Safe Haskell | None |
---|---|
Language | Haskell2010 |
- type JsonFieldName = String
- type JsonTagName = String
- data JsonOptions = JsonOptions {}
- defaultJsonOptions :: JsonOptions
- data Tag
- = NoTag
- | Tag JsonTagName
- data JsonInfo :: [*] -> * where
- jsonInfo :: forall a. (HasDatatypeInfo a, SListI (Code a)) => Proxy a -> JsonOptions -> NP JsonInfo (Code a)
- gtoJSON :: forall a. (Generic a, HasDatatypeInfo a, All2 ToJSON (Code a)) => JsonOptions -> a -> Value
- gparseJSON :: forall a. (Generic a, HasDatatypeInfo a, All2 FromJSON (Code a)) => JsonOptions -> Value -> Parser a
- class UpdateFromJSON a where
- updateFromJSON :: Value -> Parser (a -> a)
- gupdateFromJSON :: forall a xs. (Generic a, HasDatatypeInfo a, All UpdateFromJSON xs, Code a ~ `[xs]`) => JsonOptions -> Value -> Parser (a -> a)
- replaceWithJSON :: FromJSON a => Value -> Parser (a -> a)
- parseWith :: UpdateFromJSON a => a -> Value -> Parser a
- class ToJSON a where
- toJSON :: a -> Value
- toEncoding :: a -> Encoding
- class FromJSON a where
- data Proxy t :: k -> * = Proxy
Configuration
type JsonFieldName = String Source
type JsonTagName = String Source
data JsonOptions Source
JSON encoder/decoder configuration
JsonOptions | |
|
JSON view of a datatype
Constructor tag
For a datatype with a single constructor we do not need to tag values with their constructor; but for a datatype with multiple constructors we do.
jsonInfo :: forall a. (HasDatatypeInfo a, SListI (Code a)) => Proxy a -> JsonOptions -> NP JsonInfo (Code a) Source
Generic functions
gtoJSON :: forall a. (Generic a, HasDatatypeInfo a, All2 ToJSON (Code a)) => JsonOptions -> a -> Value Source
gparseJSON :: forall a. (Generic a, HasDatatypeInfo a, All2 FromJSON (Code a)) => JsonOptions -> Value -> Parser a Source
UpdateFromJSON and co
class UpdateFromJSON a where Source
For some values we can support "updating" the value with a "partial"
JSON value; record types are the prime example (and the only one supported
by the generic function). For non-record types we typically can only
replace the value with a "complete" JSON value; in this case, we simply
ignore the old value (see replaceWithJSON
). Typical class instances will
look like
instance UpdateFromJSON SomeRecordType where updateFromJSON = gupdateFromJSON <jsonOptions>
or
instance UpdateFromJSON SomeNonRecordType where updateFromJSON = replaceWithJSON
NOTE: The generic function uses one-level lenses for the object fields. We could generalize this to arbitrary paths, but then the type would change to
updateFromJSON :: Value -> Parser (a -> UpdateM a)
I.e., updating a value from JSON would, in general, involve a database write.
updateFromJSON :: Value -> Parser (a -> a) Source
gupdateFromJSON :: forall a xs. (Generic a, HasDatatypeInfo a, All UpdateFromJSON xs, Code a ~ `[xs]`) => JsonOptions -> Value -> Parser (a -> a) Source
Construct a function that updates a value of some record type, given a JSON object with new values for some (or none, or all) of the fields
replaceWithJSON :: FromJSON a => Value -> Parser (a -> a) Source
For types that we can only replace "whole", rather than update field by field
parseWith :: UpdateFromJSON a => a -> Value -> Parser a Source
Conversely, for types that we can only parse if we have a starting point
Re-exports
class ToJSON a where
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
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
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
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
data Proxy t :: k -> *
A concrete, poly-kinded proxy type
Monad (Proxy *) | |
Functor (Proxy *) | |
Applicative (Proxy *) | |
Foldable (Proxy *) | |
Bounded (Proxy k s) | |
Enum (Proxy k s) | |
Eq (Proxy k s) | |
Data t => Data (Proxy * t) | |
Ord (Proxy k s) | |
Read (Proxy k s) | |
Show (Proxy k s) | |
Ix (Proxy k s) | |
Generic (Proxy * t) | |
Monoid (Proxy k s) | |
Semigroup (Proxy k s) | |
type Rep (Proxy k t) = D1 D1Proxy (C1 C1_0Proxy U1) | |
type Code (Proxy * t0) = (:) [*] ([] *) ([] [*]) |