Text.JSON.FromJSValue

Interface for extracting data from JSValue.

Basic Parsing

class FromJSValue a where

Structures that can be parsed from JSON. Instances must declare either fromJSValue (parse directly from JSValue) or fromJSValueM (uses MonadReader).

Example implementation:

data D = D String Int

instance FromJSValue D where
  fromJSValue = do
    s <- fromJSValue "string_key"
    i <- fromJSValue "int_key"
    return (D <$> s <*> i)

Note that we make use of MonadReader instance for "(->)" and of Applicative programming style with <$> and <*>.

Note: fromJSValueM is deprecated, in future fromJSValue will be generalized to work in any MonadReader JSValue.

Minimal complete definition

Nothing

Methods

fromJSValue :: JSValue -> Maybe a

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe a)

Instances

FromJSValue Bool

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe Bool

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe Bool)

FromJSValue Double

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe Double

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe Double)

FromJSValue Float

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe Float

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe Float)

FromJSValue Int

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe Int

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe Int)

FromJSValue Int16

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe Int16

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe Int16)

FromJSValue Int32

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe Int32

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe Int32)

FromJSValue Int64

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe Int64

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe Int64)

FromJSValue Integer

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe Integer

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe Integer)

FromJSValue String

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe String

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe String)

FromJSValue ByteString

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe ByteString

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe ByteString)

FromJSValue JSValue

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe JSValue

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe JSValue)

FromJSValue a => FromJSValue [a]

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe [a]

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe [a])

FromJSValue a => FromJSValue (Maybe a)

Parsing any Maybe always returns Just

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe (Maybe a)

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe (Maybe a))

(FromJSValue a, FromJSValue b) => FromJSValue (a, b)

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe (a, b)

fromJSValueM :: (JSValueContainer c, MonadReader c m) => m (Maybe (a, b))

(FromJSValue a, FromJSValue b, FromJSValue c) => FromJSValue (a, b, c)

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe (a, b, c)

fromJSValueM :: (JSValueContainer c0, MonadReader c0 m) => m (Maybe (a, b, c))

(FromJSValue a, FromJSValue b, FromJSValue c, FromJSValue d) => FromJSValue (a, b, c, d)

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe (a, b, c, d)

fromJSValueM :: (JSValueContainer c0, MonadReader c0 m) => m (Maybe (a, b, c, d))

(FromJSValue a, FromJSValue b, FromJSValue c, FromJSValue d, FromJSValue e) => FromJSValue (a, b, c, d, e)

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe (a, b, c, d, e)

fromJSValueM :: (JSValueContainer c0, MonadReader c0 m) => m (Maybe (a, b, c, d, e))

(FromJSValue a, FromJSValue b, FromJSValue c, FromJSValue d, FromJSValue e, FromJSValue f) => FromJSValue (a, b, c, d, e, f)

Defined in Text.JSON.FromJSValue

Methods

fromJSValue :: JSValue -> Maybe (a, b, c, d, e, f)

fromJSValueM :: (JSValueContainer c0, MonadReader c0 m) => m (Maybe (a, b, c, d, e, f))

class FromJSValueWithUpdate a where

Structures that can be parsed from JSON, fields absent in the JSON will be filled in using (optional) original structure.

By convention JSON null should be treated as a request to reset structure element to default value.

Minimal complete definition

Nothing

Methods

fromJSValueWithUpdate :: Maybe a -> JSValue -> Maybe a

fromJSValueWithUpdateM :: (JSValueContainer c, MonadReader c m) => Maybe a -> m (Maybe a)

class MatchWithJSValue a where

Structures that can be matched with JSValue

Minimal complete definition

Nothing

Methods

matchesWithJSValue :: a -> JSValue -> Bool

matchesWithJSValueM :: (JSValueContainer c, MonadReader c m) => a -> m Bool

Data Extraction

jsValueField :: (JSValueContainer c, MonadReader c m, FromJSValue a) => String -> m (Maybe (Maybe a))

Reading the value that is on some field. Returns Nothing if JSON is not an object or field is present but cannot be parsed, 'Just Nothing' if absent, and 'Just (Just a)' otherwise

fromJSValueField :: (JSValueContainer c, MonadReader c m, FromJSValue a) => String -> m (Maybe a)

Reading the value that is on a field. Semantics are a bit involved, example GHCi session should clarify:

Prelude> :set -XNoMonomorphismRestriction
Prelude> let x = withJSValue (JSObject (toJSObject [("key",JSString $ toJSString "value")]))
Prelude> x (fromJSValueField "key") :: IO (Maybe Int)
Nothing
Prelude> x (fromJSValueField "key") :: IO (Maybe (Maybe Int))
Just Nothing
Prelude> x (fromJSValueField "key") :: IO (Maybe (Maybe (Maybe Int)))
Just (Just Nothing)
Prelude> x (fromJSValueField "key") :: IO (Maybe String)
Just "value"
Prelude> x (fromJSValueField "key") :: IO (Maybe (Maybe String))
Just (Just "value")
Prelude> x (fromJSValueField "key") :: IO (Maybe (Maybe (Maybe String)))
Just (Just (Just "value"))
Prelude> let x = withJSValue (JSArray [])
Prelude> x (fromJSValueField "key") :: IO (Maybe String)
Nothing
Prelude> x (fromJSValueField "key") :: IO (Maybe (Maybe String))
Nothing
Prelude> x (fromJSValueField "key") :: IO (Maybe (Maybe (Maybe String)))
Nothing

fromJSValueFieldBase64 :: (JSValueContainer c, MonadReader c m) => String -> m (Maybe ByteString)

Version of fromJSValueField for Base64 encoded data to avoid memory leak.

fromJSValueFieldCustom :: (JSValueContainer c, MonadReader c m) => String -> m (Maybe a) -> m (Maybe a)

Generalization of fromJSValueField. Does not use FromJSValue instances.

fromJSValueCustomMany :: (JSValueContainer c, MonadReader c m) => m (Maybe a) -> m (Maybe [a])

Runs parser on each element of underlaying json. Returns Just iff JSON is array.

fromJSValueCustomList :: (JSValueContainer c, MonadReader c m) => [m (Maybe a)] -> m (Maybe [a])

Generalization of fromJSValueCustomMany, where each element of array can have different parser.

fromJSValueManyWithUpdate :: (JSValueContainer c, MonadReader c m, FromJSValueWithUpdate a, MatchWithJSValue a) => [a] -> m (Maybe [a])

Runs parser on each element of underlying json. Returns Just iff JSON is an array.

Note: This method has quadratic complexity. It is better to write less general matching algorithms that use Maps.

Running

withJSValue :: Monad m => JSValue -> ReaderT JSValue m a -> m a

Simple runner. Example:

let (v :: MyStruct) = runIdentity $ withJSValue js (fromJSValueM)

or inline:

let z = runIdentity $ withJSValue js $ do
            a <- fromJSValueField "a"
            b <- fromJSValueField "b"
            c <- fromJSValueField "c"
            return ((,,) <$> a <*> b <*> <*> c)

or using the monad transformer:

z <- withJSValue js $ do
            a <- fromJSValueField "a"
            b <- fromJSValueField "b"
            c <- fromJSValueField "c"
            return ((,,) <$> a <*> b <*> c)