For the latest version of this document, please see [https://github.com/bos/aeson/blob/master/changelog.md](https://github.com/bos/aeson/blob/master/changelog.md).

1.4.4.0

New features:

• Adds a parameterized parser jsonWith that can be used to choose how to handle duplicate keys in objects, thanks to Xia Li-Yao.

• Add generic implementations of FromJSONKey and ToJSONKey, thanks to Xia Li-Yao. Example:

data Foo = Bar
  deriving Generic

opts :: JSONKeyOptions
opts = defaultJSONKeyOptions { keyModifier = toLower }

instance ToJSONKey Foo where
  toJSONKey = genericToJSONKey opts

instance FromJSONKey Foo where
  fromJSONKey = genericFromJSONKey opts

Minor:

• aeson now uses time-compat instead of time-locale-compat, thanks to Oleg Grenrus.
• Prepare for MonadFail breakages in GHC 8.8, thanks to Oleg Grenrus.
• Require bytestring >= 0.10.8.1 for newer GHCs to avoid build failures, thanks to Oleg Grenrus.
• Support primitive 0.7.*, thanks to Adam Bergmark.
• Allow semigroups 0.19.* and hashable 1.3.*, thanks to Oleg Grenrus.
• Fix a typo in the error message when parsing NonEmpty, thanks to Colin Woodbury.
• Document surprising behavior when using omitNothingFields with type variables, thanks to Xia Li-Yao.

Internal changes:

• Code cleanup by Oleg Grenrus
• Fix dependencies of the benchmarks on older GHC's, thanks to Xia Li-Yao.

1.4.3.0

• Improve error messages for FromJSON in existing instances and GHC Generic implementation. Thanks to Xia Li-Yao & Igor Pashev.
• Tweak error-reporting combinators and their documentation. Thanks to Xia Li-Yao.
  • typeMismatch is now about comparing JSON types (i.e., the expected and actual names of the Value constructor).
  • withObject and other with* combinators now also mention the JSON types they expect
  • New unexpected and prependFailure combinators.
• Add Contravariant ToJSONKeyFunction instance. Thanks to Oleg Grenrus.
• Add KeyValue instance for Object. Thanks to Robert Hensing.
• Improve performance when parsing certain large numbers, thanks to Oleg Grenrus.
• Add Data.Aeson.QQ.Simple - A limited version of aeson-qq. Thanks to Oleg Grenrus.
• Exposes internal helper functions like <?>, JSONPath, and parseIndexedJSON from Data.Aeson module. Thanks to Abid Uzair.
• Better error messages when there are syntax errors parsing objects and arrays. Thanks to Fintan Halpenny.
• Support building with th-abstraction-0.3.0.0 or later. Thanks to Ryan Scott.

1.4.2.0

• Add Data.Aeson.QQ.Simple which is a simpler version of the aeson-qq package, it does not support interpolation, thanks to Oleg Grenrus.
• Add Contravariant ToJSONKeyFunction instance, thanks to Oleg Grenrus.
• Add KeyValue Object instance, thanks to Robert Hensing
• Improved performance when parsing large numbers, thanks to Oleg Grenrus.

1.4.1.0

• Optimizations of generics, thanks to Rémy Oudompheng, here are some numbers for GHC 8.4:
  • Compilation time: G/BigProduct.hs is 25% faster, G/BigRecord.hs is 2x faster.
  • Runtime performance: BigRecord/toJSON/generic and BigProduct/encode/generic are more than 2x faster.
• Added To/FromJSON instances for Void and Generics's V1, thanks to Will Yager
• Added To/FromJSON instances for primitive's Array, SmallArray, PrimArray and UnliftedArray, thanks to Andrew Thad.
• Fixes handling of UTCTime wrt. leap seconds , thanks to Adam Schønemann
• Warning and documentation fixes thanks to tom-bop, Gabor Greif, Ian Jeffries, and Mateusz Curyło.

1.4.0.0

This release introduces bounds on the size of Scientific numbers when they are converted to other arbitrary precision types that do not represent them efficiently in memory.

This means that trying to decode a number such as 1e1000000000 into an Integer will now fail instead of using a lot of memory. If you need to represent large numbers you can add a newtype (preferably over Scientific) and providing a parser using withScientific.

The following instances are affected by this:

• FromJSON Natural
• FromJSONKey Natural
• FromJSON Integer
• FromJSONKey Integer
• FromJSON NominalDiffTime

For the same reasons the following instances & functions have been removed:

• Remove FromJSON Data.Attoparsec.Number instance. Note that Data.Attoparsec.Number is deprecated.
• Remove deprecated withNumber, use withScientific instead.

Finally, encoding integral values with large exponents now uses scientific notation, this saves space for large numbers.

1.3.1.1

• Catch 0 denominators when parsing Ratio

1.3.1.0

• Fix bug in generically derived FromJSON instances that are using unwrapUnaryRecords, thanks to Xia Li-yao
• Allow base-compat 0.10.*, thanks to Oleg Grenrus

1.3.0.0

Breaking changes:

• GKeyValue has been renamed to KeyValuePair, thanks to Xia Li-yao
• Removed unused FromJSON constraint in withEmbeddedJson, thanks to Tristan Seligmann

Other improvements:

• Optimizations of TH toEncoding, thanks to Xia Li-yao
• Optimizations of hex decoding when using the default/pure unescape implementation, thanks to Xia Li-yao
• Improved error message on Day parse failures, thanks to Gershom Bazerman
• Add encodeFile as well as decodeFile* variants, thanks to Markus Hauck
• Documentation fixes, thanks to Lennart Spitzner
• CPP cleanup, thanks to Ryan Scott

1.2.4.0

• Add Ord instance for JSONPathElement, thanks to Simon Hengel.

1.2.3.0

• Added withEmbeddedJSON to help parse JSON embedded inside a JSON string, thanks to Jesse Kempf.
• Memory usage improvements to the default (pure) parser, thanks to Jonathan Paugh. Also thanks to Neil Mitchell & Oleg Grenrus for contributing a benchmark.
• omitNothingFields now works for the Option newtype, thanks to Xia Li-yao.
• Some documentation fixes, thanks to Jonathan Paug & Philippe Crama.

1.2.2.0

• Add FromJSON and ToJSON instances for
  • DiffTime, thanks to Víctor López Juan.
  • CTime, thanks to Daniel Díaz.
• Fix handling of fractions when parsing Natural, thanks to Yuriy Syrovetskiy.
• Change text in error messages for Integral types to make them follow the common pattern, thanks to Yuriy Syrovetskiy.
• Add missing INCOHERENT pragma for RecordToPair, thanks to Xia Li-yao.
• Everything related to Options is now exported from Data.Aeson, thanks to Xia Li-yao.
• Optimizations to not escape text in clear cases, thanks to Oleg Grenrus.
• Some documentation fixes, thanks to Phil de Joux & Xia Li-yao.

1.2.1.0

• Add parserThrowError and parserCatchError combinators, thanks to Oleg Grenrus.

• Add Generic instance for Value, thanks to Xia Li-yao.

• Fix a mistake in the 1.2.0.0 changelog, the cffi flag is disabled by default! Thanks to dbaynard.

1.2.0.0

• tagSingleConstructors, an option to encode single-constructor types as tagged sums was added to Options. It is disabled by default for backward compatibility.

• The cffi flag is now turned off (False) by default, this means C FFI code is no longer used by default. You can flip the flag to get C implementation.

• The Options constructor is no longer exposed to prevent new options from being breaking changes, use defaultOptions instead.

• The contents of GToJSON and GToEncoding are no longer exposed.

• Some INLINE pragmas were removed to avoid GHC running out of simplifier ticks.

1.1.2.0

• Fix an accidental change in the format of deriveJSON. Thanks to Xia Li-yao!

• Documentation improvements regarding ToJSON, FromJSON, and SumEncoding. Thanks to Xia Li-yao and Lennart Spitzner!

1.1.1.0

• Added a pure implementation of the C FFI code, the C FFI code. If you wish to use the pure haskell version set the cffi flag to False. This should make aeson compile when C isn't available, such as for GHCJS. Thanks to James Parker & Marcin Tolysz!

• Using the fast flag can no longer cause a test case to fail. As far as we know this didn't affect any users of the library itself. Thanks to Xia Li-yao!

1.1.0.0

• Added instances for UUID.

• The operators for parsing fields now have named aliases:

  • .: => parseField
  • .:? => parseFieldMaybe
  • .:! => parseFieldMaybe'
  • These functions now also have variants with explicit parser functions: explicitParseField, explicitParseFieldMaybe, "explicitParseFieldMaybe'` Thanks to Oleg Grenrus.

• ToJSONKey (Identity a) and FromJSONKey (Identity a) no longer require the unnecessary FromJSON a constraint. Thanks to Oleg Grenrus.

• Added Data.Aeson.Encoding.pair' which is a more general version of Data.Aeson.Encoding.pair. Thanks to Andrew Martin.

• Days BCE are properly encoded and + is now a valid prefix for Days CE. Thanks to Matt Parsons.

• Some commonly used ToJSON instances are now specialized in order to improve compile time. Thanks to Bartosz Nitka.

JSONTestSuite cleanups, all motivated by tighter RFC 7159 compliance:

• The parser now rejects numbers for which the integer portion contains a leading zero.
• The parser now rejects numbers for which a decimal point is not followed by at least one digit,
• The parser now rejects documents that contain whitespace outside the set {space, newline, carriage return, tab}.

Over 90% of JSONTestSuite tests currently pass. The remainder can be categorised as follows:

• The string parser is strict with Unicode compliance where the RFC leaves room for implementation-defined behaviour (tests prefixed with "i_string_". (This is necessary because the text library cannot accommodate invalid Unicode.)

• The parser does not (and will not) support UTF-16, UTF-32, or byte order marks (BOM).

• The parser accepts unescaped control characters, even though the RFC states that control characters must be escaped. (This may change at some point, but doesn't seem important.)

1.0.2.1

• Fixes a regression where a bunch of valid characters caused an "Invalid UTF8-Stream" error when decoding. Thanks to Vladimir Shabanov who investigated and fixed this.

1.0.2.0

• Fixes a regression where it was no longer possible to derive instances for types such as data T a = T { f1 :: a, f2 :: Maybe a }.

Thanks to Sean Leather for fixing this, and to Ryan Scott for helping out.

1.0.1.0

• Decoding performance has been significantly improved (see https://github.com/bos/aeson/pull/452). Thanks to @winterland1989.

• Add ToJSON/FromJSON instances for newtypes from Data.Semigroup: Min, Max, First, Last, WrappedMonoid, Option. Thanks to Lennart Spitzner.

• Make the documentation for .:! more accurate. Thanks to Ian Jeffries.

1.0.0.0

Major enhancements:

• Introduced new FromJSONKey and ToJSONKey type classes that are used to encode maps without going through HashMap. This also allows arbitrary serialization of keys where a string-like key will encode into an object and other keys will encode into an array of key-value tuples.

• Added higher rank classes: ToJSON1, ToJSON2, FromJSON1, and FromJSON2.

• Added Data.Aeson.Encoding with functions to safely write ToJSON instances using toEncoding.

Other enhancements:

• A Cabal fast flag was added to disable building with optimizations. This drastically speeds up compiling both aeson and libraries using aeson so it is recommended to enable it during development. With cabal-install you can cabal install aeson -ffast and with stack you can add a flag section to your stack.yaml:

flags:
  aeson:
    fast: true

• Added list specific members to ToJSON and FromJSON classes. In the same way Read and Show handle lists specifically. This removes need for overlapping instances to handle String.

• Added a new sumEncoding option UntaggedValue which prevents objects from being tagged with the constructor name.

• JSONPaths are now tracked in instances derived with template-haskell and generics.

• Get rid of redundancy of JSONPath error messages in nested records.

  eitherDecode "{\"x\":{\"a\": [1,2,true]}}" :: Either String Y previously yielded Error in $.x.a[2]: failed to parse field" x: failed to parse field a: expected Int, encountered Boolean and now yields Error in $.x.a[2]: expected Int, encountered Boolean".

  Some users might prefer to insert modifyFailure themselves to customize error messages, which previously prevented the use of (.:).

• Backwards compatibility with bytestring-0.9 using the bytestring-builder compatibility package.

• Export decodeWith, decodeStrictWith, eitherDecodeWith, and eitherDecodeStrictWith from Data.Aeson.Parser. This allows decoding using explicit parsers instead of using FromJSON instances.

• Un-orphan internal instances to render them in haddocks.

Other changes:

• Integral FromJSON instances now only accept integral values. E.g. parsing 3.14 to Int fails instead of succeeding with the value 3.

• Over/underflows are now caught for bounded numeric types.

• Remove the contents field encoding with allNullaryToStringTag = False, giving us { "tag" : "c1" } instead of { "tag" : "c1", contents : [] }. The contents field is optional when parsing so this is only a breaking change for ToJSON instances.

• Fix a bug where genericToEncoding with unwrapUnaryRecords = True would produce an invalid encoding: "unwrap\":"".

• ToJSON instances using genericToEncoding and omitNothingFields no longer produce invalid JSON.

• Added instances for DList, Compose, Product, Sum.

0.11.2.0

• Enable PolyKinds to generalize Proxy, Tagged, and Const instances.
• Add unsafeToEncoding in Data.Aeson.Types, use with care!

0.11.1.4

• Fix build with base >= 4.8 and unordered-containers < 0.2.6.

0.11.1.3

• Fix build on TH-less GHCs

0.11.1.2

• Fix build with base < 4.8 and unordered-containers < 0.2.6.
• Add missing field in docs for defaultOptions.

0.11.1.1

• Fixes a bug where the hashes of equal values could differ.

0.11.1.0

The only changes are added instances.

These are new:

• ToJSON a => ToJSON (NonEmpty a)
• FromJSON a => FromJSON (NonEmpty a)
• ToJSON (Proxy a)
• FromJSON (Proxy a)
• ToJSON b => ToJSON (Tagged a b)
• FromJSON b => FromJSON (Tagged a b)
• ToJSON a => ToJSON (Const a b)
• FromJSON a => FromJSON (Const a b)

These are now available for older GHCs:

• ToJSON Natural
• FromJSON Natural

0.11.0.0

This release should be close to backwards compatible with aeson 0.9.

If you are upgrading from aeson 0.10 it might be easier to go back in history to the point you were still using 0.9.

Breaking changes:

• Revert .:? to behave like it did in 0.9. If you want the 0.10 behavior use .:! instead.

• Revert JSON format of Either to 0.9, Left and Right are now serialized with an initial uppercase letter. If you want the names in lowercase you can add a newtype with an instance.

• All ToJSON and FromJSON instances except for [a] are no longer OVERLAPPABLE. Mark your instance as OVERLAPPING if it overlaps any of the other aeson instances.

• All ToJSON and FromJSON instances except for [Char] are no longer incoherent, this means you may need to replace your incoherent instances with a newtyped instance.

Additions:

• Introduce .:! that behaves like .:? did in 0.10.

• Allow HH:MM format for ZonedTime and UTCTime. This is one of the formats allowed by ISO 8601.

• Added ToJSON and FromJSON instances for the Version, Ordering, and Natural types.

Bug fixes:

• JSONPath identifiers are now escaped if they contain invalid characters.

• Fixed JSONPath messages for Seq to include indices.

• Fixed JSONPath messages for Either to include left/right.

• Fix missing quotes surrounding time encodings.

• Fix #293: Type error in TH when using omitNothingFields = True.

Compatibility:

• Various updates to support GHC 8.

0.10.0.0

Performance improvements

• Direct encoding via the new toEncoding method is over 2x faster than toJSON. (You must write or code-gen a toEncoding implementation to unlock this speedup. See below for details.)

• Improved string decoding gives a 12% speed win in parsing string-heavy JSON payloads (very common).

• Encoding and decoding of time-related types are 10x faster (!!) as a result of bypassing Data.Time.Format and the arbitrary-precision Integer type.

• When using toEncoding, [Char] can be encoded without a conversion to Text. This is fast and efficient.

• Parsing into an Object is now 5% faster and more allocation-efficient.

SUBTLE API CHANGES, READ CAREFULLY

With the exception of long-deprecated code, the API changes below should be upwards compatible from older versions of aeson. If you run into upgrade problems, please file an issue with details.

• The ToJSON class has a new method, toEncoding, that allows direct encoding from a Haskell value to a lazy bytestring without construction of an intermediate Value.

  The performance benefits of direct encoding are significant: more than 2x faster than before, with less than 1/3 the memory usage.

  To preserve API compatibility across upgrades from older versions of this library, the default implementation of toEncoding uses toJSON. You will not see any performance improvement unless you write an implementation of toEncoding, which can be very simple:

  instance ToJSON Coord where
    toEncoding = genericToEncoding defaultOptions
  

  (Behind the scenes, the encode function uses toEncoding now, so if you implement toEncoding for your types, you should see a speedup immediately.)

  If you use Template Haskell or GHC Generics to auto-generate your ToJSON instances, you'll benefit from fast toEncoding implementations for free!

• When converting from a Value to a target Haskell type, FromJSON instances now provide much better error messages, including a complete JSON path from the root of the object to the offending element. This greatly eases debugging.

• It is now possible to use Template Haskell to generate FromJSON and ToJSON instances for types in data families.

• If you use Template Haskell or generics, and used to use the camelTo function to rename fields, the new camelTo2 function is smarter. For example, camelTo will rename CamelAPICase to camelapi_case (ugh!), while camelTo2 will map it to camel_api_case (yay!).

• New ToJSON and FromJSON instances for the following time-related types: Day, LocalTime.

• FromJSON UTCTime parser accepts the same values as for ZonedTime, but converts any time zone offset into a UTC time.

• The Result type is now an instance of Foldable and Traversable.

• The Data.Aeson.Generic module has been removed. It was deprecated in late 2013.

• GHC 7.2 and older are no longer supported.

• The instance of Monad for the Result type lacked an implementation of fail (oops). This has been corrected.

• Semantics of (.:?) operator are changed. It's doesn't anymore accept present Null value.

• Added (Foldable t, ToJSON a) => ToJSON (t a) overlappable instance. You might see No instance for (Foldable YourPolymorphicType) arising from a use of ‘.=’ -errors due this change.

0.9.0.1

• A stray export of encodeToBuilder got away!

0.9.0.0

• The json and json' parsers are now synonyms for value and value', in conformance with the looser semantics of RFC 7159.

• Renamed encodeToByteStringBuilder to the more compact encodeToBuilder.

0.8.1.1

• The dependency on the unordered-containers package was too lax, and has been corrected.

0.8.1.0

• Encoding a Scientific value with a huge exponent is now handled efficiently. (This would previously allocate a huge arbitrary-precision integer, potentially leading to a denial of service.)

• Handling of strings that contain backslash escape sequences is greatly improved. For a pathological string containing almost a megabyte of consecutive backslashes, the new implementation is 27x faster and uses 42x less memory.

• The ToJSON instance for UTCTime is rendered with higher (picosecond) resolution.

• The value parser now correctly handles leading whitespace.

• New instances of ToJSON and FromJSON for Data.Sequence and Data.Functor.Identity. The Value type now has a Read instance.

• ZonedTime parser ordering now favours the standard JSON format, increasing efficiency in the common case.

• Encoding to a Text.Builder now escapes '<' and '>' characters, to reduce XSS risk.

0.8.0.2

• Fix ToJSON instance for 15-tuples (see #223).

0.8.0.1

• Support time-1.5.

0.8.0.0

• Add ToJSON and FromJSON instances for tuples of up to 15 elements.

0.7.1.0

• Major compiler and library compatibility changes: we have dropped support for GHC older than 7.4, text older than 1.1, and bytestring older than 0.10.4.0. Supporting the older versions had become increasingly difficult, to the point where it was no longer worth it.

0.7.0.0

• The performance of encoding to and decoding of bytestrings have both improved by up to 2x, while also using less memory.

• New dependency: the scientific package lets us parse floating point numbers more quickly and accurately.

• eitherDecode, decodeStrictWith: fixed bugs.

• Added FromJSON and ToJSON instances for Tree and Scientific.

• Fixed the ToJSON instances for UTCTime and ZonedTime.

0.6 series

• Much improved documentation.

• Angle brackets are now escaped in JSON strings, to help avoid XSS attacks.

• Fixed up handling of nullary constructors when using generic encoding.

• Added ToJSON/FromJSON instances for:

  • The Fixed class
  • ISO-8601 dates: UTCTime, ZonedTime, and TimeZone

• Added accessor functions for inspecting Values.

• Added eitherDecode function that returns an error message if decoding fails.

0.5 to 0.6

• This release introduces a slightly obscure, but backwards-incompatible, change.

  In the generic APIs of versions 0.4 and 0.5, fields whose names began with a "_" character would have this character removed. This no longer occurs, as it was both buggy and surprising (https://github.com/bos/aeson/issues/53).

• Fixed a bug in generic decoding of nullary constructors (https://github.com/bos/aeson/issues/62).

0.4 to 0.5

• When used with the UTF-8 encoding performance improvements introduced in version 0.11.1.12 of the text package, this release improves aeson's JSON encoding performance by 33% relative to aeson 0.4.

  As part of achieving this improvement, an API change was necessary. The fromValue function in the Data.Aeson.Encode module now uses the text package's Builder type instead of the blaze-builder package's Builder type.

0.3 to 0.4

• The new decode function complements the longstanding encode function, and makes the API simpler.

• New examples make it easier to learn to use the package (https://github.com/bos/aeson/tree/master/examples).

• Generics support

  aeson's support for data-type generic programming makes it possible to use JSON encodings of most data types without writing any boilerplate instances.

  Thanks to Bas Van Dijk, aeson now supports the two major schemes for doing datatype-generic programming:

  • the modern mechanism, built into GHC itself (http://www.haskell.org/ghc/docs/latest/html/users_guide/generic-programming.html)

  • the older mechanism, based on SYB (aka "scrap your boilerplate")

  The modern GHC-based generic mechanism is fast and terse: in fact, its performance is generally comparable in performance to hand-written and TH-derived ToJSON and FromJSON instances. To see how to use GHC generics, refer to examples/Generic.hs.

  The SYB-based generics support lives in Data.Aeson.Generic and is provided mainly for users of GHC older than 7.2. SYB is far slower (by about 10x) than the more modern generic mechanism. To see how to use SYB generics, refer to examples/GenericSYB.hs.

• We switched the intermediate representation of JSON objects from Data.Map to Data.HashMap which has improved type conversion performance.

• Instances of ToJSON and FromJSON for tuples are between 45% and 70% faster than in 0.3.

• Evaluation control

  This version of aeson makes explicit the decoupling between identifying an element of a JSON document and converting it to Haskell. See the Data.Aeson.Parser documentation for details.

  The normal aeson decode function performs identification strictly, but defers conversion until needed. This can result in improved performance (e.g. if the results of some conversions are never needed), but at a cost in increased memory consumption.

  The new decode' function performs identification and conversion immediately. This incurs an up-front cost in CPU cycles, but reduces reduce memory consumption.