package melange-json

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Compositional JSON encode/decode library for Melange


Dune Dependency






Provides encoders and decoders to convert JSON values into typed values. With the possibility to create custom encoders and decoders.

Published: 06 Feb 2024



Compositional JSON encode/decode library for BuckleScript.

Based on @glennsl/bs-json.

The Decode module in particular provides a basic set of decoder functions to be composed into more complex decoders. A decoder is a function that takes a Js.Json.t and either returns a value of the desired type if successful or raises a DecodeError exception if not. Other functions accept a decoder and produce another decoder. Like array, which when given a decoder for type t will return a decoder that tries to produce a value of type t array. So to decode an int array you combine with Json.Decode.array into Json.Decode.(array int). An array of arrays of ints? Json.Decode.(array (array int)). Dict containing arrays of ints? Json.Decode.(dict (array int)).


type line = {
  start: point,
  end_: point,
  thickness: option(int)
and point = {
  x: int,
  y: int

module Decode = {
  let point = json =>
      x: json |> field("x", int),
      y: json |> field("y", int)

  let line = json =>
      start:     json |> field("start", point),
      end_:      json |> field("end", point),
      thickness: json |> optional(field("thickness", int))

let data = {| {
  "start": { "x": 1, "y": -4 },
  "end":   { "x": 5, "y": 8 }
} |};

let line = data |> Json.parseOrRaise
                |> Decode.line;

NOTE: Json.Decode.{ ... } creates an ordinary record, but also opens the Json.Decode module locally, within the scope delimited by the curly braces, so we don't have to qualify the functions we use from it, like field, int and optional here. You can also use Json.Decode.( ... ) to open the module locally within the parentheses, if you're not creating a record.

See examples for more.


Install opam package manager.


opam install melange-json


Add melange-json to the libraries field in your dune file:

; ...
  (libraries melange-json)
; ...



For the moment, please see the interface files:

Writing custom decoders and encoders

If you look at the type signature of Json.Decode.array, for example, you'll see it takes an 'a decoder and returns an 'a array decoder. 'a decoder is just an alias for Js.Json.t -> 'a, so if we expand the type signature of array we'll get (Js.Json.t -> 'a) -> Js.Json.t -> 'a array. We can now see that it is a function that takes a decoder and returns a function, itself a decoder. Applying the int decoder to array will give us an int array decoder, a function Js.Json.t -> int array.

If you've written a function that takes just Js.Json.t and returns user-defined types of your own, you've already been writing composable decoders! Let's look at Decode.point from the example above:

let point = json => {
  open! Json.Decode;
    x: json |> field("x", int),
    y: json |> field("y", int)

This is a function Js.Json.t -> point, or a point decoder. So if we'd like to decode an array of points, we can just pass it to Json.Decode.array to get a point array decoder in return.


To write a decoder builder like Json.Decode.array we need to take another decoder as an argument, and thanks to currying we just need to apply it where we'd otherwise use a fixed decoder. Say we want to be able to decode both int points and float points. First we'd have to parameterize the type:

type point('a) = {
  x: 'a,
  y: 'a

Then we can change our point function from above to take and use a decoder argument:

let point = (decodeNumber, json) => {
  open! Json.Decode;
    x: json |> field("x", decodeNumber),
    y: json |> field("y", decodeNumber)

And if we wish we can now create aliases for each variant:

let intPoint = point(;
let floatPoint = point(Json.Decode.float);

Encoders work exactly the same way, just in reverse. 'a encoder is just an alias for 'a -> Js.Json.t, and this also transfers to composition: 'a encoder -> 'a array encoder expands to ('a -> Js.Json.t) -> 'a array -> Js.Json.t.


This work is dual-licensed under LGPL 3.0 and MPL 2.0. You can choose between one of them if you use this work.

Please see LICENSE.LGPL-3.0 and LICENSE.MPL-2.0 for the full text of each license.

SPDX-License-Identifier: LGPL-3.0 OR MPL-2.0



  • Rewrote Encode.list to be stack-safe and much faster.


  • Added


  • Dual licensed as LGPL-3.0 and MPL-2.0. MPL is mostly equivalent to LGPL but relaxes its restriction on linking, which works better with the JavaScript packaging and distribution model.


  • Removed deprecated arrayOf encoder

  • Renamed dict encoder to jsonDict

  • Added new dict encoder that takes an additional encoder argument used to encode the contained values, and so it's consistent with the respective dict decoder.


  • Bumped bs-platform peer dependency to 5.0.4 to stop the compiler's complaining.


  • Replace usage of Js.Date.toJSON with Js.Date.toJSONUsafe, which is exactly the same, just to avoid deprecation warnings for end users (Thanks Bob!)

  • Requires bs-platform >= 4.0.2


  • Removed Json.Decode.boolean, Json.Encode.boolean, Json.Encode.booleanArray

  • Requires bs-platform >= 3.0.0


  • Reverted commits that broke backwards compatibility despite only affecting the implementation


  • Deprecated Json.Decode.boolean, Json.Encode.boolean, Json.Encode.booleanArray

  • Added Json.Encode.boolArray


  • Added Json.Encode.char and Json.Decode.char


  • Added "stack traces" to higher-order decoders, making it easier to find the location of an error.


  • Moved repository from reasonml-community/bs-json to glennsl/bs-json

  • Renamed NPM package from bs-json to @glennsl/bs-json


  • Replaced Json.Encoder.array with Json.Encode.arrayOf renamed to array. Deprecated arrayOf alias.

  • Added Json.parse, Json.parseOrRaise, Json.stringify

  • Added date encoder and decoder

  • Added tuple2/tuple3/tuple4 encoders and decoders

  • Fixed bug where js integers > 32-bit were rejected as integers by (#15)


  • Added Json.Encode.bool

  • Added Json.Encode.pair

  • Added Json.Encode.withDefault

  • Added Json.Encode.nullable

  • Added Json.Encode.arrayOf

  • Added Json.Encode.jsonArray as replacement for Json.Encode.array

  • Deprecated Json.Encode.array


  • Fixed embarrassing bug where an API was used that isn't available on IE (honestly more embarrassed on behalf of IE though)


  • Added Json.Decode.pair


  • Added Json.Encode.list


  • Breaking: Renamed Json.Encode.object_ to Json.Encode.dict

  • Added Json.Encode.object_ taking a list of properties instead of a Json.Dict.t as before

Dependencies (3)

  1. melange >= "3.0.0"
  2. ocaml
  3. dune >= "3.9"

Dev Dependencies (5)

  1. odoc with-doc
  2. ocaml-lsp-server with-test
  3. opam-check-npm-deps with-test
  4. reason >= "3.10.0" & with-test
  5. melange-jest with-test




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