Yesod.Core.Json

Convert from a JSON value

defaultLayoutJson

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

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

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

Since 1.2.1

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

Provide a JSON representation for usage with selectReps, using aeson's toJSON function to perform the conversion.

Since 1.2.1

Convert to a JSON value

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

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

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

Deprecated: Use requireJsonBody instead

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

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

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

Produce JSON values

data Value

A JSON value represented as a Haskell value.

Constructors

Object !Object

Array !Array

String !Text

Number !Scientific

Bool !Bool

Null

Instances

Eq Value

Data Value

Show Value

Typeable Value

IsString Value

ToJSON Value

FromJSON Value

Hashable Value

ToJavascript Value

NFData Value

ToTypedContent Value

HasContentType Value

ToContent Value

class ToJSON a where

A type that can be converted to JSON.

An example type and instance:

{-# LANGUAGE OverloadedStrings #-}

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

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

Note the use of the OverloadedStrings language extension which enables Text values to be written as string literals.

Instead of manually writing your ToJSON instance, there are three 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 will probably be more efficient than the following two options:
• Data.Aeson.Generic provides a generic toJSON function that accepts any type which is an instance of Data.
• If your compiler has support for the DeriveGeneric and DefaultSignatures language extensions (GHC 7.2 and newer), toJSON will have a default generic implementation.

To use the latter option, simply add a deriving Generic clause to your datatype and declare a ToJSON instance for your datatype without giving a definition for toJSON.

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 ToJSON Coord

Note that, instead of using DefaultSignatures, it's also possible to parameterize the generic encoding using genericToJSON applied to your encoding/decoding Options:

 instance ToJSON Coord where
     toJSON = genericToJSON defaultOptions

Methods

toJSON :: a -> Value

Instances

ToJSON Bool

ToJSON Char

ToJSON Double

ToJSON Float

ToJSON Int

ToJSON Int8

ToJSON Int16

ToJSON Int32

ToJSON Int64

ToJSON Integer

ToJSON Word

ToJSON Word8

ToJSON Word16

ToJSON Word32

ToJSON Word64

ToJSON ()

ToJSON Scientific

ToJSON Number

ToJSON Text

ToJSON UTCTime

ToJSON Value

ToJSON DotNetTime

ToJSON Text

ToJSON IntSet

ToJSON ZonedTime

ToJSON [Char]

ToJSON a => ToJSON [a]

ToJSON (Ratio Integer)

ToJSON a => ToJSON (Maybe a)

ToJSON a => ToJSON (Dual a)

ToJSON a => ToJSON (First a)

ToJSON a => ToJSON (Last a)

ToJSON a => ToJSON (HashSet a)

ToJSON a => ToJSON (Vector a)

(Vector Vector a, ToJSON a) => ToJSON (Vector a)

(Storable a, ToJSON a) => ToJSON (Vector a)

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

ToJSON v => ToJSON (Tree v)

ToJSON a => ToJSON (Set a)

ToJSON a => ToJSON (IntMap a)

HasResolution a => ToJSON (Fixed a)

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

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

ToJSON v => ToJSON (HashMap String v)

ToJSON v => ToJSON (HashMap Text v)

ToJSON v => ToJSON (HashMap Text v)

ToJSON v => ToJSON (Map String v)

ToJSON v => ToJSON (Map Text v)

ToJSON v => ToJSON (Map Text v)

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

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

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

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

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

class FromJSON a where

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

When writing an instance, use empty, mzero, or fail to make a conversion fail, e.g. if an Object is missing a required key, or the value is of the wrong type.

An example type and instance:

{-# LANGUAGE OverloadedStrings #-}

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

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

-- A non-Object value is of the wrong type, so use mzero to fail.
   parseJSON _          = mzero

Note the use of the OverloadedStrings language extension which enables Text values to be written as string literals.

Instead of manually writing your FromJSON instance, there are three 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 will probably be more efficient than the following two options:
• Data.Aeson.Generic provides a generic fromJSON function that parses to any type which is an instance of Data.
• If your compiler has support for the DeriveGeneric and DefaultSignatures language extensions, parseJSON will have a default generic implementation.

To use this, 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

Note that, instead of using DefaultSignatures, it's also possible to parameterize the generic decoding using genericParseJSON applied to your encoding/decoding Options:

 instance FromJSON Coord where
     parseJSON = genericParseJSON defaultOptions

Methods

parseJSON :: 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 Word

FromJSON Word8

FromJSON Word16

FromJSON Word32

FromJSON Word64

FromJSON ()

FromJSON Scientific

FromJSON Number

FromJSON Text

FromJSON UTCTime

FromJSON Value

FromJSON DotNetTime

FromJSON Text

FromJSON IntSet

FromJSON ZonedTime

FromJSON [Char]

FromJSON a => FromJSON [a]

FromJSON (Ratio Integer)

FromJSON a => FromJSON (Maybe a)

FromJSON a => FromJSON (Dual a)

FromJSON a => FromJSON (First a)

FromJSON a => FromJSON (Last a)

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

FromJSON a => FromJSON (Vector a)

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

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

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

FromJSON v => FromJSON (Tree v)

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

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

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

FromJSON v => FromJSON (HashMap String v)

FromJSON v => FromJSON (HashMap Text v)

FromJSON v => FromJSON (HashMap Text v)

FromJSON v => FromJSON (Map String v)

FromJSON v => FromJSON (Map Text v)

FromJSON v => FromJSON (Map Text v)

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

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

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

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

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

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.

(.=) :: ToJSON a => Text -> a -> Pair

Construct a Pair from a key and a value.

(.:) :: 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

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.

acceptsJson :: MonadHandler m => m Bool

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