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Jzon is a library for ReScript to encode and decode JSON data with type safety.


  1. yarn add rescript-jzon
  2. Add "rescript-jzon" item to the dependencies key of bsconfig.json

Quick start

Imagine you have the following ReScript types to encode and decode:

type style = {
  size: float,
  color: string,

type point = {
  x: float,
  y: float,
  z: float,
  style: option<style>,

First, define their codecs:

module Codecs = {
  let style = Jzon.object2(
    // Function to encode original object to linear tuple
    ({size, color}) => (size, color),

    // Function to decode linear tuple back to object
    ((size, color)) => {size, color}->Ok,

    // Field names and codecs for the tuple elements
    Jzon.field("size", Jzon.float),
    Jzon.field("color", Jzon.string),

  // Similar codec for another record type
  let point = Jzon.object4(
    ({x, y, z, style}) => (x, y, z, style),
    ((x, y, z, style)) => {x, y, z, style}->Ok,
    Jzon.field("x", Jzon.float),
    Jzon.field("y", Jzon.float),
    // ... supports default values
    Jzon.field("z", Jzon.float)->Jzon.default(0.0),
    // ... may refer your other codecs
    Jzon.field("style", style)->Jzon.optional,

Next, convert between the ReScript types and Js.Json.t with:

let myPoint = {
  x: 1.0,
  y: 2.0,
  z: 3.0,
  style: Some({size: 4.0, color: "#fd0"}),

let json = myPoint->Jzon.encodeWith(Codecs.point)

and back with:

let myPoint = json->Jzon.decodeWith(Codecs.point)