Compositional JSON encode/decode library for BuckleScript
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Latest commit d545caf Aug 1, 2018


Compositional JSON encode/decode library for BuckleScript.

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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: float,
  y: float

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

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

let data = {| {
  "start": { "x": 1.1, "y": -0.4 },
  "end":   { "x": 5.3, "y": 3.8 }
} |};

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

See examples for more.


npm install --save @glennsl/bs-json

Then add @glennsl/bs-json to bs-dependencies in your bsconfig.json:

  "bs-dependencies": ["@glennsl/bs-json"]



For the moment, please see the interface files:

Writing custom decoders and encoders

If you look at the type signature of Js.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", float),
    y: json |> field("y", float)

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.



  • 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