Safe Haskell | None |
---|---|
Language | Haskell98 |
- defaultLayoutJson :: (Yesod site, ToJSON a) => WidgetT site IO () -> HandlerT site IO a -> HandlerT site IO 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)
- parseJsonBody_ :: (MonadHandler m, FromJSON a) => m a
- requireJsonBody :: (MonadHandler m, FromJSON a) => m a
- data Value :: *
- class ToJSON a where
- toJSON :: a -> Value
- toEncoding :: a -> Encoding
- class FromJSON a where
- array :: ToJSON a => [a] -> Value
- object :: [Pair] -> Value
- (.=) :: KeyValue kv => forall v. 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
:: (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
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.
Produce JSON values
A JSON value represented as a Haskell value.
A type that can be converted to JSON.
An example type and instance:
-- Allow ourselves to writeText
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 will probably be more efficient than the following two options:
- 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 Generic
ToJSON
instance for your datatype without giving
definitions for toJSON
or toEncoding
.
For example, the previous example can be simplified to a more minimal instance:
{-# LANGUAGE DeriveGeneric #-} import GHC.Generics data Coord = Coord { x :: Double, y :: Double } derivingGeneric
instance ToJSON Coord where toEncoding =genericToEncoding
defaultOptions
Why do we provide an implementation for toEncoding
here? The
toEncoding
function is a relatively new addition to this class.
To allow users of older versions of this library to upgrade without
having to edit all of their instances or encounter surprising
incompatibilities, the default implementation of toEncoding
uses
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
. Our one-liner definition of
toEncoding
above bypasses the intermediate Value
.
If DefaultSignatures
doesn't give exactly the results you want,
you can customize the generic encoding with only a tiny amount of
effort, using genericToJSON
and genericToEncoding
with your
preferred Options
:
instance ToJSON Coord where toJSON =genericToJSON
defaultOptions
toEncoding =genericToEncoding
defaultOptions
Nothing
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
andmzero
work, but are terse and uninformativefail
yields a custom error messagetypeMismatch
produces an informative message for cases when the value encountered is not of the expected type
An example type and instance:
-- Allow ourselves to writeText
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 usemzero
to fail, buttypeMismatch
-- gives a much more informative error message. parseJSON invalid =typeMismatch
"Coord" invalid
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 will probably be more efficient than the following two options:
- 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 Generic
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
If DefaultSignatures
doesn't give exactly the results you want,
you can customize the generic decoding with only a tiny amount of
effort, using genericParseJSON
with your preferred Options
:
instance FromJSON Coord where parseJSON =genericParseJSON
defaultOptions
Nothing
(.:) :: 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
:: (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 #
:: (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.