JSON to Haskell Converter

JSON is a lightweight, human-readable key-value format — widely used for REST API responses, configuration files, NoSQL documents. Converting it to strongly-typed Haskell structures eliminates runtime surprises and speeds up development. This tool does it in one click, entirely in your browser.

Generate Haskell data types with Aeson FromJSON / ToJSON instances for type-safe API parsing.

How to use this converter

1. Paste your JSON into the left editor panel
2. Click Generate
3. Copy the generated Haskell code from the right panel

No account. No upload. No tracking. Runs entirely in your browser.

Example Haskell output

{-# LANGUAGE DeriveGeneric #-}
 
import GHC.Generics (Generic)
import Data.Aeson (FromJSON, ToJSON)
import Data.Text (Text)
 
data Address = Address
  { city :: Text
  , zip  :: Text
  } deriving (Show, Generic)
 
instance FromJSON Address
instance ToJSON Address
 
data User = User
  { userId  :: Int
  , name    :: Text
  , email   :: Maybe Text
  , address :: Address
  } deriving (Show, Generic)

The problem with manual JSON-to-Haskell mapping

Haskell is a purely functional, lazy evaluation language — typed data models are central to how it works. Yet copying fields from JSON payloads into Haskell data / newtype definitions introduces subtle errors:

• Mistyped field names cause silent deserialization failures
• Missing optional fields trigger runtime panics or null errors
• Schema drift between API and model goes undetected until production

Haskell's aeson library uses FromJSON/ToJSON type classes — this converter generates instances compatible with GHC's DeriveGeneric.

This converter eliminates the manual step entirely.

Haskell and JSON: what you need to know

Haskell is a purely functional, lazy evaluation language, used in financial systems, compilers, and research. It uses data record types with Aeson for structured data — making it a natural fit for JSON-driven applications.

What the converter generates

Output produces Haskell data declarations with deriving (Generic, Show) and Aeson FromJSON / ToJSON instances. Optional fields use Maybe and field names follow camelCase conventions.

A common gotcha

Haskell field naming conventions differ from JSON — aeson handles snake_case to camelCase automatically via defaultOptions.

JSON input characteristics

JSON supports nested objects, arrays, strings, numbers, booleans, and null. Tools like jq, Postman, and browser DevTools make JSON the most developer-friendly data exchange format.

When should I use this converter?

Use this when building Haskell web services or scripts that consume external JSON/YAML data and need fully typed, Aeson-compatible data types.

Common use cases

• Generating Aeson-compatible types for Servant or Yesod API handlers
• Creating typed data structures for Haskell CLI tools parsing config files
• Producing FromJSON instances for Haskell data pipeline scripts
• Validating JSON contract compatibility with Haskell type definitions
• Generating Haskell models from financial modeling API responses
• Creating typed DTOs for compiler tooling
• Rapid prototyping with real JSON payloads
• Keeping Haskell structs in sync when JSON schemas evolve
• Onboarding new team members by auto-generating the data layer

Frequently asked questions

Is my JSON data sent to a server?

No. All conversion runs locally in your browser using client-side JavaScript. Your JSON data never leaves your machine.

What JSON inputs does this converter accept?

Paste any valid JSON — including REST API responses, configuration files, NoSQL documents. The converter infers types and generates a matching Haskell model.

What serialization library does the generated Haskell code assume?

The generated code is compatible with the standard Haskell serialization ecosystem — Haskell data types with Generic-derived Aeson instances. No unusual dependencies required.

What version of Haskell does the output target?

The converter targets modern Haskell conventions — Haskell data types with Generic-derived Aeson instances. If you need output for an older version, the generated code can typically be adapted with minor changes.

Related tools on LangStop

• JSON Formatter & Validator
• JSON to Python Converter
• JSON to Go Converter
• JSON to Java Converter
• JSON to JSON Schema

If you work frequently with JSON and Haskell, bookmark this page to skip the manual model-writing step entirely.