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aeson: Working with JSON data

aeson is an efficient and easy to use library for decoding (parsing) and encoding JSON. The ways to work with aeson ranges from simple (but inflexible) to complex (but flexible).

API docs

The API documentation can be found here. There’s plenty of good information in the API docs.

Decoding and encoding standard data types

JSON strings can trivially be decoded to Haskell values, provided that the types of those values are instances of FromJSON. Analogously, types that are instances of ToJSON can easily be encoded back to JSON strings. aeson comes with convenient FromJSON and ToJSON instances for the standard data types (numbers, booleans, lists, tuples, dates, and so on).

Let’s try these instances out in GHCi. You’ll need to import Data.Aeson, Data.Map, Data.Text and Data.Time.Clock in order to try out the examples above. You’ll also need to launch GHCi with ghci -XOverloadedStrings.

λ> decode "true" :: Maybe Bool
Just True

λ> encode True
"true"

λ> decode "[1, 2, 3]" :: Maybe [Int]
Just [1,2,3]

λ> encode [1, 2, 3]
"[1,2,3]"

λ> decode ""1984-10-15T00:00:00Z"" :: Maybe UTCTime
Just 1984-10-15 00:00:00 UTC

λ> encode (read "1984-10-15 00:00:00 UTC" :: UTCTime)
""1984-10-15T00:00:00Z""

λ> decode "{ "foo": 0, "bar":1, "baz": 2 }" :: Maybe (Map Text Int)
Just (fromList [("bar",1),("baz",2),("foo",0)])

λ> encode (fromList [ ("bar" :: Text, 1 :: Int)
                    , ("baz" :: Text, 2 :: Int)
        , ("foo" :: Text, 0 :: Int) ])
"{"bar":1,"baz":2,"foo":0}"

Automatic decoding and encoding

So how do you define FromJSON and ToJSON instances for types that don’t have them?

One simple way to do this is to use the DeriveGeneric extension and have the compiler generate the implementation for you.

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE OverloadedStrings #-}
import Data.Aeson
import Data.Text
import GHC.Generics

data Person = Person { name :: Text, age :: Int } deriving (Generic, Show)

instance FromJSON Person
instance ToJSON Person

main :: IO ()
main = do
    putStrLn $ "Encode: " ++ (show (encode (Person { name = "Joe", age = 12 })))
    putStrLn $ "Decode: " ++
        (show (decode "{ "name": "Joe", "age": 12 }" :: Maybe Person))

For compatibility reasons, the default DeriveGeneric ToJSON implementation is relatively slow. So for better efficiency, you might want to override the toEncoding function ToJSON instance with:

instance ToJSON Person where
    toEncoding = genericToEncoding defaultOptions

You can also derive instances at compile-time, using Data.Aeson.TH and the TemplateHaskell extension.

Decoding and encoding in your own way

FromJSON and ToJSON instances can also be written by hand.

For FromJSON (decoding), you need to define the parseJSON function, which takes a JSON value (of the aeson type Value) and returns a Haskell value of your type. In order to define your own ToJSON instance, you need to do the reverse – create a Value from a Haskell value of your type.

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
{-# LANGUAGE OverloadedStrings #-}
import Control.Applicative
import Data.Aeson
import Data.Text (Text)

-- Same type as before, but without the Generic instance.
data Person = Person { name :: Text, age :: Int } deriving Show

-- We expect a JSON object, so we fail at any non-Object value.
instance FromJSON Person where
    parseJSON (Object v) = Person <$> v .: "name" <*> v .: "age"
    parseJSON _ = empty

instance ToJSON Person where
    toJSON (Person name age) = object ["name" .= name, "age" .= age]

-- The main function is unchanged from before.
main :: IO ()
main = do
    putStrLn $ "Encode: " ++ (show (encode (Person { name = "Joe", age = 12 })))
    putStrLn $ "Decode: " ++
        (show (decode "{ "name": "Joe", "age": 12 }" :: Maybe Person))

Above, parseJSON constructs the Haskell value using a JSON object. If some other kind of JSON value is provided, decoding will fail. Note that parseJSON above operates in an Applicative Parser context. This is what parseJSON could look like using a monadic computation:

instance FromJSON Person where
    parseJSON (Object v) = do
        name <- v .: "name"
        age <- v .: "age"
        return (Person { name = name, age = age })
    parseJSON _ = empty

The toJSON function above uses the object function to define a JSON object, which takes a list of key-value Pair values. The Pair type is simply defined as (Text, Value), and .= is a helper to encode the right-hand side to its corresponding JSON value.

Working with a arbitrary JSON data

Sometimes you want to work with the JSON data without actually converting it to some specific type first. One way to do this is to work with the JSON abstract syntax tree (AST). This can be done by simply decoding it to a Value, like so:

λ> decode "{ "foo": false, "bar": [1, 2, 3] }" :: Maybe Value
Just (Object (fromList [("foo",Bool False),("bar",Array [Number 1.0,Number
2.0,Number 3.0])]))

You can also decode your JSON string to any of the JSON types that Value encapsulates:

JSON aeson Type Standard Type
Array Array Vector Value
Boolean Bool Bool
Number Number Scientific
Object Object HashMap Text Value
String String Text

Once you have a JSON value, you can define a Parser like we did in parseJSON above, and run it using parse, parseEither or parseMaybe.

Decoding JSON to a specific Haskell type is actually a two-step process – first, the JSON string is converted to a Value, and then the FromJSON instance is used to convert that Value to the specific type.

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