prolude-0.0.0.8: ITProTV's custom prelude
Safe HaskellNone
LanguageHaskell2010

Prolude.Json

Synopsis

Documentation

eitherDecode :: FromJSON a => ByteString -> Either String a #

Like decode but returns an error message when decoding fails.

encode :: ToJSON a => a -> ByteString #

Efficiently serialize a JSON value as a lazy ByteString.

This is implemented in terms of the ToJSON class's toEncoding method.

class ToJSON a #

A type that can be converted to JSON.

Instances in general must specify toJSON and should (but don't need to) specify toEncoding.

An example type and instance:

-- Allow ourselves to write Text literals.
{-# LANGUAGE OverloadedStrings #-}

data Coord = Coord { x :: Double, y :: Double }

instance ToJSON Coord where
  toJSON (Coord x y) = object ["x" .= x, "y" .= y]

  toEncoding (Coord x y) = pairs ("x" .= x <> "y" .= y)

Instead of manually writing your ToJSON instance, there are two options to do it automatically:

  • Data.Aeson.TH provides Template Haskell functions which will derive an instance at compile time. The generated instance is optimized for your type so it will probably be more efficient than the following option.
  • The compiler can provide a default generic implementation for toJSON.

To use the second, simply add a deriving Generic clause to your datatype and declare a ToJSON instance. If you require nothing other than defaultOptions, it is sufficient to write (and this is the only alternative where the default toJSON implementation is sufficient):

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics

data Coord = Coord { x :: Double, y :: Double } deriving Generic

instance ToJSON Coord where
    toEncoding = genericToEncoding defaultOptions

If on the other hand you wish to customize the generic decoding, you have to implement both methods:

customOptions = defaultOptions
                { fieldLabelModifier = map toUpper
                }

instance ToJSON Coord where
    toJSON     = genericToJSON customOptions
    toEncoding = genericToEncoding customOptions

Previous versions of this library only had the toJSON method. Adding toEncoding had two reasons:

  1. toEncoding is more efficient for the common case that the output of toJSON is directly serialized to a ByteString. Further, expressing either method in terms of the other would be non-optimal.
  2. The choice of defaults allows a smooth transition for existing users: Existing instances that do not define toEncoding still compile and have the correct semantics. This is ensured by making the default implementation of toEncoding use toJSON. This produces correct results, but since it performs an intermediate conversion to a Value, it will be less efficient than directly emitting an Encoding. (this also means that specifying nothing more than instance ToJSON Coord would be sufficient as a generically decoding instance, but there probably exists no good reason to not specify toEncoding in new instances.)

Instances

Instances details
ToJSON Bool 
Instance details

Defined in Data.Aeson.Types.ToJSON

ToJSON Char 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Double 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Float 
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ToJSON Int 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Int8 
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ToJSON Int16 
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ToJSON Int32 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Int64 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Integer 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Natural 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Ordering 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Word 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Word8 
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ToJSON Word16 
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ToJSON Word32 
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ToJSON Word64 
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Defined in Data.Aeson.Types.ToJSON

ToJSON () 
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Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: () -> Value #

toEncoding :: () -> Encoding #

toJSONList :: [()] -> Value #

toEncodingList :: [()] -> Encoding #

ToJSON Scientific 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Text 
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Defined in Data.Aeson.Types.ToJSON

ToJSON UTCTime 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Value 
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Defined in Data.Aeson.Types.ToJSON

ToJSON DotNetTime 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Text 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Number 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Void 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Version 
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Defined in Data.Aeson.Types.ToJSON

ToJSON CTime 
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Defined in Data.Aeson.Types.ToJSON

ToJSON IntSet 
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Defined in Data.Aeson.Types.ToJSON

ToJSON PersistValue 
Instance details

Defined in Database.Persist.Types.Base

ToJSON ZonedTime 
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Defined in Data.Aeson.Types.ToJSON

ToJSON LocalTime 
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Defined in Data.Aeson.Types.ToJSON

ToJSON TimeOfDay 
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Defined in Data.Aeson.Types.ToJSON

ToJSON CalendarDiffTime 
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Defined in Data.Aeson.Types.ToJSON

ToJSON SystemTime

Encoded as number

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Defined in Data.Aeson.Types.ToJSON

ToJSON NominalDiffTime 
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Defined in Data.Aeson.Types.ToJSON

ToJSON DiffTime 
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Defined in Data.Aeson.Types.ToJSON

ToJSON DayOfWeek 
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Defined in Data.Aeson.Types.ToJSON

ToJSON Day 
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Defined in Data.Aeson.Types.ToJSON

ToJSON CalendarDiffDays 
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Defined in Data.Aeson.Types.ToJSON

ToJSON UUID 
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ToJSON a => ToJSON [a] 
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Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: [a] -> Value #

toEncoding :: [a] -> Encoding #

toJSONList :: [[a]] -> Value #

toEncodingList :: [[a]] -> Encoding #

ToJSON a => ToJSON (Maybe a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

(ToJSON a, Integral a) => ToJSON (Ratio a) 
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Defined in Data.Aeson.Types.ToJSON

ToJSON a => ToJSON (Min a) 
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Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Min a -> Value #

toEncoding :: Min a -> Encoding #

toJSONList :: [Min a] -> Value #

toEncodingList :: [Min a] -> Encoding #

ToJSON a => ToJSON (Max a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Max a -> Value #

toEncoding :: Max a -> Encoding #

toJSONList :: [Max a] -> Value #

toEncodingList :: [Max a] -> Encoding #

ToJSON a => ToJSON (First a) 
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Defined in Data.Aeson.Types.ToJSON

ToJSON a => ToJSON (Last a) 
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Defined in Data.Aeson.Types.ToJSON

ToJSON a => ToJSON (WrappedMonoid a) 
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Defined in Data.Aeson.Types.ToJSON

ToJSON a => ToJSON (Option a) 
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ToJSON a => ToJSON (Identity a) 
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ToJSON a => ToJSON (First a) 
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ToJSON a => ToJSON (Last a) 
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ToJSON a => ToJSON (Dual a) 
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ToJSON a => ToJSON (NonEmpty a) 
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ToJSON a => ToJSON (IntMap a) 
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Defined in Data.Aeson.Types.ToJSON

ToJSON v => ToJSON (Tree v) 
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Defined in Data.Aeson.Types.ToJSON

ToJSON a => ToJSON (Seq a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Seq a -> Value #

toEncoding :: Seq a -> Encoding #

toJSONList :: [Seq a] -> Value #

toEncodingList :: [Seq a] -> Encoding #

ToJSON a => ToJSON (Set a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Set a -> Value #

toEncoding :: Set a -> Encoding #

toJSONList :: [Set a] -> Value #

toEncodingList :: [Set a] -> Encoding #

ToJSON1 f => ToJSON (Fix f)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Fix f -> Value #

toEncoding :: Fix f -> Encoding #

toJSONList :: [Fix f] -> Value #

toEncodingList :: [Fix f] -> Encoding #

(ToJSON1 f, Functor f) => ToJSON (Mu f)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Mu f -> Value #

toEncoding :: Mu f -> Encoding #

toJSONList :: [Mu f] -> Value #

toEncodingList :: [Mu f] -> Encoding #

(ToJSON1 f, Functor f) => ToJSON (Nu f)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Nu f -> Value #

toEncoding :: Nu f -> Encoding #

toJSONList :: [Nu f] -> Value #

toEncodingList :: [Nu f] -> Encoding #

ToJSON a => ToJSON (DNonEmpty a)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.ToJSON

ToJSON a => ToJSON (DList a) 
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Defined in Data.Aeson.Types.ToJSON

(Prim a, ToJSON a) => ToJSON (PrimArray a) 
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ToJSON a => ToJSON (SmallArray a) 
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Defined in Data.Aeson.Types.ToJSON

ToJSON a => ToJSON (Array a) 
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Defined in Data.Aeson.Types.ToJSON

ToJSON a => ToJSON (Maybe a)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.ToJSON

ToJSON a => ToJSON (HashSet a) 
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Defined in Data.Aeson.Types.ToJSON

(Vector Vector a, ToJSON a) => ToJSON (Vector a) 
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Defined in Data.Aeson.Types.ToJSON

(Storable a, ToJSON a) => ToJSON (Vector a) 
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Defined in Data.Aeson.Types.ToJSON

(Prim a, ToJSON a) => ToJSON (Vector a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

ToJSON a => ToJSON (Vector a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

(ToJSON a, ToJSON b) => ToJSON (Either a b) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Either a b -> Value #

toEncoding :: Either a b -> Encoding #

toJSONList :: [Either a b] -> Value #

toEncodingList :: [Either a b] -> Encoding #

(ToJSON a, ToJSON b) => ToJSON (a, b) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b) -> Value #

toEncoding :: (a, b) -> Encoding #

toJSONList :: [(a, b)] -> Value #

toEncodingList :: [(a, b)] -> Encoding #

(ToJSON v, ToJSONKey k) => ToJSON (HashMap k v) 
Instance details

Defined in Data.Aeson.Types.ToJSON

(ToJSON v, ToJSONKey k) => ToJSON (Map k v) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Map k v -> Value #

toEncoding :: Map k v -> Encoding #

toJSONList :: [Map k v] -> Value #

toEncodingList :: [Map k v] -> Encoding #

HasResolution a => ToJSON (Fixed a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

ToJSON (Proxy a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

(ToJSON a, ToJSON b) => ToJSON (Pair a b)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Pair a b -> Value #

toEncoding :: Pair a b -> Encoding #

toJSONList :: [Pair a b] -> Value #

toEncodingList :: [Pair a b] -> Encoding #

(ToJSON a, ToJSON b) => ToJSON (These a b)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: These a b -> Value #

toEncoding :: These a b -> Encoding #

toJSONList :: [These a b] -> Value #

toEncodingList :: [These a b] -> Encoding #

(ToJSON a, ToJSON b) => ToJSON (Either a b)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Either a b -> Value #

toEncoding :: Either a b -> Encoding #

toJSONList :: [Either a b] -> Value #

toEncodingList :: [Either a b] -> Encoding #

(ToJSON a, ToJSON b) => ToJSON (These a b)

Since: aeson-1.5.1.0

Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: These a b -> Value #

toEncoding :: These a b -> Encoding #

toJSONList :: [These a b] -> Value #

toEncodingList :: [These a b] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c) => ToJSON (a, b, c) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c) -> Value #

toEncoding :: (a, b, c) -> Encoding #

toJSONList :: [(a, b, c)] -> Value #

toEncodingList :: [(a, b, c)] -> Encoding #

ToJSON a => ToJSON (Const a b) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Const a b -> Value #

toEncoding :: Const a b -> Encoding #

toJSONList :: [Const a b] -> Value #

toEncodingList :: [Const a b] -> Encoding #

ToJSON b => ToJSON (Tagged a b) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Tagged a b -> Value #

toEncoding :: Tagged a b -> Encoding #

toJSONList :: [Tagged a b] -> Value #

toEncodingList :: [Tagged a b] -> Encoding #

(ToJSON1 f, ToJSON1 g, ToJSON a) => ToJSON (These1 f g a)

Since: aeson-1.5.1.0

Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: These1 f g a -> Value #

toEncoding :: These1 f g a -> Encoding #

toJSONList :: [These1 f g a] -> Value #

toEncodingList :: [These1 f g a] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c, ToJSON d) => ToJSON (a, b, c, d) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c, d) -> Value #

toEncoding :: (a, b, c, d) -> Encoding #

toJSONList :: [(a, b, c, d)] -> Value #

toEncodingList :: [(a, b, c, d)] -> Encoding #

(ToJSON1 f, ToJSON1 g, ToJSON a) => ToJSON (Product f g a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Product f g a -> Value #

toEncoding :: Product f g a -> Encoding #

toJSONList :: [Product f g a] -> Value #

toEncodingList :: [Product f g a] -> Encoding #

(ToJSON1 f, ToJSON1 g, ToJSON a) => ToJSON (Sum f g a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Sum f g a -> Value #

toEncoding :: Sum f g a -> Encoding #

toJSONList :: [Sum f g a] -> Value #

toEncodingList :: [Sum f g a] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c, ToJSON d, ToJSON e) => ToJSON (a, b, c, d, e) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c, d, e) -> Value #

toEncoding :: (a, b, c, d, e) -> Encoding #

toJSONList :: [(a, b, c, d, e)] -> Value #

toEncodingList :: [(a, b, c, d, e)] -> Encoding #

(ToJSON1 f, ToJSON1 g, ToJSON a) => ToJSON (Compose f g a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Compose f g a -> Value #

toEncoding :: Compose f g a -> Encoding #

toJSONList :: [Compose f g a] -> Value #

toEncodingList :: [Compose f g a] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c, ToJSON d, ToJSON e, ToJSON f) => ToJSON (a, b, c, d, e, f) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c, d, e, f) -> Value #

toEncoding :: (a, b, c, d, e, f) -> Encoding #

toJSONList :: [(a, b, c, d, e, f)] -> Value #

toEncodingList :: [(a, b, c, d, e, f)] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c, ToJSON d, ToJSON e, ToJSON f, ToJSON g) => ToJSON (a, b, c, d, e, f, g) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c, d, e, f, g) -> Value #

toEncoding :: (a, b, c, d, e, f, g) -> Encoding #

toJSONList :: [(a, b, c, d, e, f, g)] -> Value #

toEncodingList :: [(a, b, c, d, e, f, g)] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c, ToJSON d, ToJSON e, ToJSON f, ToJSON g, ToJSON h) => ToJSON (a, b, c, d, e, f, g, h) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c, d, e, f, g, h) -> Value #

toEncoding :: (a, b, c, d, e, f, g, h) -> Encoding #

toJSONList :: [(a, b, c, d, e, f, g, h)] -> Value #

toEncodingList :: [(a, b, c, d, e, f, g, h)] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c, ToJSON d, ToJSON e, ToJSON f, ToJSON g, ToJSON h, ToJSON i) => ToJSON (a, b, c, d, e, f, g, h, i) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c, d, e, f, g, h, i) -> Value #

toEncoding :: (a, b, c, d, e, f, g, h, i) -> Encoding #

toJSONList :: [(a, b, c, d, e, f, g, h, i)] -> Value #

toEncodingList :: [(a, b, c, d, e, f, g, h, i)] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c, ToJSON d, ToJSON e, ToJSON f, ToJSON g, ToJSON h, ToJSON i, ToJSON j) => ToJSON (a, b, c, d, e, f, g, h, i, j) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c, d, e, f, g, h, i, j) -> Value #

toEncoding :: (a, b, c, d, e, f, g, h, i, j) -> Encoding #

toJSONList :: [(a, b, c, d, e, f, g, h, i, j)] -> Value #

toEncodingList :: [(a, b, c, d, e, f, g, h, i, j)] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c, ToJSON d, ToJSON e, ToJSON f, ToJSON g, ToJSON h, ToJSON i, ToJSON j, ToJSON k) => ToJSON (a, b, c, d, e, f, g, h, i, j, k) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c, d, e, f, g, h, i, j, k) -> Value #

toEncoding :: (a, b, c, d, e, f, g, h, i, j, k) -> Encoding #

toJSONList :: [(a, b, c, d, e, f, g, h, i, j, k)] -> Value #

toEncodingList :: [(a, b, c, d, e, f, g, h, i, j, k)] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c, ToJSON d, ToJSON e, ToJSON f, ToJSON g, ToJSON h, ToJSON i, ToJSON j, ToJSON k, ToJSON l) => ToJSON (a, b, c, d, e, f, g, h, i, j, k, l) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c, d, e, f, g, h, i, j, k, l) -> Value #

toEncoding :: (a, b, c, d, e, f, g, h, i, j, k, l) -> Encoding #

toJSONList :: [(a, b, c, d, e, f, g, h, i, j, k, l)] -> Value #

toEncodingList :: [(a, b, c, d, e, f, g, h, i, j, k, l)] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c, ToJSON d, ToJSON e, ToJSON f, ToJSON g, ToJSON h, ToJSON i, ToJSON j, ToJSON k, ToJSON l, ToJSON m) => ToJSON (a, b, c, d, e, f, g, h, i, j, k, l, m) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c, d, e, f, g, h, i, j, k, l, m) -> Value #

toEncoding :: (a, b, c, d, e, f, g, h, i, j, k, l, m) -> Encoding #

toJSONList :: [(a, b, c, d, e, f, g, h, i, j, k, l, m)] -> Value #

toEncodingList :: [(a, b, c, d, e, f, g, h, i, j, k, l, m)] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c, ToJSON d, ToJSON e, ToJSON f, ToJSON g, ToJSON h, ToJSON i, ToJSON j, ToJSON k, ToJSON l, ToJSON m, ToJSON n) => ToJSON (a, b, c, d, e, f, g, h, i, j, k, l, m, n) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c, d, e, f, g, h, i, j, k, l, m, n) -> Value #

toEncoding :: (a, b, c, d, e, f, g, h, i, j, k, l, m, n) -> Encoding #

toJSONList :: [(a, b, c, d, e, f, g, h, i, j, k, l, m, n)] -> Value #

toEncodingList :: [(a, b, c, d, e, f, g, h, i, j, k, l, m, n)] -> Encoding #

(ToJSON a, ToJSON b, ToJSON c, ToJSON d, ToJSON e, ToJSON f, ToJSON g, ToJSON h, ToJSON i, ToJSON j, ToJSON k, ToJSON l, ToJSON m, ToJSON n, ToJSON o) => ToJSON (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) -> Value #

toEncoding :: (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) -> Encoding #

toJSONList :: [(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)] -> Value #

toEncodingList :: [(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)] -> Encoding #

withText :: String -> (Text -> Parser a) -> Value -> Parser a #

withText name f value applies f to the Text when value is a String and fails otherwise.

Error message example

withText "MyType" f Null
-- Error: "parsing MyType failed, expected String, but encountered Null"

withObject :: String -> (Object -> Parser a) -> Value -> Parser a #

withObject name f value applies f to the Object when value is an Object and fails otherwise.

Error message example

withObject "MyType" f (String "oops")
-- Error: "parsing MyType failed, expected Object, but encountered String"

class FromJSON a #

A type that can be converted from JSON, with the possibility of failure.

In many cases, you can get the compiler to generate parsing code for you (see below). To begin, let's cover writing an instance by hand.

There are various reasons a conversion could fail. For example, an Object could be missing a required key, an Array could be of the wrong size, or a value could be of an incompatible type.

The basic ways to signal a failed conversion are as follows:

  • fail yields a custom error message: it is the recommended way of reporting a failure;
  • empty (or mzero) is uninformative: use it when the error is meant to be caught by some (<|>);
  • typeMismatch can be used to report a failure when the encountered value is not of the expected JSON type; unexpected is an appropriate alternative when more than one type may be expected, or to keep the expected type implicit.

prependFailure (or modifyFailure) add more information to a parser's error messages.

An example type and instance using typeMismatch and prependFailure:

-- Allow ourselves to write Text literals.
{-# LANGUAGE OverloadedStrings #-}

data Coord = Coord { x :: Double, y :: Double }

instance FromJSON Coord where
    parseJSON (Object v) = Coord
        <$> v .: "x"
        <*> v .: "y"

    -- We do not expect a non-Object value here.
    -- We could use empty to fail, but typeMismatch
    -- gives a much more informative error message.
    parseJSON invalid    =
        prependFailure "parsing Coord failed, "
            (typeMismatch "Object" invalid)

For this common case of only being concerned with a single type of JSON value, the functions withObject, withScientific, etc. are provided. Their use is to be preferred when possible, since they are more terse. Using withObject, we can rewrite the above instance (assuming the same language extension and data type) as:

instance FromJSON Coord where
    parseJSON = withObject "Coord" $ \v -> Coord
        <$> v .: "x"
        <*> v .: "y"

Instead of manually writing your FromJSON instance, there are two options to do it automatically:

  • Data.Aeson.TH provides Template Haskell functions which will derive an instance at compile time. The generated instance is optimized for your type so it will probably be more efficient than the following option.
  • The compiler can provide a default generic implementation for parseJSON.

To use the second, simply add a deriving Generic clause to your datatype and declare a FromJSON instance for your datatype without giving a definition for parseJSON.

For example, the previous example can be simplified to just:

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics

data Coord = Coord { x :: Double, y :: Double } deriving Generic

instance FromJSON Coord

The default implementation will be equivalent to parseJSON = genericParseJSON defaultOptions; if you need different options, you can customize the generic decoding by defining:

customOptions = defaultOptions
                { fieldLabelModifier = map toUpper
                }

instance FromJSON Coord where
    parseJSON = genericParseJSON customOptions

Instances

Instances details
FromJSON Bool 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Char 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Double 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Float 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Int 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Int8 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Int16 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Int32 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Int64 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Integer

This instance includes a bounds check to prevent maliciously large inputs to fill up the memory of the target system. You can newtype Scientific and provide your own instance using withScientific if you want to allow larger inputs.

Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Natural 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Ordering 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Word 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Word8 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Word16 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Word32 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Word64 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON () 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser () #

parseJSONList :: Value -> Parser [()] #

FromJSON Scientific 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Text 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON UTCTime 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Value 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON DotNetTime 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Text 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Void 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Version 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON CTime 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON IntSet 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON PersistValue 
Instance details

Defined in Database.Persist.Types.Base

FromJSON MongoConf 
Instance details

Defined in Database.Persist.MongoDB

FromJSON ZonedTime

Supported string formats:

YYYY-MM-DD HH:MM Z YYYY-MM-DD HH:MM:SS Z YYYY-MM-DD HH:MM:SS.SSS Z

The first space may instead be a T, and the second space is optional. The Z represents UTC. The Z may be replaced with a time zone offset of the form +0000 or -08:00, where the first two digits are hours, the : is optional and the second two digits (also optional) are minutes.

Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON LocalTime 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON TimeOfDay 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON CalendarDiffTime 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON SystemTime 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON NominalDiffTime

This instance includes a bounds check to prevent maliciously large inputs to fill up the memory of the target system. You can newtype Scientific and provide your own instance using withScientific if you want to allow larger inputs.

Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON DiffTime

This instance includes a bounds check to prevent maliciously large inputs to fill up the memory of the target system. You can newtype Scientific and provide your own instance using withScientific if you want to allow larger inputs.

Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON DayOfWeek 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON Day 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON CalendarDiffDays 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON UUID 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON NoOrphanNominalDiffTime 
Instance details

Defined in Database.Persist.MongoDB

Methods

parseJSON :: Value -> Parser NoOrphanNominalDiffTime #

parseJSONList :: Value -> Parser [NoOrphanNominalDiffTime] #

FromJSON NoOrphanPortID 
Instance details

Defined in Database.Persist.MongoDB

Methods

parseJSON :: Value -> Parser NoOrphanPortID #

parseJSONList :: Value -> Parser [NoOrphanPortID] #

FromJSON a => FromJSON [a] 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser [a] #

parseJSONList :: Value -> Parser [[a]] #

FromJSON a => FromJSON (Maybe a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

(FromJSON a, Integral a) => FromJSON (Ratio a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (Min a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Min a) #

parseJSONList :: Value -> Parser [Min a] #

FromJSON a => FromJSON (Max a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Max a) #

parseJSONList :: Value -> Parser [Max a] #

FromJSON a => FromJSON (First a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (Last a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (WrappedMonoid a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (Option a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (Identity a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (First a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (Last a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (Dual a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (NonEmpty a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (IntMap a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON v => FromJSON (Tree v) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (Seq a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Seq a) #

parseJSONList :: Value -> Parser [Seq a] #

(Ord a, FromJSON a) => FromJSON (Set a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Set a) #

parseJSONList :: Value -> Parser [Set a] #

FromJSON1 f => FromJSON (Fix f)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Fix f) #

parseJSONList :: Value -> Parser [Fix f] #

(FromJSON1 f, Functor f) => FromJSON (Mu f)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Mu f) #

parseJSONList :: Value -> Parser [Mu f] #

(FromJSON1 f, Functor f) => FromJSON (Nu f)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Nu f) #

parseJSONList :: Value -> Parser [Nu f] #

FromJSON a => FromJSON (DNonEmpty a)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (DList a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

(Prim a, FromJSON a) => FromJSON (PrimArray a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (SmallArray a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (Array a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (Maybe a)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.FromJSON

(Eq a, Hashable a, FromJSON a) => FromJSON (HashSet a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

(Vector Vector a, FromJSON a) => FromJSON (Vector a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

(Storable a, FromJSON a) => FromJSON (Vector a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

(Prim a, FromJSON a) => FromJSON (Vector a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON a => FromJSON (Vector a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

(FromJSON a, FromJSON b) => FromJSON (Either a b) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Either a b) #

parseJSONList :: Value -> Parser [Either a b] #

(FromJSON a, FromJSON b) => FromJSON (a, b) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b) #

parseJSONList :: Value -> Parser [(a, b)] #

(FromJSON v, FromJSONKey k, Eq k, Hashable k) => FromJSON (HashMap k v) 
Instance details

Defined in Data.Aeson.Types.FromJSON

(FromJSONKey k, Ord k, FromJSON v) => FromJSON (Map k v) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Map k v) #

parseJSONList :: Value -> Parser [Map k v] #

HasResolution a => FromJSON (Fixed a)

This instance includes a bounds check to prevent maliciously large inputs to fill up the memory of the target system. You can newtype Scientific and provide your own instance using withScientific if you want to allow larger inputs.

Instance details

Defined in Data.Aeson.Types.FromJSON

FromJSON (Proxy a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

(FromJSON a, FromJSON b) => FromJSON (Pair a b)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Pair a b) #

parseJSONList :: Value -> Parser [Pair a b] #

(FromJSON a, FromJSON b) => FromJSON (These a b)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (These a b) #

parseJSONList :: Value -> Parser [These a b] #

(FromJSON a, FromJSON b) => FromJSON (Either a b)

Since: aeson-1.5.3.0

Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Either a b) #

parseJSONList :: Value -> Parser [Either a b] #

(FromJSON a, FromJSON b) => FromJSON (These a b)

Since: aeson-1.5.1.0

Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (These a b) #

parseJSONList :: Value -> Parser [These a b] #

(FromJSON a, FromJSON b, FromJSON c) => FromJSON (a, b, c) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c) #

parseJSONList :: Value -> Parser [(a, b, c)] #

FromJSON a => FromJSON (Const a b) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Const a b) #

parseJSONList :: Value -> Parser [Const a b] #

FromJSON b => FromJSON (Tagged a b) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Tagged a b) #

parseJSONList :: Value -> Parser [Tagged a b] #

(FromJSON1 f, FromJSON1 g, FromJSON a) => FromJSON (These1 f g a)

Since: aeson-1.5.1.0

Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (These1 f g a) #

parseJSONList :: Value -> Parser [These1 f g a] #

(FromJSON a, FromJSON b, FromJSON c, FromJSON d) => FromJSON (a, b, c, d) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c, d) #

parseJSONList :: Value -> Parser [(a, b, c, d)] #

(FromJSON1 f, FromJSON1 g, FromJSON a) => FromJSON (Product f g a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Product f g a) #

parseJSONList :: Value -> Parser [Product f g a] #

(FromJSON1 f, FromJSON1 g, FromJSON a) => FromJSON (Sum f g a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Sum f g a) #

parseJSONList :: Value -> Parser [Sum f g a] #

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e) => FromJSON (a, b, c, d, e) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c, d, e) #

parseJSONList :: Value -> Parser [(a, b, c, d, e)] #

(FromJSON1 f, FromJSON1 g, FromJSON a) => FromJSON (Compose f g a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Compose f g a) #

parseJSONList :: Value -> Parser [Compose f g a] #

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f) => FromJSON (a, b, c, d, e, f) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c, d, e, f) #

parseJSONList :: Value -> Parser [(a, b, c, d, e, f)] #

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g) => FromJSON (a, b, c, d, e, f, g) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c, d, e, f, g) #

parseJSONList :: Value -> Parser [(a, b, c, d, e, f, g)] #

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h) => FromJSON (a, b, c, d, e, f, g, h) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c, d, e, f, g, h) #

parseJSONList :: Value -> Parser [(a, b, c, d, e, f, g, h)] #

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i) => FromJSON (a, b, c, d, e, f, g, h, i) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c, d, e, f, g, h, i) #

parseJSONList :: Value -> Parser [(a, b, c, d, e, f, g, h, i)] #

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j) => FromJSON (a, b, c, d, e, f, g, h, i, j) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c, d, e, f, g, h, i, j) #

parseJSONList :: Value -> Parser [(a, b, c, d, e, f, g, h, i, j)] #

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k) => FromJSON (a, b, c, d, e, f, g, h, i, j, k) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c, d, e, f, g, h, i, j, k) #

parseJSONList :: Value -> Parser [(a, b, c, d, e, f, g, h, i, j, k)] #

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c, d, e, f, g, h, i, j, k, l) #

parseJSONList :: Value -> Parser [(a, b, c, d, e, f, g, h, i, j, k, l)] #

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l, FromJSON m) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l, m) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c, d, e, f, g, h, i, j, k, l, m) #

parseJSONList :: Value -> Parser [(a, b, c, d, e, f, g, h, i, j, k, l, m)] #

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l, FromJSON m, FromJSON n) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l, m, n) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c, d, e, f, g, h, i, j, k, l, m, n) #

parseJSONList :: Value -> Parser [(a, b, c, d, e, f, g, h, i, j, k, l, m, n)] #

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l, FromJSON m, FromJSON n, FromJSON o) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) #

parseJSONList :: Value -> Parser [(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)] #

data Parser a #

A JSON parser. N.B. This might not fit your usual understanding of "parser". Instead you might like to think of Parser as a "parse result", i.e. a parser to which the input has already been applied.

Instances

Instances details
Monad Parser 
Instance details

Defined in Data.Aeson.Types.Internal

Methods

(>>=) :: Parser a -> (a -> Parser b) -> Parser b #

(>>) :: Parser a -> Parser b -> Parser b #

return :: a -> Parser a #

Functor Parser 
Instance details

Defined in Data.Aeson.Types.Internal

Methods

fmap :: (a -> b) -> Parser a -> Parser b #

(<$) :: a -> Parser b -> Parser a #

MonadFail Parser 
Instance details

Defined in Data.Aeson.Types.Internal

Methods

fail :: String -> Parser a #

Applicative Parser 
Instance details

Defined in Data.Aeson.Types.Internal

Methods

pure :: a -> Parser a #

(<*>) :: Parser (a -> b) -> Parser a -> Parser b #

liftA2 :: (a -> b -> c) -> Parser a -> Parser b -> Parser c #

(*>) :: Parser a -> Parser b -> Parser b #

(<*) :: Parser a -> Parser b -> Parser a #

MonadPlus Parser 
Instance details

Defined in Data.Aeson.Types.Internal

Methods

mzero :: Parser a #

mplus :: Parser a -> Parser a -> Parser a #

Alternative Parser 
Instance details

Defined in Data.Aeson.Types.Internal

Methods

empty :: Parser a #

(<|>) :: Parser a -> Parser a -> Parser a #

some :: Parser a -> Parser [a] #

many :: Parser a -> Parser [a] #

Semigroup (Parser a) 
Instance details

Defined in Data.Aeson.Types.Internal

Methods

(<>) :: Parser a -> Parser a -> Parser a #

sconcat :: NonEmpty (Parser a) -> Parser a #

stimes :: Integral b => b -> Parser a -> Parser a #

Monoid (Parser a) 
Instance details

Defined in Data.Aeson.Types.Internal

Methods

mempty :: Parser a #

mappend :: Parser a -> Parser a -> Parser a #

mconcat :: [Parser a] -> Parser a #