yesod-core-1.4.36: Creation of type-safe, RESTful web applications.

Safe HaskellNone
LanguageHaskell98

Yesod.Core.Json

Contents

Synopsis

Convert from a JSON value

defaultLayoutJson Source #

Arguments

:: (Yesod site, ToJSON a) 
=> WidgetT site IO ()

HTML

-> HandlerT site IO a

JSON

-> HandlerT site IO TypedContent 

Provide both an HTML and JSON representation for a piece of data, using the default layout for the HTML output (defaultLayout).

Since: 0.3.0

jsonToRepJson :: (Monad m, ToJSON a) => a -> m Value Source #

Deprecated: Use returnJson instead

Wraps a data type in a RepJson. The data type must support conversion to JSON via ToJSON.

Since: 0.3.0

returnJson :: (Monad m, ToJSON a) => a -> m Value Source #

Convert a value to a JSON representation via aeson's toJSON function.

Since: 1.2.1

returnJsonEncoding :: (Monad m, ToJSON a) => a -> m Encoding Source #

Convert a value to a JSON representation via aeson's toEncoding function.

Since: 1.4.21

provideJson :: (Monad m, ToJSON a) => a -> Writer (Endo [ProvidedRep m]) () Source #

Provide a JSON representation for usage with selectReps, using aeson's toJSON (aeson >= 0.11: toEncoding) function to perform the conversion.

Since: 1.2.1

Convert to a JSON value

parseJsonBody :: (MonadHandler m, FromJSON a) => m (Result a) Source #

Parse the request body to a data type as a JSON value. The data type must support conversion from JSON via FromJSON. If you want the raw JSON value, just ask for a Result Value.

Note that this function will consume the request body. As such, calling it twice will result in a parse error on the second call, since the request body will no longer be available.

Since: 0.3.0

parseCheckJsonBody :: (MonadHandler m, FromJSON a) => m (Result a) Source #

Same as parseJsonBody, but ensures that the mime type indicates JSON content.

parseJsonBody_ :: (MonadHandler m, FromJSON a) => m a Source #

Deprecated: Use requireJsonBody instead

Same as parseJsonBody, but return an invalid args response on a parse error.

requireJsonBody :: (MonadHandler m, FromJSON a) => m a Source #

Same as parseJsonBody, but return an invalid args response on a parse error.

requireCheckJsonBody :: (MonadHandler m, FromJSON a) => m a Source #

Same as requireJsonBody, but ensures that the mime type indicates JSON content.

Produce JSON values

data Value :: * #

A JSON value represented as a Haskell value.

Instances

Eq Value 

Methods

(==) :: Value -> Value -> Bool #

(/=) :: Value -> Value -> Bool #

Data Value 

Methods

gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Value -> c Value #

gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c Value #

toConstr :: Value -> Constr #

dataTypeOf :: Value -> DataType #

dataCast1 :: Typeable (* -> *) t => (forall d. Data d => c (t d)) -> Maybe (c Value) #

dataCast2 :: Typeable (* -> * -> *) t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c Value) #

gmapT :: (forall b. Data b => b -> b) -> Value -> Value #

gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Value -> r #

gmapQr :: (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Value -> r #

gmapQ :: (forall d. Data d => d -> u) -> Value -> [u] #

gmapQi :: Int -> (forall d. Data d => d -> u) -> Value -> u #

gmapM :: Monad m => (forall d. Data d => d -> m d) -> Value -> m Value #

gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Value -> m Value #

gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Value -> m Value #

Read Value 
Show Value 

Methods

showsPrec :: Int -> Value -> ShowS #

show :: Value -> String #

showList :: [Value] -> ShowS #

IsString Value 

Methods

fromString :: String -> Value #

Generic Value 

Associated Types

type Rep Value :: * -> * #

Methods

from :: Value -> Rep Value x #

to :: Rep Value x -> Value #

Lift Value 

Methods

lift :: Value -> Q Exp #

FromString Encoding 
FromString Value 

Methods

fromString :: String -> Value

Hashable Value 

Methods

hashWithSalt :: Int -> Value -> Int #

hash :: Value -> Int #

ToJSON Value 
KeyValue Pair 

Methods

(.=) :: ToJSON v => Text -> v -> Pair #

FromJSON Value 
NFData Value 

Methods

rnf :: Value -> () #

ToJavascript Value 
ToTypedContent Encoding Source # 
ToTypedContent Value Source # 
HasContentType Encoding Source # 
HasContentType Value Source # 
ToContent Encoding Source # 
ToContent Value Source # 
FromPairs Encoding Series 
GKeyValue Encoding Series 

Methods

gPair :: String -> Encoding -> Series

GToJSON Encoding arity U1 

Methods

gToJSON :: Options -> ToArgs Encoding arity a -> U1 a -> Encoding

GToJSON Value arity U1 

Methods

gToJSON :: Options -> ToArgs Value arity a -> U1 a -> Value

ToJSON1 f => GToJSON Encoding One (Rec1 f) 

Methods

gToJSON :: Options -> ToArgs Encoding One a -> Rec1 f a -> Encoding

ToJSON1 f => GToJSON Value One (Rec1 f) 

Methods

gToJSON :: Options -> ToArgs Value One a -> Rec1 f a -> Value

ToJSON a => GToJSON Encoding arity (K1 i a) 

Methods

gToJSON :: Options -> ToArgs Encoding arity a -> K1 i a a -> Encoding

(EncodeProduct arity a, EncodeProduct arity b) => GToJSON Encoding arity ((:*:) a b) 

Methods

gToJSON :: Options -> ToArgs Encoding arity a -> (a :*: b) a -> Encoding

(ToJSON1 f, GToJSON Encoding One g) => GToJSON Encoding One ((:.:) f g) 

Methods

gToJSON :: Options -> ToArgs Encoding One a -> (f :.: g) a -> Encoding

ToJSON a => GToJSON Value arity (K1 i a) 

Methods

gToJSON :: Options -> ToArgs Value arity a -> K1 i a a -> Value

(WriteProduct arity a, WriteProduct arity b, ProductSize a, ProductSize b) => GToJSON Value arity ((:*:) a b) 

Methods

gToJSON :: Options -> ToArgs Value arity a -> (a :*: b) a -> Value

(ToJSON1 f, GToJSON Value One g) => GToJSON Value One ((:.:) f g) 

Methods

gToJSON :: Options -> ToArgs Value One a -> (f :.: g) a -> Value

FromPairs Value (DList Pair) 

Methods

fromPairs :: DList Pair -> Value

ToJSON v => GKeyValue v (DList Pair) 

Methods

gPair :: String -> v -> DList Pair

(GToJSON Encoding arity a, ConsToJSON Encoding arity a, Constructor Meta c) => SumToJSON' * TwoElemArray Encoding arity (C1 c a) 

Methods

sumToJSON' :: Options -> ToArgs arity (C1 c a) a -> f a -> Tagged TwoElemArray Encoding arity

(GToJSON Value arity a, ConsToJSON Value arity a, Constructor Meta c) => SumToJSON' * TwoElemArray Value arity (C1 c a) 

Methods

sumToJSON' :: Options -> ToArgs arity (C1 c a) a -> f a -> Tagged TwoElemArray Value arity

type Rep Value 

class ToJSON a where #

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 to 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.)

Methods

toJSON :: a -> Value #

Convert a Haskell value to a JSON-friendly intermediate type.

toEncoding :: a -> Encoding #

Encode a Haskell value as JSON.

The default implementation of this method creates an intermediate Value using toJSON. This provides source-level compatibility for people upgrading from older versions of this library, but obviously offers no performance advantage.

To benefit from direct encoding, you must provide an implementation for this method. The easiest way to do so is by having your types implement Generic using the DeriveGeneric extension, and then have GHC generate a method body as follows.

instance ToJSON Coord where
    toEncoding = genericToEncoding defaultOptions

toJSONList :: [a] -> Value #

toEncodingList :: [a] -> Encoding #

Instances

ToJSON Bool 
ToJSON Char 
ToJSON Double 
ToJSON Float 
ToJSON Int 
ToJSON Int8 
ToJSON Int16 
ToJSON Int32 
ToJSON Int64 
ToJSON Integer 
ToJSON Ordering 
ToJSON Word 
ToJSON Word8 
ToJSON Word16 
ToJSON Word32 
ToJSON Word64 
ToJSON () 

Methods

toJSON :: () -> Value #

toEncoding :: () -> Encoding #

toJSONList :: [()] -> Value #

toEncodingList :: [()] -> Encoding #

ToJSON Scientific 
ToJSON Number 
ToJSON Text 
ToJSON UTCTime 
ToJSON Value 
ToJSON DotNetTime 
ToJSON Text 
ToJSON Natural 
ToJSON Version 
ToJSON IntSet 
ToJSON LocalTime 
ToJSON ZonedTime 
ToJSON TimeOfDay 
ToJSON NominalDiffTime 
ToJSON Day 
ToJSON UUID 
ToJSON a => ToJSON [a] 

Methods

toJSON :: [a] -> Value #

toEncoding :: [a] -> Encoding #

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

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

ToJSON a => ToJSON (Maybe a) 
(ToJSON a, Integral a) => ToJSON (Ratio a) 
ToJSON a => ToJSON (Identity a) 
ToJSON a => ToJSON (Min a) 

Methods

toJSON :: Min a -> Value #

toEncoding :: Min a -> Encoding #

toJSONList :: [Min a] -> Value #

toEncodingList :: [Min a] -> Encoding #

ToJSON a => ToJSON (Max a) 

Methods

toJSON :: Max a -> Value #

toEncoding :: Max a -> Encoding #

toJSONList :: [Max a] -> Value #

toEncodingList :: [Max a] -> Encoding #

ToJSON a => ToJSON (First a) 
ToJSON a => ToJSON (Last a) 
ToJSON a => ToJSON (WrappedMonoid a) 
ToJSON a => ToJSON (Option a) 
ToJSON a => ToJSON (NonEmpty a) 
HasResolution a => ToJSON (Fixed a) 
ToJSON a => ToJSON (Dual a) 
ToJSON a => ToJSON (First a) 
ToJSON a => ToJSON (Last a) 
ToJSON a => ToJSON (IntMap a) 
ToJSON v => ToJSON (Tree v) 
ToJSON a => ToJSON (Seq a) 

Methods

toJSON :: Seq a -> Value #

toEncoding :: Seq a -> Encoding #

toJSONList :: [Seq a] -> Value #

toEncodingList :: [Seq a] -> Encoding #

ToJSON a => ToJSON (Set a) 

Methods

toJSON :: Set a -> Value #

toEncoding :: Set a -> Encoding #

toJSONList :: [Set a] -> Value #

toEncodingList :: [Set a] -> Encoding #

ToJSON a => ToJSON (DList a) 
ToJSON a => ToJSON (HashSet a) 
ToJSON a => ToJSON (Vector a) 
(Storable a, ToJSON a) => ToJSON (Vector a) 
(Vector Vector a, ToJSON a) => ToJSON (Vector a) 
(Prim a, ToJSON a) => ToJSON (Vector a) 
(ToJSON a, ToJSON b) => ToJSON (Either a b) 

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) 

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

Methods

toJSON :: Map k v -> Value #

toEncoding :: Map k v -> Encoding #

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

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

ToJSON (Proxy k a) 

Methods

toJSON :: Proxy k a -> Value #

toEncoding :: Proxy k a -> Encoding #

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

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

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

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 k a b) 

Methods

toJSON :: Const k a b -> Value #

toEncoding :: Const k a b -> Encoding #

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

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

ToJSON b => ToJSON (Tagged k a b) 

Methods

toJSON :: Tagged k a b -> Value #

toEncoding :: Tagged k a b -> Encoding #

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

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

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

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 (Sum * f g a) 

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 #

(ToJSON1 f, ToJSON1 g, ToJSON a) => ToJSON (Product * f g a) 

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 #

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

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) 

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) 

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) 

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) 

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) 

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) 

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) 

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) 

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) 

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) 

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) 

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 #

class FromJSON a where #

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:

  • empty and mzero work, but are terse and uninformative;
  • fail yields a custom error message;
  • typeMismatch produces an informative message for cases when the value encountered is not of the expected type.

An example type and instance using typeMismatch:

-- 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 mzero to fail, but typeMismatch
    -- gives a much more informative error message.
    parseJSON invalid    = typeMismatch "Coord" invalid

For this common case of only being concerned with a single type of JSON value, the functions withObject, withNumber, 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

Methods

parseJSON :: Value -> Parser a #

parseJSONList :: Value -> Parser [a] #

Instances

FromJSON Bool 
FromJSON Char 
FromJSON Double 
FromJSON Float 
FromJSON Int 
FromJSON Int8 
FromJSON Int16 
FromJSON Int32 
FromJSON Int64 
FromJSON Integer

WARNING: Only parse Integers from trusted input since an attacker could easily fill up the memory of the target system by specifying a scientific number with a big exponent like 1e1000000000.

FromJSON Ordering 
FromJSON Word 
FromJSON Word8 
FromJSON Word16 
FromJSON Word32 
FromJSON Word64 
FromJSON () 

Methods

parseJSON :: Value -> Parser () #

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

FromJSON Scientific 
FromJSON Number 
FromJSON Text 
FromJSON UTCTime 
FromJSON Value 
FromJSON DotNetTime 
FromJSON Text 
FromJSON Natural 
FromJSON Version 
FromJSON IntSet 
FromJSON LocalTime 
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.

FromJSON TimeOfDay 
FromJSON NominalDiffTime

WARNING: Only parse lengths of time from trusted input since an attacker could easily fill up the memory of the target system by specifying a scientific number with a big exponent like 1e1000000000.

FromJSON Day 
FromJSON UUID 
FromJSON a => FromJSON [a] 

Methods

parseJSON :: Value -> Parser [a] #

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

FromJSON a => FromJSON (Maybe a) 
(FromJSON a, Integral a) => FromJSON (Ratio a) 
FromJSON a => FromJSON (Identity a) 
FromJSON a => FromJSON (Min a) 

Methods

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

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

FromJSON a => FromJSON (Max a) 

Methods

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

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

FromJSON a => FromJSON (First a) 
FromJSON a => FromJSON (Last a) 
FromJSON a => FromJSON (WrappedMonoid a) 
FromJSON a => FromJSON (Option a) 
FromJSON a => FromJSON (NonEmpty a) 
HasResolution a => FromJSON (Fixed a)

WARNING: Only parse fixed-precision numbers from trusted input since an attacker could easily fill up the memory of the target system by specifying a scientific number with a big exponent like 1e1000000000.

FromJSON a => FromJSON (Dual a) 
FromJSON a => FromJSON (First a) 
FromJSON a => FromJSON (Last a) 
FromJSON a => FromJSON (IntMap a) 
FromJSON v => FromJSON (Tree v) 
FromJSON a => FromJSON (Seq a) 

Methods

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

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

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

Methods

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

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

FromJSON a => FromJSON (DList a) 
(Eq a, Hashable a, FromJSON a) => FromJSON (HashSet a) 
FromJSON a => FromJSON (Vector a) 
(Storable a, FromJSON a) => FromJSON (Vector a) 
(Vector Vector a, FromJSON a) => FromJSON (Vector a) 
(Prim a, FromJSON a) => FromJSON (Vector a) 
(FromJSON a, FromJSON b) => FromJSON (Either a b) 

Methods

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

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

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

Methods

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

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

(FromJSON v, FromJSONKey k, Eq k, Hashable k) => FromJSON (HashMap k v) 
(FromJSONKey k, Ord k, FromJSON v) => FromJSON (Map k v) 

Methods

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

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

FromJSON (Proxy k a) 

Methods

parseJSON :: Value -> Parser (Proxy k a) #

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

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

Methods

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

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

FromJSON a => FromJSON (Const k a b) 

Methods

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

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

FromJSON b => FromJSON (Tagged k a b) 

Methods

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

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

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

Methods

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

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

(FromJSON1 f, FromJSON1 g, FromJSON a) => FromJSON (Sum * f g a) 

Methods

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

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

(FromJSON1 f, FromJSON1 g, FromJSON a) => FromJSON (Product * f g a) 

Methods

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

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

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

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) 

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) 

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) 

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) 

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) 

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) 

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) 

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) 

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) 

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) 

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) 

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)] #

array :: ToJSON a => [a] -> Value Source #

Convert a list of values to an Array.

object :: [Pair] -> Value #

Create a Value from a list of name/value Pairs. If duplicate keys arise, earlier keys and their associated values win.

(.=) :: KeyValue kv => forall v. ToJSON v => Text -> v -> kv #

(.:) :: FromJSON a => Object -> Text -> Parser a #

Retrieve the value associated with the given key of an Object. The result is empty if the key is not present or the value cannot be converted to the desired type.

This accessor is appropriate if the key and value must be present in an object for it to be valid. If the key and value are optional, use .:? instead.

Convenience functions

jsonOrRedirect Source #

Arguments

:: (MonadHandler m, ToJSON a) 
=> Route (HandlerSite m)

Redirect target

-> a

Data to send via JSON

-> m Value 

jsonOrRedirect simplifies the scenario where a POST handler sends a different response based on Accept headers:

  1. 200 with JSON data if the client prefers application/json (e.g. AJAX, see acceptsJSON).
  2. 3xx otherwise, following the PRG pattern.

jsonEncodingOrRedirect Source #

Arguments

:: (MonadHandler m, ToJSON a) 
=> Route (HandlerSite m)

Redirect target

-> a

Data to send via JSON

-> m Encoding 

jsonEncodingOrRedirect simplifies the scenario where a POST handler sends a different response based on Accept headers:

  1. 200 with JSON data if the client prefers application/json (e.g. AJAX, see acceptsJSON).
  2. 3xx otherwise, following the PRG pattern. @since 1.4.21

acceptsJson :: MonadHandler m => m Bool Source #

Returns True if the client prefers application/json as indicated by the Accept HTTP header.