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Please see the README on GitHub at https://github.com/ChrisPenner/json-to-haskell#readme

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Versions [faq],,,,
Change log CHANGELOG.md
Dependencies aeson, aeson-extra, ansi-wl-pprint, base (>=4.7 && <5), bimap, bytestring, casing, containers, json-to-haskell, microlens-platform, mtl, nonempty-containers, optparse-applicative, raw-strings-qq, recursion-schemes, text, unordered-containers, vector [details]
License BSD-3-Clause
Copyright 2020 Chris Penner
Author Chris Penner
Maintainer christopher.penner@gmail.com
Home page https://github.com/ChrisPenner/json-to-haskell#readme
Bug tracker https://github.com/ChrisPenner/json-to-haskell/issues
Source repo head: git clone https://github.com/ChrisPenner/json-to-haskell
Uploaded by ChrisPenner at 2020-11-09T03:44:15Z
Distributions NixOS:
Executables json-to-haskell
Downloads 202 total (15 in the last 30 days)
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Status Hackage Matrix CI
Docs available [build log]
Last success reported on 2020-11-09 [all 1 reports]


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Maintainer's Corner

For package maintainers and hackage trustees

Readme for json-to-haskell-

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<!-- toc GFM --> <!-- tocstop -->

Web interface

Click here for the web interface Web interface source code (Miso)

What is it?

In goes JSON, out comes Haskell!

A handy tool for quickly spec'ing out types for a given JSON type.

Note: This tool isn't perfect, but it should get you most of the way there.

$ cat > company.json
{ "company": 
 { "employees": 
    [ {"name": "Jon", "age": 32} 
    , {"name": "Alice", "age": 27} 
 , "star_rating": 4.7 
$ cat company.json | json-to-haskell
{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE OverloadedStrings #-}
module Model where

import Data.Aeson (ToJSON(..), FromJSON(..), Value(..), (.:), (.=), object)
import Data.Aeson.Types (prependFailure, typeMismatch)
import Data.Text (Text)

data Company = Company
  { companyStarRating :: Double
  , companyEmployees :: [Employees]
  } deriving (Show, Eq, Ord)

data Employees = Employees
  { employeesAge :: Int
  , employeesName :: Text
  } deriving (Show, Eq, Ord)

data Model = Model
  { modelCompany :: Company
  } deriving (Show, Eq, Ord)

instance ToJSON Company where
  toJSON Company{..} = object
    [ "star_rating" .= companyStarRating
    , "employees" .= companyEmployees

instance ToJSON Employees where
  toJSON Employees{..} = object
    [ "age" .= employeesAge
    , "name" .= employeesName

instance ToJSON Model where
  toJSON Model{..} = object
    [ "company" .= modelCompany

instance FromJSON Company where
  parseJSON (Object v) = do
    companyStarRating <- v .: "star_rating"
    companyEmployees <- v .: "employees"
    pure $ Company{..}
  parseJSON invalid = do
    prependFailure "parsing Company failed, "
      (typeMismatch "Object" invalid)

instance FromJSON Employees where
  parseJSON (Object v) = do
    employeesAge <- v .: "age"
    employeesName <- v .: "name"
    pure $ Employees{..}
  parseJSON invalid = do
    prependFailure "parsing Employees failed, "
      (typeMismatch "Object" invalid)

instance FromJSON Model where
  parseJSON (Object v) = do
    modelCompany <- v .: "company"
    pure $ Model{..}
  parseJSON invalid = do
    prependFailure "parsing Model failed, "
      (typeMismatch "Object" invalid)


Take your pick:

cabal install haskell-to-json
# OR
stack install haskell-to-json


There's a web interface here if you prefer.

Otherwise, install the cli and ask for help; the cli will have the most up to date help message:

$ json-to-haskell --help
Usage: json-to-haskell [-t|--tab-stop ARG] [-n|--numbers ARG] [-s|--strings ARG]
                       [-m|--maps ARG] [-l|--lists ARG] [--no-module-header]
                       [--no-instances] [--no-prefix-record-fields] [--strict]

Available options:
  -t,--tab-stop ARG        Number of spaces to indent each level.
  -n,--numbers ARG         Type to use for numbers.

                               Use floats for numbers with decimals, Int for whole numbers.
                               Use floats for numbers with decimals, Int for whole numbers.
                               Use floats for all numbers.
                               Use doubles for all numbers.
                               Use scientific for all numbers.

  -s,--strings ARG         Type to use for strings.

                             Use String for strings.
                             Use Text for strings.
                             Use ByteString for strings.

  -m,--maps ARG            Type to use for maps.

                             Use Data.Map for maps.
                             Use Data.HashMap for maps.

  -l,--lists ARG           Type to use for lists.

                             Use [] for lists.
                             Use Data.Vector for lists.

  --no-module-header       Omit the module header containing language extensions, module definition and imports.
  --no-instances           Omit the ToJSON and FromJSON instances.
                           Omit record field prefixes.
  --strict                 Use strict record fields.
  -h,--help                Show this help text

Help Wanted

Want to help out?

I'd love to have a Purescript version of the app! It should be pretty easy to port, you'd just need to adapt the "printer" for the parsed data type. Check out Printer.hs .

It'd also be really nice to port the whole project to Purescript, the current miso build is difficult to set up locally.