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mjson - An Erlang library for encoding and decoding JSON

Small (less than 100 lines each) and quite performant JSON decoder and encoder. No external dependencies. Works on AtomVM and OTP >= R16.

RFC 8259 compliant.

Passes all tests in https://github.com/nst/JSONTestSuite.

Useful when a simple, basic JSON encoder / decoder is needed.

Erlang representation of JSON

JSON type Erlang type
object map()
array list()
string binary()
number integer() | float()
boolean boolean()
null null

Note that there are three JSON literals true, false, and null, and they are decoded into the corresponding atoms 'true', 'false', and 'null'.

For example, the following JSON value:

{
  "id": 198030,
  "sizes": [7.0, 12.0],
  "valid": false
}

is decoded to the Erlang term:

#{
  <<"id">> => 19830,
  <<"sizes">> => [7.0, 12.0],
  <<"valid">> => false
 }

If the option key_as_existing_atom is given when decoding, object member names are converted to existing atoms. If the atom doesn't exist, the object member name is returned as a binary.

The key_as_existing_atom option is useful when the schema for the JSON document is known, and all keys are already present in the code.

Encoding

In addition to the representation described above, the encoder encodes Erlang atoms into JSON strings (except for the atoms 'true', 'false', and 'null'), encodes calendar:datetime() values as RFC3339 / ISO8601 strings, and also encodes Erlang map keys of type iodata to JSON strings.

For example, the following Erlang term:

#{
  "bar" => baz,
  foo => null,
  time => {{2024,4,1},{3,47,0}}
 }

is encoded to:

{
  "bar": "baz",
  "foo": null,
  "time": "2024-04-01T03:47:00Z"
}

which is decoded to:

#{
  <<"bar">> => <<"baz">>,
  <<"foo">> => null,
  <<"time">> => <<"2024-04-01T03:47:00Z">>
 }

API documentation

See mjson.md.

Build for AtomVM

AtomVM doesn't implement erlang:list_to_integer/2 and the module re. Define the variables below to compile without using these functions. Without erlang:list_to_integer/2, mjson:decode/1 will not translate unicode-escaped characters.

ERLC_OPTS="-D NO_LIST_TO_INTEGER_2 -D NO_RE_RUN_2" make