1For 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).
5* Fix bug in generically derived `FromJSON` instances that are using `unwrapUnaryRecords`, thanks to Xia Li-yao
6* Allow base-compat 0.10.*, thanks to Oleg Grenrus
11* `GKeyValue` has been renamed to `KeyValuePair`, thanks to Xia Li-yao
12* Removed unused `FromJSON` constraint in `withEmbeddedJson`, thanks to Tristan Seligmann
15* Optimizations of TH toEncoding, thanks to Xia Li-yao
16* Optimizations of hex decoding when using the default/pure unescape implementation, thanks to Xia Li-yao
17* Improved error message on `Day` parse failures, thanks to Gershom Bazerman
18* Add `encodeFile` as well as `decodeFile*` variants, thanks to Markus Hauck
19* Documentation fixes, thanks to Lennart Spitzner
20* CPP cleanup, thanks to Ryan Scott
24* Add `Ord` instance for `JSONPathElement`, thanks to Simon Hengel.
29* Added `withEmbeddedJSON` to help parse JSON embedded inside a JSON string, thanks to Jesse Kempf.
30* Memory usage improvements to the default (pure) parser, thanks to Jonathan Paugh. Also thanks to Neil Mitchell & Oleg Grenrus for contributing a benchmark.
31* `omitNothingFields` now works for the `Option` newtype, thanks to Xia Li-yao.
32* Some documentation fixes, thanks to Jonathan Paug & Philippe Crama.
36* Add `FromJSON` and `ToJSON` instances for
37 * `DiffTime`, thanks to Víctor López Juan.
38 * `CTime`, thanks to Daniel Díaz.
39* Fix handling of fractions when parsing Natural, thanks to Yuriy Syrovetskiy.
40* Change text in error messages for Integral types to make them follow the common pattern, thanks to Yuriy Syrovetskiy.
41* Add missing `INCOHERENT` pragma for `RecordToPair`, thanks to Xia Li-yao.
42* Everything related to `Options` is now exported from `Data.Aeson`, thanks to Xia Li-yao.
43* Optimizations to not escape text in clear cases, thanks to Oleg Grenrus.
44* Some documentation fixes, thanks to Phil de Joux & Xia Li-yao.
48* Add `parserThrowError` and `parserCatchError` combinators, thanks to Oleg Grenrus.
50* Add `Generic` instance for `Value`, thanks to Xia Li-yao.
52* Fix a mistake in the 1.2.0.0 changelog, the `cffi` flag is disabled by default! Thanks to dbaynard.
56* `tagSingleConstructors`, an option to encode single-constructor types as tagged sums was added to `Options`. It is disabled by default for backward compatibility.
58* 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.
60* The `Options` constructor is no longer exposed to prevent new options from being breaking changes, use `defaultOptions` instead.
62* The contents of `GToJSON` and `GToEncoding` are no longer exposed.
64* Some INLINE pragmas were removed to avoid GHC running out of simplifier ticks.
68* Fix an accidental change in the format of `deriveJSON`. Thanks to Xia Li-yao!
70* Documentation improvements regarding `ToJSON`, `FromJSON`, and `SumEncoding`. Thanks to Xia Li-yao and Lennart Spitzner!
74* 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!
76* 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!
80* Added instances for `UUID`.
82* The operators for parsing fields now have named aliases:
83 - `.:` => `parseField`
84 - `.:?` => `parseFieldMaybe`
85 - `.:!` => `parseFieldMaybe'`
86 - These functions now also have variants with explicit parser functions: `explicitParseField`, `explicitParseFieldMaybe`, "explicitParseFieldMaybe'`
87Thanks to Oleg Grenrus.
89* `ToJSONKey (Identity a)` and `FromJSONKey (Identity a)` no longer require the unnecessary `FromJSON a` constraint. Thanks to Oleg Grenrus.
91* Added `Data.Aeson.Encoding.pair'` which is a more general version of `Data.Aeson.Encoding.pair`. Thanks to Andrew Martin.
93* `Day`s BCE are properly encoded and `+` is now a valid prefix for `Day`s CE. Thanks to Matt Parsons.
95* Some commonly used ToJSON instances are now specialized in order to improve compile time. Thanks to Bartosz Nitka.
98[JSONTestSuite](https://github.com/nst/JSONTestSuite) cleanups, all
99motivated by tighter RFC 7159 compliance:
101* The parser now rejects numbers for which
102 [the integer portion contains a leading zero](https://github.com/bos/aeson/commit/3fb7c155f2255482b1b9566ec5c1eaf9895d630e).
103* The parser now rejects numbers for which
104 [a decimal point is not followed by at least one digit](https://github.com/bos/aeson/commit/ecfca35a45286dbe2bbaf5f62354be393bc59b66),
105* The parser now rejects documents that contain [whitespace outside the
106 set {space, newline, carriage return, tab}](https://github.com/bos/aeson/commit/8ef622c2ad8d4a109884e17c2792238a2a320e44).
108Over 90% of JSONTestSuite tests currently pass. The remainder can be
109categorised as follows:
111* The string parser is strict with Unicode compliance where the RFC
112 leaves room for implementation-defined behaviour (tests prefixed
113 with "`i_string_`". (This is necessary because the `text` library
114 cannot accommodate invalid Unicode.)
116* The parser does not (and will not) support UTF-16, UTF-32, or byte
118* The parser accepts unescaped control characters, even though the RFC
119 states that control characters must be escaped. (This may change at
120 some point, but doesn't seem important.)
124* Fixes a regression where a bunch of valid characters caused an
125 "Invalid UTF8-Stream" error when decoding. Thanks to Vladimir
126 Shabanov who investigated and fixed this.
130* Fixes a regression where it was no longer possible to derive
131 instances for types such as `data T a = T { f1 :: a, f2 :: Maybe a
134Thanks to Sean Leather for fixing this, and to Ryan Scott for helping out.
138* Decoding performance has been significantly improved (see
139 https://github.com/bos/aeson/pull/452). Thanks to @winterland1989.
141* Add `ToJSON`/`FromJSON` instances for newtypes from
142 `Data.Semigroup`: `Min`, `Max`, `First`, `Last`, `WrappedMonoid`,
143 `Option`. Thanks to Lennart Spitzner.
145* Make the documentation for `.:!` more accurate. Thanks to Ian Jeffries.
151* Introduced new `FromJSONKey` and `ToJSONKey` type classes that are
152 used to encode maps without going through HashMap. This also allows arbitrary
153 serialization of keys where a string-like key will encode into an object and
154 other keys will encode into an array of key-value tuples.
156* Added higher rank classes: `ToJSON1`, `ToJSON2`, `FromJSON1`, and
159* Added `Data.Aeson.Encoding` with functions to safely write `ToJSON`
160 instances using `toEncoding`.
164* 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:
171* Added list specific members to `ToJSON` and `FromJSON` classes. In
172 the same way `Read` and `Show` handle lists specifically. This
173 removes need for overlapping instances to handle `String`.
175* Added a new `sumEncoding` option `UntaggedValue` which prevents
176 objects from being tagged with the constructor name.
178* JSONPaths are now tracked in instances derived with template-haskell
181* Get rid of redundancy of JSONPath error messages in nested records.
183 `eitherDecode "{\"x\":{\"a\": [1,2,true]}}" :: Either String Y`
185 `Error in $.x.a[2]: failed to parse field" x: failed to parse field a: expected Int, encountered Boolean`
186 and now yields `Error in $.x.a[2]: expected Int, encountered Boolean"`.
188 Some users might prefer to insert `modifyFailure` themselves to
189 customize error messages, which previously prevented the use of
192* Backwards compatibility with `bytestring-0.9` using the
193 `bytestring-builder` compatibility package.
195* Export `decodeWith`, `decodeStrictWith`, `eitherDecodeWith`, and
196 `eitherDecodeStrictWith` from `Data.Aeson.Parser`. This allows
197 decoding using explicit parsers instead of using `FromJSON`
200* Un-orphan internal instances to render them in haddocks.
204* Integral `FromJSON` instances now only accept integral
205 values. E.g. parsing `3.14` to `Int` fails instead of succeeding
208* Over/underflows are now caught for bounded numeric types.
210* Remove the `contents` field encoding with `allNullaryToStringTag = False`,
211 giving us `{ "tag" : "c1" }` instead of `{ "tag" : "c1", contents : [] }`.
212 The contents field is optional when parsing so this is only a breaking
213 change for ToJSON instances.
215* Fix a bug where `genericToEncoding` with `unwrapUnaryRecords = True`
216 would produce an invalid encoding: `"unwrap\":""`.
218* `ToJSON` instances using `genericToEncoding` and `omitNothingFields`
219 no longer produce invalid JSON.
221* Added instances for `DList`, `Compose`, `Product`, `Sum`.
225* Enable `PolyKinds` to generalize `Proxy`, `Tagged`, and `Const` instances.
226* Add `unsafeToEncoding` in `Data.Aeson.Types`, use with care!
230* Fix build with `base >= 4.8` and `unordered-containers < 0.2.6`.
234* Fix build on TH-less GHCs
238* Fix build with `base < 4.8` and `unordered-containers < 0.2.6`.
239* Add missing field in docs for `defaultOptions`.
243* Fixes a bug where the hashes of equal values could differ.
247The only changes are added instances.
250* `ToJSON a => ToJSON (NonEmpty a)`
251* `FromJSON a => FromJSON (NonEmpty a)`
253* `FromJSON (Proxy a)`
254* `ToJSON b => ToJSON (Tagged a b)`
255* `FromJSON b => FromJSON (Tagged a b)`
256* `ToJSON a => ToJSON (Const a b)`
257* `FromJSON a => FromJSON (Const a b)`
259These are now available for older GHCs:
265This release should be close to backwards compatible with aeson 0.9.
267If you are upgrading from aeson 0.10 it might be easier to go back in
268history to the point you were still using 0.9.
272* Revert `.:?` to behave like it did in 0.9. If you want the 0.10
273 behavior use `.:!` instead.
275* Revert JSON format of `Either` to 0.9, `Left` and `Right` are now
276 serialized with an initial uppercase letter. If you want the names
277 in lowercase you can add a newtype with an instance.
279* All `ToJSON` and `FromJSON` instances except for `[a]` are no longer
280 `OVERLAPPABLE`. Mark your instance as `OVERLAPPING` if it overlaps
281 any of the other aeson instances.
283* All `ToJSON` and `FromJSON` instances except for `[Char]` are no
284 longer incoherent, this means you may need to replace your
285 incoherent instances with a newtyped instance.
289* Introduce `.:!` that behaves like `.:?` did in 0.10.
291* Allow `HH:MM` format for `ZonedTime` and `UTCTime`.
292 This is one of the formats allowed by
293 [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601#Times).
295* Added `ToJSON` and `FromJSON` instances for the
296 `Version`, `Ordering`, and `Natural` types.
300* JSONPath identifiers are now escaped if they contain invalid characters.
302* Fixed JSONPath messages for Seq to include indices.
304* Fixed JSONPath messages for Either to include `left`/`right`.
306* Fix missing quotes surrounding time encodings.
308* Fix #293: Type error in TH when using `omitNothingFields = True`.
312* Various updates to support GHC 8.
317## Performance improvements
319* Direct encoding via the new `toEncoding` method is over 2x faster
320 than `toJSON`. (You must write or code-gen a `toEncoding`
321 implementation to unlock this speedup. See below for details.)
323* Improved string decoding gives a 12% speed win in parsing
324 string-heavy JSON payloads (very common).
326* Encoding and decoding of time-related types are 10x faster (!!) as a
327 result of bypassing `Data.Time.Format` and the arbitrary-precision
330* When using `toEncoding`, `[Char]` can be encoded without a conversion to
331 `Text`. This is fast and efficient.
333* Parsing into an `Object` is now 5% faster and more
334 allocation-efficient.
336## SUBTLE API CHANGES, READ CAREFULLY
338With the exception of long-deprecated code, the API changes below
339**should be upwards compatible** from older versions of `aeson`. If you run
340into upgrade problems, please file an issue with details.
342* The `ToJSON` class has a new method, `toEncoding`, that allows
343 direct encoding from a Haskell value to a lazy bytestring without
344 construction of an intermediate `Value`.
346 The performance benefits of direct encoding are significant: more
347 than 2x faster than before, with less than 1/3 the memory usage.
349 To preserve API compatibility across upgrades from older versions of
350 this library, the default implementation of `toEncoding` uses
351 `toJSON`. You will *not* see any performance improvement unless you
352 write an implementation of `toEncoding`, which can be very simple:
355 instance ToJSON Coord where
356 toEncoding = genericToEncoding defaultOptions
359 (Behind the scenes, the `encode` function uses `toEncoding` now, so
360 if you implement `toEncoding` for your types, you should see a
361 speedup immediately.)
363 If you use Template Haskell or GHC Generics to auto-generate your
364 `ToJSON` instances, you'll benefit from fast toEncoding
365 implementations for free!
367* When converting from a `Value` to a target Haskell type, `FromJSON`
368 instances now provide much better error messages, including a
369 complete JSON path from the root of the object to the offending
370 element. This greatly eases debugging.
372* It is now possible to use Template Haskell to generate `FromJSON`
373 and `ToJSON` instances for types in data families.
375* If you use Template Haskell or generics, and used to use the
376 `camelTo` function to rename fields, the new `camelTo2` function is
377 smarter. For example, `camelTo` will rename `CamelAPICase` to
378 `camelapi_case` (ugh!), while `camelTo2` will map it to
379 `camel_api_case` (yay!).
381* New `ToJSON` and `FromJSON` instances for the following time-related
382 types: `Day`, `LocalTime`.
384* `FromJSON` `UTCTime` parser accepts the same values as for `ZonedTime`,
385 but converts any time zone offset into a UTC time.
387* The `Result` type is now an instance of `Foldable` and `Traversable`.
389* The `Data.Aeson.Generic` module has been removed. It was deprecated in
392* GHC 7.2 and older are no longer supported.
394* The instance of `Monad` for the `Result` type lacked an implementation
395 of `fail` (oops). This has been corrected.
397* Semantics of `(.:?)` operator are changed. It's doesn't anymore accept
398 present `Null` value.
400* Added `(Foldable t, ToJSON a) => ToJSON (t a)` overlappable instance.
401 You might see `No instance for (Foldable YourPolymorphicType) arising from a
402 use of ‘.=’` -errors due this change.
406* A stray export of `encodeToBuilder` got away!
410* The `json` and `json'` parsers are now synonyms for `value` and
411 `value'`, in conformance with the looser semantics of RFC 7159.
413* Renamed `encodeToByteStringBuilder` to the more compact
418* The dependency on the `unordered-containers` package was too lax,
419 and has been corrected.
423* Encoding a `Scientific` value with a huge exponent is now handled
424 efficiently. (This would previously allocate a huge
425 arbitrary-precision integer, potentially leading to a denial of
428* Handling of strings that contain backslash escape sequences is
429 greatly improved. For a pathological string containing almost a
430 megabyte of consecutive backslashes, the new implementation is 27x
431 faster and uses 42x less memory.
433* The `ToJSON` instance for `UTCTime` is rendered with higher
434 (picosecond) resolution.
436* The `value` parser now correctly handles leading whitespace.
438* New instances of `ToJSON` and `FromJSON` for `Data.Sequence` and
439 `Data.Functor.Identity`. The `Value` type now has a `Read` instance.
441* `ZonedTime` parser ordering now favours the standard `JSON` format,
442 increasing efficiency in the common case.
444* Encoding to a `Text.Builder` now escapes `'<'` and `'>'` characters,
449* Fix `ToJSON` instance for 15-tuples (see #223).
457* Add `ToJSON` and `FromJSON` instances for tuples of up to 15
462* Major compiler and library compatibility changes: we have dropped
463 support for GHC older than 7.4, `text` older than 1.1, and
464 `bytestring` older than 0.10.4.0. Supporting the older versions had
465 become increasingly difficult, to the point where it was no longer
470* The performance of encoding to and decoding of bytestrings have both
471 improved by up to 2x, while also using less memory.
473* New dependency: the `scientific` package lets us parse floating point
474 numbers more quickly and accurately.
476* `eitherDecode`, `decodeStrictWith`: fixed bugs.
478* Added `FromJSON` and `ToJSON` instances for `Tree` and `Scientific`.
480* Fixed the `ToJSON` instances for `UTCTime` and `ZonedTime`.
484* Much improved documentation.
486* Angle brackets are now escaped in JSON strings, to help avoid XSS
489* Fixed up handling of nullary constructors when using generic
492* Added `ToJSON`/`FromJSON` instances for:
495 * ISO-8601 dates: `UTCTime`, `ZonedTime`, and `TimeZone`
497* Added accessor functions for inspecting `Value`s.
499* Added `eitherDecode` function that returns an error message if
504* This release introduces a slightly obscure, but
505 backwards-incompatible, change.
507 In the generic APIs of versions 0.4 and 0.5, fields whose names
508 began with a `"_"` character would have this character removed. This
509 no longer occurs, as it was both buggy and surprising
510 (https://github.com/bos/aeson/issues/53).
512* Fixed a bug in generic decoding of nullary constructors
513 (https://github.com/bos/aeson/issues/62).
517* When used with the UTF-8 encoding performance improvements
518 introduced in version 0.11.1.12 of the `text` package, this release
519 improves `aeson`'s JSON encoding performance by 33% relative to
522 As part of achieving this improvement, an API change was necessary.
523 The `fromValue` function in the `Data.Aeson.Encode` module now uses
524 the `text` package's `Builder` type instead of the `blaze-builder`
525 package's `Builder` type.
529* The new `decode` function complements the longstanding `encode`
530 function, and makes the API simpler.
532* New examples make it easier to learn to use the package
533 (https://github.com/bos/aeson/tree/master/examples).
537 `aeson`'s support for data-type generic programming makes it
538 possible to use JSON encodings of most data types without writing
539 any boilerplate instances.
541 Thanks to Bas Van Dijk, `aeson` now supports the two major schemes
542 for doing datatype-generic programming:
544 * the modern mechanism, built into GHC itself
545 (http://www.haskell.org/ghc/docs/latest/html/users_guide/generic-programming.html)
547 * the older mechanism, based on SYB (aka "scrap your
550 The modern GHC-based generic mechanism is fast and terse: in fact,
551 its performance is generally comparable in performance to
552 hand-written and TH-derived `ToJSON` and `FromJSON` instances. To
553 see how to use GHC generics, refer to `examples/Generic.hs`.
555 The SYB-based generics support lives in `Data.Aeson.Generic` and is
556 provided mainly for users of GHC older than 7.2. SYB is far slower
557 (by about 10x) than the more modern generic mechanism. To see how
558 to use SYB generics, refer to `examples/GenericSYB.hs`.
560* We switched the intermediate representation of JSON objects from
561 `Data.Map` to `Data.HashMap` which has improved type conversion
564* Instances of `ToJSON` and `FromJSON` for tuples are between 45% and 70%
569 This version of aeson makes explicit the decoupling between
570 *identifying* an element of a JSON document and *converting* it to
571 Haskell. See the `Data.Aeson.Parser` documentation for details.
573 The normal `aeson` `decode` function performs identification
574 strictly, but defers conversion until needed. This can result in
575 improved performance (e.g. if the results of some conversions are
576 never needed), but at a cost in increased memory consumption.
578 The new `decode'` function performs identification and conversion
579 immediately. This incurs an up-front cost in CPU cycles, but
580 reduces reduce memory consumption.
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