VtUtils.JSON

JSON utilities

Example of the data type, that supports JSON encoding and decoding with Aeson:

  data Foo = Foo
      { foo :: Int
      , bar :: Text
      } deriving Generic
  instance FromJSON Foo
  instance ToJSON Foo

Documentation

jsonDecodeFile :: forall a. FromJSON a => Text -> IO a

Parses contents of a specified JSON file into a typed data

Data type should be specified with a type annotation:

Example:

dt <- jsonDecodeFile "path/to/foo.json" :: IO Foo

Data must be an instance of FromJSON

File contents are decoded as UTF-8

Throws an error if file cannot be read or data cannot be decoded

Arguments:

• path :: Text: Path to JSON file

Return value: Decoded data

jsonDecodeText :: forall a. FromJSON a => Text -> a

Parses a JSON Text string into a typed data

Data type should be specified with a type annotation:

Example:

let dt = jsonDecodeText text :: Foo

Data must be an instance of FromJSON

Throws an error if data cannot be decoded

Arguments:

• text :: Text: JSON Text string to parse

Return value: Decoded data

jsonEncodeText :: ToJSON a => a -> Text

Encodes a data into a JSON Text string

Data must be an instance of ToJSON

Throws an error if data cannot be encoded

Arguments:

• data :: ToJSON: some data that supports JSON serialization with Aeson

Return value: JSON Text string

jsonGet :: forall a. FromJSON a => Value -> Text -> a

Extract the field value from the specified JSON object

Throws an error, if specified JSON Value is not a JSON object, if it does't contain a specified field, if field type is different from the one specified in type annotation

Data type should be specified with a type annotation:

let obj = object
        [ "foo" .= (42 :: Int)
        , "bar" .= ("baz" :: Text)
        ]
let fooval = jsonGet obj "foo" :: Int
let barval = jsonGet obj "bar" :: Text

Arguments:

• val :: Aeson.Value: JSON value, must be a JSON object
• field :: Text: Field name

Return value: Field value