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

### 2.2.3.0

* Support `hashable-1.4.6.0`.
* Fix an issue where `Hashable Key` wasn't newtype instance over underlying `Text`,
  so with `-ordered-keymap` there were correctness issues.
* Add instances for `Data.Semigroup.Sum`, `Product`, `Any`, `All`

### 2.2.2.0

* Support GHC-8.6.5...9.10.1
* Depend on `character-ps` instead of defining own Word8 pattern synonyms

### 2.2.1.0

* Add `Data.Aeson.RFC8785`, a JSON Canonicalization Scheme implementation
  https://datatracker.ietf.org/doc/html/rfc8785
* Add Data.Aeson.Decoding.Text, decodeStrictText :: Text -> ...

  We avoid intermediate `ByteString` copy by not doing
  `decode .  TE.encodeUtf8`, but instead working on `Text` value directly.
  As we know that the stream is valid Unicode (UTF8 or UTF16),
  we can also take some shortcuts.

  One gotcha is that internal `Text` values (in `Key`s or `Value` `String`s)
  will most likely retain the original input `Text` value (its underlying `Array`).
  It shouldn't be an issue if the `Value` is then decoded to something else so these
  `Text` values disapper, but if not (e.g. `Object` keys survive)
  then users might want to use `Data.Text.copy`.

### 2.2.0.0

* Rework how `omitNothingFields` works. Add `allowOmittedFields` as a parsing counterpart.

  New type-class members were added: `omitField :: a -> Bool` to `ToJSON` and `omittedField :: Maybe a` to `FromJSON`.
  These control which fields can be omitted.
  The `.:?=`, `.:!=` and `.?=` operators were added to make use of these new members.

  GHC.Generics and Template Haskell deriving has been updated accordingly.
  Note: They behave as the parsers have been written with `.:!=`, i.e.
  if the field value is `null` it's passed to the underlying parser.
  This doesn't make difference for `Maybe` or `Option`, but does make for
  types which parser doesn't accept `null`.
  (`()` parser accepts everything and `Proxy` accepts `null).

  In addition to `Maybe` (and `Option`) fields the `Data.Monoid.First` and `Data.Monoid.Last` are also omitted,
  as well as the most newtype wrappers, when their wrap omittable type (e.g. newtypes in `Data.Monoid` and `Data.Semigroup`, `Identity`, `Const`, `Tagged`, `Compose`).
  Additionall "boring" types like `()` and `Proxy` are omitted as well.
  As the omitting is now uniform, type arguments are also omitted (also in `Generic1` derived instance).

  Resolves issues:

  -  [#687](https://github.com/haskell/aeson/issues/687) Derived ToJSON1 instance does not respect omitNothingFields = True,
  -  [#571](https://github.com/haskell/aeson/issues/571) omitNothingFields not used in Generic Decode,
  -  [#792](https://github.com/haskell/aeson/issues/792) Make Proxy fields optional.

* Use `Data.Aeson.Decoding` parsing functions (introduced in version 2.1.2.0) as default in `Data.Aeson`.
  As one side-effect, `decode` and `decode'` etc pair functions are operationally the same.
  All variants use an intermediate `Value` in normal form.

  The lazier variant could had `Value` thunks inside `Array` (i.e. `Vector`), but the record had been value strict since version `0.4.0.0` (before that the lazy `Data.Map` was used as `Object`).

* Move `Data.Aeson.Parser` module into separate [`attoparsec-aeson`](https://hackage.haskell.org/package/attoparsec-aeson) package, as these parsers are not used by `aeson` itself anymore.
* Use [`text-iso8601`](https://hackage.haskell.org/package/text-iso8601) package for parsing `time` types. These are slightly faster than previously used (copy of) `attoparsec-iso8601`.
  Formats accepted is slightly changed:
  - The space between time and timezone offset (in `UTCTime` and `ZonedTime`) is disallowed. ISO8601 explictly forbidds it.
  - The timezone offsets can be in range -23:59..23:59. This is how Python, joda-time etc seems to do. (Previously the range was -12..+14)

* Remove internal `Data.Aeson.Internal` and `Data.Aeson.Internal.Time` modules. Everything from the former is exported elsewhere (`Data.Aeson.Types`), the latter was truly internal.
* Remove `cffi` flag. Toggling the flag made `aeson` use a C implementation for string unescaping (used for `text <2` versions).
  The new native Haskell implementation (introduced in version 2.0.3.0) is at least as fast.
* Drop instances for `Number` from `attoparsec` package.
* Improve `Arbitrary Value` instance.
* Add instances for `URI` from `network-uri`.
* add instances for `Down` from `Data.Ord`.
* Use `integer-conversion` for converting `Text` and `ByteString`s into `Integer`s.
* Bump lower bounds of non GHC-boot lib dependencies.

### 2.1.2.1

* Support `th-abstraction-0.5`

### 2.1.2.0

* Add `throwDecode :: (MonadThrow m, FromJSON a) => ByteString -> m a`
  and variants.
* Add `Data.Aeson.Decoding` which uses new underlying tokenizer / parser.
  This parser seems to be faster, and the intermediate `Tokens` streams
  allow to differentiate more than `Value` if needed.
  If no critical issues is found, this parser will become the default
  in next major `aeson` version.
* Support deriving for empty datatypes (such as `Void` and `V1`)
  in `FromJSON` and `ToJSON`.
* Add `To/FromJSONKey Void` instances
* Fix `FromJSONKey Double` handling of infinities

### 2.1.1.0

- Add `Data.Aeson.KeyMap.!?` (flipped) alias to `Data.Aeson.KeyMap.lookup`.
- Add `Data.Aeson.KeyMap.insertWith` function.
- Use `unsafeDupablePerformIO` instead of incorrect `accursedUnutterablePerformIO` in creation of keys in TH serialisation.
  This fixes a bug in TH deriving, e.g. when `Strict` pragma was enabled.

### 2.1.0.0

- Change time instances of types with year (`Day`, `UTCTime`) to require years with at least 4 digits.
- Change `KeyValue` instances to be more general (and use equality to constraint them) instead of being more lax flexible instances.
- Export `Key` type also from `Data.Aeson.KeyMap` module.
- Export `mapWithKey` from `Data.Aeson.KeyMap` module.
- Export `ifromJSON` and `iparse` from `Data.Aeson.Types`. Add `iparseEither`.
- Add `MonadFix Parser` instance.
- Make `Semigroup Series` slightly lazier
- Add instances for `Generically` type

### 2.0.3.0

* `text-2.0` support
* `bytestring-0.11.2.0` support
* Rewrite pure text literal unescaper.
* Add `QuickCheck`'s `Arbitrary`, `CoArbitrary` and `Function` instances

### 2.0.2.0

* Add `IsList (KeyMap v)` instance.
* Add `toMapText` and `fromMapText` to `Data.Aeson.KeyMap`.
* Add `ShortText` instances
* Add `Solo` instances

### 2.0.1.0

* Add `FromJSON KeyMap` instance.
* Make `ordered-keymap` on by default.

### 2.0.0.0

* Remove forced `-O2` and then unneeded `fast` flag.
  Also remove most of `INLINE` pragmas.
  In the effect, `aeson` compiles almost twice as fast.

  To get `fast` compilation effect cabal-install users may specify `optimization: False`.

* Make map type used by Object abstract so the underlying implementation can
  be modified, thanks to Callan McGill

* Add `ordered-keymap` flag allowing to change the underlying implementation of object `KeyMap`.

* Drop GHC-7 support

* Remove Data.Aeson.Encode module

* `Double` and `Float` infinities are encoded as `"+inf"` and `"-inf"`.
  Change `To/FromJSONKey` instances to use `"+inf"` and `"-inf"` too.

* `FromJSON ()` and `FromJSON (Proxy tag)` accept any JSON value.


### 1.5.6.0
* Make `Show Value` instance print object keys in lexicographic order.

### 1.5.5.1
* Fix a bug in `FromJSON QuarterOfYear` instance.

### 1.5.5.0
* Add instances for `Month`, `Quarter` and `QuarterOfYear` (from `time-1.11`), thanks to Oleg Grenrus.

* The aeson repository has been moved to the haskell github organization!

#### 1.5.4.1
* Use `Text.Encoding.decodeLatin1` to speed up ASCII string decoding, thanks to Dmitry Ivanov.
* Support `bytestring 0.11.*` and `th-abstraction 0.4.*`, thanks to Oleg Grenrus.

### 1.5.4.0

* Add instances for `ToJSONKey` and `FromJSONKey` to `Const`, thanks to Dan Fithian.
* Add support for template-haskell 2.17, thanks to Galen Huntington.
* Documentation typo fix, thanks to Jean-Baptiste Mazon.

### 1.5.3.0

* Add instances for types in `strict` and `data-fix` packages, thanks to Oleg Grenrus.
* CPP cleanup, thanks to Oleg Grenrus.
* Instances for `dlist`'s `Data.DList.DNonEmpty.DNonEmpty`, thanks to Oleg Grenrus.

### 1.5.2.0

* Add `Ord Value` instance, thanks to Oleg Grenrus.
* Export `rejectUnknownFields` from `Data.Aeson`

### 1.5.1.0

* Add instances for `these`, thanks to Oleg Grenrus.

## 1.5.0.0

* Fix bug in `rejectUnknownFields` not respecting `fieldLabelModifier`, thanks to Markus Schirp.
* `GFromJSON` members are no longer exported from `Data.Aeson(.Types)`, if you are using `gParseJSON` consider switching to `gParseJSON'`, thanks to Oleg Grenrus.
* Aeson no longer accepts unescaped control characters, thanks to Oleg Grenrus.
* Remove `CoerceText` since GHC >=7.8 has `Coercible`, thanks to Oleg Grenrus.
* Rename the `GToJSON` class to `GToJSON'` and expose it, thanks to Oleg Grenrus.

Closed tickets: https://github.com/bos/aeson/milestone/21


#### 1.4.7.1

* GHC 8.10 compatibility, thanks to Ryan Scott.

### 1.4.7.0

Long overdue release (once again), so there's quite a bit of stuff
included even though it's a "minor" release. Big thanks to all the
contributors, the project would not exist without you!

Special thanks to Oleg Grenrus and Xia Li-Yao for reviewing tons
of stuff.

New stuff:

* Add `rejectUnknownFields` to Options which rejects unknown fields on
  deserialization. Useful to find errors during development, but
  enabling this should be considered a breaking change as previously
  accepted inputs may now be rejected. Thanks to rmanne.

```
instance FromJSON Foo where
  parseJSON = gParseJSON defaultOptions { rejectUnknownFields = True }
```

* `FromJSON` instance of `Ratio a` now parses numbers in addtion to
  standard `{numerator=..., denumerator=...}` encoding. Thanks to
  Aleksey Khudyakov.

* Add more information to parse errors, including a sample of the
  surrounding text. Hopefully this will lead to less "Failed to read:
  satisfy" confusion! Thanks to Sasha Bogicevic. We expect some
  downstream test suites to break because of this, apologies in
  advance. Hopefully you will like the improvement anyway :-)

* Add `parseFail` to `Data.Aeson.Types`. `parseFail = fail` but
  doesn't require users to know about `MonadFail`. Thanks to Colin
  Woodbury.

* Make Template Haskell type family detection smarter when deriving
  `ToJSON1` instances, thanks to Ryan Scott.

* Optimize string parsing for the common case of strings without
  escapes, thanks to Yuras.


Misc:

* Clean up compiler warnings and switch from base-compat to
  base-compat-batteries. Thanks to Colin Woodbury & Oleg Grenrus.

* Clarification & fixes to documentation regarding treatment of Maybe fields, thanks to Roman Cheplyaka.

* Add documentation for internal development workflows. Thanks to Guru
  Devanla.

* Drop support for GHC < 7.8. We've chosen to support older GHCs as
  long as it doesn't prevent us from adding new features, but now it
  does!  Thanks to Oleg Grenrus for the patch.

* Allow generic-deriving 1.13 in test suite.

* Some DRY fixes thanks to Mark Fajkus.

### 1.4.6.0

* Provide a clearer error message when a required tagKey for a constructor is missing, thanks to Guru Devanla.
  The error message now looks like this: `Error in $: parsing Types.SomeType failed, expected Object with key "tag" containing one of ["nullary","unary","product","record","list"], key "tag" not found`

* Add `formatPath` and `formatRelativePath` functions to turn a `JSONPath` into a `String`, thanks to Robbie McMichael


### 1.4.5.0

* Expose `(<?>)`, `JSONPath` and `JSONPathElement(..)` from `Data.Aeson.Types`. Previously only available through internal modules. Thanks to Luke Clifton.

* Support for base-compat 0.11, thanks to Ryan Scott.

* Travis build for GHC 8.8, thanks to Oleg Grenrus.

### 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:

```haskell
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.

* `Day`s BCE are properly encoded and `+` is now a valid prefix for `Day`s CE. Thanks to Matt Parsons.

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


[JSONTestSuite](https://github.com/nst/JSONTestSuite) cleanups, all
motivated by tighter RFC 7159 compliance:

* The parser now rejects numbers for which
  [the integer portion contains a leading zero](https://github.com/bos/aeson/commit/3fb7c155f2255482b1b9566ec5c1eaf9895d630e).
* The parser now rejects numbers for which
  [a decimal point is not followed by at least one digit](https://github.com/bos/aeson/commit/ecfca35a45286dbe2bbaf5f62354be393bc59b66),
* The parser now rejects documents that contain [whitespace outside the
  set {space, newline, carriage return, tab}](https://github.com/bos/aeson/commit/8ef622c2ad8d4a109884e17c2792238a2a320e44).

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](https://en.wikipedia.org/wiki/ISO_8601#Times).

* 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:

  ```haskell
  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 `Value`s.

* 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.
