Fable.TypedJson

Type-driven JSON validation and serialization for F# on BEAM, Python, JavaScript, and .NET, with Pydantic-style validation through refined types.

Derive a decoder and encoder from your F# model, then generate JSON Schema using the same planning logic. Add constraints through custom codecs, choose field naming conventions, and report validation errors with paths.

Install

Install the core package and the adapter for your target:

dotnet add package Fable.TypedJson
dotnet add package Fable.TypedJson.Beam

Target	Adapter package	JSON implementation
BEAM (Erlang)	Fable.TypedJson.Beam	jsx
Python	Fable.TypedJson.Python	json
JavaScript	Fable.TypedJson.JS	JSON.parse / JSON.stringify
.NET	Fable.TypedJson.DotNet	System.Text.Json

The core and Fable adapters target netstandard2.0; the .NET adapter targets net10.0. See Getting started for imports and a complete round trip.

Quick start

open Fable.TypedJson.Json
open Fable.TypedJson.Beam.Json // or .Python.Json / .JS.Json / .DotNet.Json

type Reading = { Location: string; AirTemperature: float }

let codec = auto<Reading> ()

let decoded =
    decodeText codec """{"location":"Oslo","airTemperature":22.5}"""
// Ok { Location = "Oslo"; AirTemperature = 22.5 }

let encoded = codec.encode { Location = "Oslo"; AirTemperature = 22.5 }
// JSON text: {"location":"Oslo","airTemperature":22.5}

auto accepts primitive coercion, such as "22.5" for a float field. Use autoStrict when JSON primitive types must match the model. decodeText distinguishes malformed JSON from typed validation errors.

Build and configure a codec once, then reuse it. On BEAM, retain it in an application or actor owner; a compiled module-level accessor can rebuild its value on each call.

Documentation

Start with the Overview, then follow the concepts you need:

Guide	What you will learn
Getting started	Install an adapter and decode, encode, and describe a model
Decoding	JSON text, parsed values, string maps, strictness, and errors
Validation	Refined types, custom codecs, registries, and model validators
Wire format	Naming, aliases, options, collections, scalars, and tagged unions
JSON Schema	Constraints, recursive definitions, and OpenAPI integration
Architecture	Codec planning, backend interfaces, and optimization boundaries
Performance	Codec ownership and how to interpret benchmarks
Choosing a JSON library	How this approach relates to Thoth.Json and Fable.SimpleJson

For repository setup and changes, read Contributing.