Generics.SOP.JSON

Configuration

type JsonFieldName = String

type JsonTagName = String

data JsonOptions

JSON encoder/decoder configuration

Constructors

JsonOptions

Fields

jsonFieldName :: DatatypeName -> FieldName -> JsonFieldName

Construct the name for JSON object fields (not for the tags that are used for sum-types, however)

The default just uses the name of the corresponding Haskell constructor

jsonTagName :: ConstructorName -> JsonTagName

Construct the name for a tag for sum-types.

The default just uses the name of the Haskell constructor.

defaultJsonOptions :: JsonOptions

JSON view of a datatype

data Tag

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.

Constructors

NoTag

Tag JsonTagName

data JsonInfo :: [*] -> * where

Constructors

JsonZero :: ConstructorName -> JsonInfo `[]`

JsonOne :: Tag -> JsonInfo `[a]`

JsonMultiple :: SListI xs => Tag -> JsonInfo xs

JsonRecord :: SListI xs => Tag -> NP (K String) xs -> JsonInfo xs

jsonInfo :: forall a. (HasDatatypeInfo a, SListI (Code a)) => Proxy a -> JsonOptions -> NP JsonInfo (Code a)

Generic functions

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

UpdateFromJSON and co

class UpdateFromJSON a where

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.

Methods

updateFromJSON :: Value -> Parser (a -> a)

Instances

UpdateFromJSON Bool

UpdateFromJSON Double

UpdateFromJSON Int

UpdateFromJSON Rational

UpdateFromJSON String

UpdateFromJSON Text

FromJSON a => UpdateFromJSON [a]

FromJSON a => UpdateFromJSON (Maybe a)

gupdateFromJSON :: forall a xs. (Generic a, HasDatatypeInfo a, All UpdateFromJSON xs, Code a ~ `[xs]`) => JsonOptions -> Value -> Parser (a -> a)

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)

For types that we can only replace "whole", rather than update field by field

parseWith :: UpdateFromJSON a => a -> Value -> Parser a

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 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 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 Generic clause to your datatype and declare a 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 } deriving Generic

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

Minimal complete definition

Nothing

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

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:

-- 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

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 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

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

Minimal complete definition

Nothing

Methods

parseJSON :: Value -> Parser a

data Proxy t :: k -> *

A concrete, poly-kinded proxy type

Constructors

Proxy

Instances

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) = (:) [*] ([] *) ([] [*])