| Safe Haskell | None |
|---|---|
| Language | Haskell98 |
Yesod.Core.Json
Synopsis
- defaultLayoutJson :: (Yesod site, ToJSON a) => WidgetFor site () -> HandlerFor site a -> HandlerFor site TypedContent
- jsonToRepJson :: (Monad m, ToJSON a) => a -> m Value
- returnJson :: (Monad m, ToJSON a) => a -> m Value
- returnJsonEncoding :: (Monad m, ToJSON a) => a -> m Encoding
- provideJson :: (Monad m, ToJSON a) => a -> Writer (Endo [ProvidedRep m]) ()
- parseJsonBody :: (MonadHandler m, FromJSON a) => m (Result a)
- parseCheckJsonBody :: (MonadHandler m, FromJSON a) => m (Result a)
- parseJsonBody_ :: (MonadHandler m, FromJSON a) => m a
- requireJsonBody :: (MonadHandler m, FromJSON a) => m a
- requireCheckJsonBody :: (MonadHandler m, FromJSON a) => m a
- data Value
- class ToJSON a where
- class FromJSON a where
- array :: ToJSON a => [a] -> Value
- object :: [Pair] -> Value
- (.=) :: (KeyValue kv, ToJSON v) => Text -> v -> kv
- (.:) :: FromJSON a => Object -> Text -> Parser a
- jsonOrRedirect :: (MonadHandler m, ToJSON a) => Route (HandlerSite m) -> a -> m Value
- jsonEncodingOrRedirect :: (MonadHandler m, ToJSON a) => Route (HandlerSite m) -> a -> m Encoding
- acceptsJson :: MonadHandler m => m Bool
Convert from a JSON value
Arguments
| :: (Yesod site, ToJSON a) | |
| => WidgetFor site () | HTML |
| -> HandlerFor site a | JSON |
| -> HandlerFor site TypedContent |
Provide both an HTML and JSON representation for a piece of
data, using the default layout for the HTML output
(defaultLayout).
Since: yesod-core-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: yesod-core-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: yesod-core-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: yesod-core-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: yesod-core-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: yesod-core-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. Requiring a JSON content-type helps secure your site against
CSRF attacks (browsers will perform POST requests for form and text/plain content-types
without doing a CORS check, and those content-types can easily contain valid JSON).
Produce JSON values
A JSON value represented as a Haskell value.
Instances
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 writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceToJSONCoord wheretoJSON(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 clause to your
datatype and declare a GenericToJSON 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=maptoUpper} instanceToJSONCoord wheretoJSON=genericToJSONcustomOptionstoEncoding=genericToEncodingcustomOptions
Previous versions of this library only had the toJSON method. Adding
toEncoding had two reasons:
- toEncoding is more efficient for the common case that the output of
toJSONis directly serialized to aByteString. Further, expressing either method in terms of the other would be non-optimal. - The choice of defaults allows a smooth transition for existing users:
Existing instances that do not define
toEncodingstill compile and have the correct semantics. This is ensured by making the default implementation oftoEncodingusetoJSON. This produces correct results, but since it performs an intermediate conversion to aValue, it will be less efficient than directly emitting anEncoding. (this also means that specifying nothing more thaninstance ToJSON Coordwould be sufficient as a generically decoding instance, but there probably exists no good reason to not specifytoEncodingin new instances.)
Methods
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.
instanceToJSONCoord wheretoEncoding=genericToEncodingdefaultOptions
toJSONList :: [a] -> Value #
toEncodingList :: [a] -> Encoding #
Instances
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:
emptyandmzerowork, but are terse and uninformative;failyields a custom error message;typeMismatchproduces 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 writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceFromJSONCoord whereparseJSON(Objectv) = Coord<$>v.:"x"<*>v.:"y" -- We do not expect a non-Objectvalue here. -- We could usemzeroto fail, buttypeMismatch-- gives a much more informative error message.parseJSONinvalid =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:
instanceFromJSONCoord whereparseJSON=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 clause to your
datatype and declare a GenericFromJSON 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 = ; If you need different
options, you can customize the generic decoding by defining:genericParseJSON defaultOptions
customOptions =defaultOptions{fieldLabelModifier=maptoUpper} instanceFromJSONCoord whereparseJSON=genericParseJSONcustomOptions
Instances
(.:) :: 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
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:
- 200 with JSON data if the client prefers
application/json(e.g. AJAX, seeacceptsJSON). - 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:
- 200 with JSON data if the client prefers
application/json(e.g. AJAX, seeacceptsJSON). - 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.