      "menuitem": [
        {"value": "New", "onclick": "CreateNewDoc()"},
        {"value": "Open", "onclick": "OpenDoc()"},
        {"value": "Close", "onclick": "CloseDoc()"}
      ]
    }
  }}
#+END_SRC

=mochijson2:decode/1= translates the JSON into Erlang terms like this:

#+BEGIN_SRC erlang
  {struct,
   [{<<"menu">>,
     {struct,
      [{<<"id">>,<<"file">>},
       {<<"value">>,<<"File">>},
       {<<"popup">>,
        {struct,
         [{<<"menuitem">>,
           [{struct,[{<<"value">>,<<"New">>},
                     {<<"onclick">>,<<"CreateNewDoc()">>}]},
            {struct,[{<<"value">>,<<"Open">>},
                     {<<"onclick">>,<<"OpenDoc()">>}]},
            {struct,[{<<"value">>,<<"Close">>},
                     {<<"onclick">>,<<"CloseDoc()">>}]}]}]}}]}}]}
#+END_SRC

And here's ej in action:

#+BEGIN_SRC txt
% specify the path you want to access as a tuple of keys (you can use
% strings or binaries)
4> ej:get({"menu", "value"}, Obj).
<<"File">>

% you can access list elements by index
> ej:get({"menu", "popup", "menuitem", 2, "onclick"}, Obj).
<<"OpenDoc()">>

% The atoms 'first' and 'last' can be used for lists as well
> ej:get({"menu", "popup", "menuitem", first, "value"}, Obj).  
<<"New">>

% you can filter a list of objects by specifying a property (key/value
% pair) to match on:
ej:get({"menu", "popup", "menuitem", {select, {"value", "New"}}}, Obj).

% set a value
Obj2 = ej:set({"menu", "id"}, Obj, <<"abc123">>).

% add a value
Obj3 = ej:set({"menu", "new_key"}, Obj, <<"something">>).

% add a value to a list
NewItem = {struct,[{<<"value">>,<<"Save">>}, {<<"onclick">>,<<"SaveDoc()">>}]}.
Obj4 = ej:set({"menu", "popup", "menuitem", new}, Obj, NewItem).

#+END_SRC

The idea for this helper module was inspired by [[http://groups.google.com/group/erlang-programming/browse_thread/thread/7af6f99e740df979/97c50c0df25502cd?lnk=gst&q=Javascript+parse+transform#97c50c0df25502cd][this thread on the
Erlang Questions]] mailing list and, in particular, by the reply from
Richard O'Keefe.  Additional motivation from the very similar helper
module =struct= included in the [[http://beebole.com/en/blog/erlang/tutorial-web-application-erlang/][sticky notes example application]] from
the folks at BeeBole.

** THANKS

- Christopher Brown
- Christopher Maier
- John Keiser
- Sebastian Probst Eide

** Build status

#+ATTR_HTML: alt="Build status images" title="Build status on Travis-CI"
[[https://travis-ci.org/seth/ej.png]]


** License

ej is available under the Apache License, Version 2.0.

#+BEGIN_EXAMPLE
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
#+END_EXAMPLE