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json_of_jsonm_lib

json_of_jsonm_lib is a JSON encoder and decoder library that converts text to and from a json type. The library has the following features:

  • Uses jsonm to do the actual stream encoding and decoding
  • The json type is compatible with and a subset of yojson's json type
  • Provides both string and channel interfaces by default
  • The Json_string module provides a standard type t interface in addition to the json type
  • Both result and exception functions are provided in most cases
  • The Json_encoder_decoder functor allows additional IO mechanisms, including Async, to be defined easily. Note the Async version is not included to prevent dependencies on the Async libraries. See the examples for a basic implementation

json type

The json type is defined as follows:

type json =
  [ `Null
  | `Bool of bool
  | `Float of float
  | `String of string
  | `List of json list
  | `Assoc of (string * json) list
  ]

Note that unlike yosjon there is no integer type as jsonm is a strict implementation of JSON and JSON itself does not support integers. However, jsonm is round trip safe for values that are integers on input and are within the JSON integer range (-2^53+1..2^53-1). See RFC 7159 for more details

API

The API provides two mechanisms for reporting errors: The standard functions return a result type while the _exn versions raise an exception. Note that the json encoding functions can also return an error as bad floating point values are detected.

Module Json_string

The Json_string module provices the following types and functions

  • type json
  • type t is an alias for the json type
  • json_of_string_exn str and of_string str convert str to type json raising an exception on error
  • json_of_string str is the same as json_of_string_exn but returns a (json, string) result rather than raising an exception
  • json_to_string_exn json and to_string json convert json to a string raising an exception on error
  • json_to_string str is the same as json_to_string_exn but returns a (unit, string) result rather than raising an exception

Module Json_channel

The Json_string module provices the following types and functions

  • type json
  • json_of_channel in_channel converts the text from in_channel into type json returning a (json, string) result
  • json_of_channel str_exn is the same as json_of_channel but raises an exception rather than returning a result type
  • json_to_channel out_channel json encodes json and writes the result to out_channel returning a (unit, string) result
  • json_to_channel_exn str is the same as json_to_channel_exn but raises an exception rather than returning a result type

Example usage

Basic string

open Json_of_jsonm_lib

let () =
  let json = Json_string.of_string "{ \"a\": 1 }" in
  let json_s = Json_string.to_string json in
  Printf.printf "%s\n" json_s

Note that unlike the jsonm encoder the json_of_jsonm_lib encoder detects bad floats (NaN and Inf) and returns an error, in this case the to_string function generates an exception

Channel input and output

open Json_of_jsonm_lib

let () =
  let inp = open_in "./x" in
  let outp = open_out "./z" in
  Json_channel.json_of_channel_exn inp
  |> Json_channel.json_to_channel_exn outp

Using the Jsom_encoder_decoder functor

The following is an example of implementing the Async version using the Jsom_encoder_decoder functor

open Async
open Json_of_jsonm_lib

module Json_async = struct
  module Json_of_async = Json_of_jsonm_monad.Make(struct
      type 'a t = 'a Deferred.t

      let return = Deferred.return
      let (>>=) = Deferred.Monad_infix.(>>=)
    end)


  let reader rd buf size =
    Reader.read rd ~len:size buf
    >>= function
    | `Eof -> return 0
    | `Ok len -> return len

  let read rd =
    let reader = reader rd in
    Json_of_async.decode ~reader

  let read_exn rd =
    let reader = reader rd in
    Json_of_async.decode_exn ~reader

  let write wr =
    let writer buf size = Writer.write ~len:size wr buf |> return in
    Json_of_async.encode ~writer

  let write_exn wr =
    let writer buf size = Writer.write ~len:size wr buf |> return in
    Json_of_async.encode_exn ~writer
end

The following shows a sample usage of the module

Reader.open_file "./x"
>>= fun rd -> Json_async.read_exn rd
>>= fun json -> Writer.open_file "./z"
>>= fun wr -> Json_async.write wr json

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A simple access library for jsonm

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