Monthly Downloads: 0
Programming language: Erlang
License: MIT License
Tags: JSON    
Latest version: v1.2

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Helper module for working with common Erlang representations of JSON: eep 18, map,

mochijson-style, and proplists.

Build Status hex.pm version

This work is inspired by ej, but handles all common JSON representations: eep-18, map, mochijson-style and proplists, the types returned by jsone, jiffy, and mochijson, for example.

Anything good about this is probably due to the contributors and maintainers of ej, anything bad or awkward is completely my fault.


The source code is a single file, and there are no dependencies other than:

  • Erlang 19.0 +
  • Rebar3, for building

Hex package manager is used for package management.



$ git clone git://github.com/jr0senblum/jwalk.git $ cd jwalk


$ make compile

run tests

$ make eunit


$ make dialyze

Erlang shell

$ make start

1> jwalk:get({"one"},#{<<"one">> => 1}).
2> jwalk:get({"one"},[{<<"one">>, 1}]).
3> jwalk:get({"one"},{[{<<"one">>, 1}]}).
4>  jwalk:get({"one"},{struct, [{<<"one">>, 1}]}).


Jwalk functions always take at least two parameters: a first parameter which is a tuple of elements representing a Path into a JSON Object, and a second parameter which is expected to be a valid JSON representation (map, proplist, etc.).

  • jwalk:delete(Path, Object) -> Result Removes the value at the location specified by Path from Object and returns a new structure
  • jwalk:get(Path, Object) -> Result | undefined, jwalk:get(Path, Object, Default) -> Result | Default Returns the Value at the specificed Path from Object, or undefined or Default if not found
  • jwalk:set(Path, Object, Value) -> Result Sets a Value in Object at the specified Path and returns the new structure
  • jwalk:set_p(Path, Object, Value) -> Result Sets a Value in an Object at the specified Path creating intermediate nodes as necessary and returns the new structure


Paths into JSON Objects are expressed using a tuple of Path elements, a representation of a javascript-like path: i.e., Obj.weapons.edged.distance becomes {"weapons","edged","distance"}. Path elements representing JSON Member Names can be strings or binary, they will be internally converted to binary regardless.

In addition to the string/binary representations of Member Names, a Path element can be

  • An integer index, or the atoms first and last, which will select an element out of a JSON Array.
  • {select, {"name","value"|value}} which will select a subset of JSON objects from an Array that have a Member {"Name": "Value"|Value}
  • The atom new, used in conjunction with the functions set/2 and set_p/2, as the final element of a Path will add the supplied value to the stucture as the first element of an Array, the Array is created if necessary

Path, string elements can be binary or not, they will be converetd to binary regardless.

Examples follow.

Usage Examples

Weapons = [{<<"edged">>, [[{<<"type">>, <<"swords">>}, {<<"distance">>, <<"medium">>}],   
                          [{<<"type">>, <<"bayonets">>},  {<<"distance">>, <<"medium">>}],
                          [{<<"type">>, <<"daggers">>}, {<<"distance">>, <<"close">>}]                            


1> jwalk:get({"edged", {select, {"distance","medium"}}},Weapons).

2> jwalk:get({"edged",{select,{"distance","medium"}},1},Weapons).

3> jwalk:get({"edged",{select,{"distance","medium"}},1,"type"},Weapons).

% setting an element ...
4> W2 = jwalk:set({"edged",3, "distance"}, Weapons, <<"very close">>).
  [{<<"type">>,<<"daggers">>},{<<"distance">>,<<"very close">>}]]}]

5> jwalk:get({"edged","distance"},W2).
[<<"medium">>,<<"medium">>,<<"very close">>]

Given a map:

Obj = #{<<"widget">> => 
        #{<<"debug">> => <<"on">>,
          <<"image">> => 
              #{<<"alignment">> => <<"center">>,
                <<"hOffset">> => 250,
                <<"name">> => <<"sun1">>,
                <<"src">> => <<"Images/Sun.png">>,
                <<"vOffset">> => 250},
          <<"keys">> => [],
          <<"text">> => 
              #{<<"alignment">> => <<"center">>,
                <<"data">> => <<"Click Here">>,
                <<"hOffset">> => 250,
                <<"name">> => <<"text1">>,
                <<"onMouseUp">> => <<"sun1.opacity = (sun1.opacity / 100) * 90;">>,
                <<"size">> => 36,
                <<"style">> => <<"bold">>,
                <<"vOffset">> => 100},
          <<"values">> => [1,2,3,4,5],
          <<"version">> => <<"1">>,
          <<"window">> => 
              #{<<"height">> => 500,
                <<"name">> => <<"main_window">>,
                <<"title">> => <<"Sample Konfabulator Widget">>,
                <<"width">> => 500}}}.


1> jwalk:get({"widget","debug"},Obj).

2> jwalk:get({"widget","text"},Obj).
#{<<"alignment">> => <<"center">>,
  <<"data">> => <<"Click Here">>,
  <<"hOffset">> => 250,
  <<"name">> => <<"text1">>,
  <<"onMouseUp">> => <<"sun1.opacity = (sun1.opacity / 100) * 90;">>,
  <<"size">> => 36,
  <<"style">> => <<"bold">>,
  <<"vOffset">> => 100}

set_p creates intermediary nodes:

1> jwalk:set_p({"users", {select, {"name", "sebastian"}}, "location"}, #{}, <<"Germany">>).
#{<<"users">> => [#{<<"location">> => <<"Germany">>,<<"name">> => <<"sebastian">>}]}

2> jwalk:set_p({"users", {select, {"name", "sebastian"}}, "location"}, [{}],    <<"Germany">>).

3> jwalk:set_p({"users", {select, {"name", "sebastian"}}, "location"}, {[]},    <<"Germany">>).

4> jwalk:set_p({"users", {select, {"name", "sebastian"}}, "location"}, {struct,[]}, <<"Germany">>).