Like original decode but in arbitrary MonadThrow.

Parse a top-level JSON value, i.e. also strings, numbers etc.

Like original decode' but in arbitrary MonadThrow.

Like decode but returns an error message when decoding fails.

Like decode' but returns an error message when decoding fails.

Efficiently serialize a JSON value as a lazy ByteString.

This is implemented in terms of the ToJSON class's toEncoding method.

Like original decodeStrict but in arbitrary MonadThrow.

Like original decodeStrict' but in arbitrary MonadThrow.

Like decodeStrict but returns an error message when decoding fails.

Like decodeStrict' but returns an error message when decoding fails.

A JSON value represented as a Haskell value.

An encoding of a JSON value.

Acquire the underlying bytestring builder.

A JSON "array" (sequence).

A JSON "object" (key/value map).

A newtype wrapper for UTCTime that uses the same non-standard serialization format as Microsoft .NET, whose System.DateTime type is by default serialized to JSON as in the following example:

/Date(1302547608878)/

The number represents milliseconds since the Unix epoch.

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

The result of running a Parser.

Convert a value from JSON, failing if the types do not match.

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

A key-value pair for encoding a JSON object.

Class of generic representation types (Rep) that can be converted from JSON.

Class of generic representation types (Rep) that can be converted to JSON.

Class of generic representation types (Rep) that can be converted to a JSON Encoding.

A configurable generic JSON creator. This function applied to defaultOptions is used as the default for toJSON when the type is an instance of Generic.

A configurable generic JSON encoder. This function applied to defaultOptions is used as the default for toEncoding when the type is an instance of Generic.

A configurable generic JSON decoder. This function applied to defaultOptions is used as the default for parseJSON when the type is an instance of Generic.

Default encoding Options:

Options
{ fieldLabelModifier      = id
, constructorTagModifier  = id
, allNullaryToStringTag   = True
, omitNothingFields       = False
, sumEncoding             = defaultTaggedObject
, unwrapUnaryRecords      = False
}

withObject expected f value applies f to the Object when value is an Object and fails using typeMismatch expected otherwise.

withText expected f value applies f to the Text when value is a String and fails using typeMismatch expected otherwise.

withArray expected f value applies f to the Array when value is an Array and fails using typeMismatch expected otherwise.

withNumber expected f value applies f to the Number when value is a Number. and fails using typeMismatch expected otherwise.

withScientific expected f value applies f to the Scientific number when value is a Number. and fails using typeMismatch expected otherwise.

withBool expected f value applies f to the Bool when value is a Bool and fails using typeMismatch expected otherwise.

A series of values that, when encoded, should be separated by commas. Since 0.11.0.0, the .= operator is overloaded to create either (Text, Value) or Series. You can use Series when encoding directly to a bytestring builder as in the following example:

toEncoding (Person name age) = pairs ("name" .= name <> "age" .= age)

Encode a series of key/value pairs, separated by commas.

Encode a Foldable as a JSON array.

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.

Retrieve the value associated with the given key of an Object. The result is Nothing if the key is not present, or empty if the value cannot be converted to the desired type.

This accessor is most useful if the key and value can be absent from an object without affecting its validity. If the key and value are mandatory, use .: instead.

This operator is consistent in aeson >=0.7 && <0.11

Like .:?, but the resulting parser will fail, if the key is present but is Null.

Helper for use in combination with .:? to provide default values for optional JSON object fields.

This combinator is most useful if the key and value can be absent from an object without affecting its validity and we know a default value to assign in that case. If the key and value are mandatory, use .: instead.

Example usage:

 v1 <- o .:? "opt_field_with_dfl" .!= "default_val"
 v2 <- o .:  "mandatory_field"
 v3 <- o .:? "opt_field2"

Create a Value from a list of name/value Pairs. If duplicate keys arise, earlier keys and their associated values win.

Parse a top-level JSON value.

The conversion of a parsed value to a Haskell value is deferred until the Haskell value is needed. This may improve performance if only a subset of the results of conversions are needed, but at a cost in thunk allocation.

This function is an alias for value. In aeson 0.8 and earlier, it parsed only object or array types, in conformance with the now-obsolete RFC 4627.

Parse a top-level JSON value.

This is a strict version of json which avoids building up thunks during parsing; it performs all conversions immediately. Prefer this version if most of the JSON data needs to be accessed.

This function is an alias for value'. In aeson 0.8 and earlier, it parsed only object or array types, in conformance with the now-obsolete RFC 4627.

Parse any JSON value. You should usually json in preference to this function, as this function relaxes the object-or-array requirement of RFC 4627.

In particular, be careful in using this function if you think your code might interoperate with Javascript. A naïve Javascript library that parses JSON data using eval is vulnerable to attack unless the encoded data represents an object or an array. JSON implementations in other languages conform to that same restriction to preserve interoperability and security.

Strict version of value. See also json'.

A JSON parser.