json-to-haskell
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)
Installation
Take your pick:
cabal install haskell-to-json
# OR
stack install haskell-to-json
Usage
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.
'smart-floats':
Use floats for numbers with decimals, Int for whole numbers.
'smart-doubles':
Use floats for numbers with decimals, Int for whole numbers.
'floats':
Use floats for all numbers.
'doubles':
Use doubles for all numbers.
'scientific':
Use scientific for all numbers.
-s,--strings ARG Type to use for strings.
'string':
Use String for strings.
'text':
Use Text for strings.
'bytestring':
Use ByteString for strings.
-m,--maps ARG Type to use for maps.
'map':
Use Data.Map for maps.
'hashmap':
Use Data.HashMap for maps.
-l,--lists ARG Type to use for lists.
'list':
Use [] for lists.
'vector':
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.
--no-prefix-record-fields
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.
