aeson
Fast JSON parsing and encoding
http://github.com/haskell/aeson
LTS Haskell 23.20: | 2.2.3.0@rev:3 |
Stackage Nightly 2025-05-02: | 2.2.3.0@rev:3 |
Latest on Hackage: | 2.2.3.0@rev:3 |
aeson-2.2.3.0@sha256:61e559f2bad61a98aac564f2a4fd892d21e9e1712e14e3db93326317aa944a9b,6378
Module documentation for 2.2.3.0
Welcome to aeson
aeson is a fast Haskell library for working with JSON data.
Join in!
We are happy to receive bug reports, fixes, documentation enhancements, and other improvements.
Please report bugs via the github issue tracker.
Master git repository:
git clone git://github.com/haskell/aeson.git
See what’s changed in recent (and upcoming) releases:
(You can create and contribute changes using either git or Mercurial.)
Authors
This library was originally written by Bryan O’Sullivan.
Changes
For the latest version of this document, please see http://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 underlyingText
, 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 http://datatracker.ietf.org/doc/html/rfc8785 -
Add Data.Aeson.Decoding.Text, decodeStrictText :: Text -> …
We avoid intermediate
ByteString
copy by not doingdecode . TE.encodeUtf8
, but instead working onText
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 (inKey
s orValue
String
s) will most likely retain the original inputText
value (its underlyingArray
). It shouldn’t be an issue if theValue
is then decoded to something else so theseText
values disapper, but if not (e.g.Object
keys survive) then users might want to useData.Text.copy
.
2.2.0.0
-
Rework how
omitNothingFields
works. AddallowOmittedFields
as a parsing counterpart.New type-class members were added:
omitField :: a -> Bool
toToJSON
andomittedField :: Maybe a
toFromJSON
. 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 isnull
it’s passed to the underlying parser. This doesn’t make difference forMaybe
orOption
, but does make for types which parser doesn’t acceptnull
. (()
parser accepts everything andProxy
accepts `null).In addition to
Maybe
(andOption
) fields theData.Monoid.First
andData.Monoid.Last
are also omitted, as well as the most newtype wrappers, when their wrap omittable type (e.g. newtypes inData.Monoid
andData.Semigroup
,Identity
,Const
,Tagged
,Compose
). Additionall “boring” types like()
andProxy
are omitted as well. As the omitting is now uniform, type arguments are also omitted (also inGeneric1
derived instance).Resolves issues:
-
Use
Data.Aeson.Decoding
parsing functions (introduced in version 2.1.2.0) as default inData.Aeson
. As one side-effect,decode
anddecode'
etc pair functions are operationally the same. All variants use an intermediateValue
in normal form.The lazier variant could had
Value
thunks insideArray
(i.e.Vector
), but the record had been value strict since version0.4.0.0
(before that the lazyData.Map
was used asObject
). -
Move
Data.Aeson.Parser
module into separateattoparsec-aeson
package, as these parsers are not used byaeson
itself anymore. -
Use
text-iso8601
package for parsingtime
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
andZonedTime
) 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)
- The space between time and timezone offset (in
-
Remove internal
Data.Aeson.Internal
andData.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 madeaeson
use a C implementation for string unescaping (used fortext <2
versions). The new native Haskell implementation (introduced in version 2.0.3.0) is at least as fast. -
Drop instances for
Number
fromattoparsec
package. -
Improve
Arbitrary Value
instance. -
Add instances for
URI
fromnetwork-uri
. -
add instances for
Down
fromData.Ord
. -
Use
integer-conversion
for convertingText
andByteString
s intoInteger
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 intermediateTokens
streams allow to differentiate more thanValue
if needed. If no critical issues is found, this parser will become the default in next majoraeson
version. - Support deriving for empty datatypes (such as
Void
andV1
) inFromJSON
andToJSON
. - Add
To/FromJSONKey Void
instances - Fix
FromJSONKey Double
handling of infinities
2.1.1.0
- Add
Data.Aeson.KeyMap.!?
(flipped) alias toData.Aeson.KeyMap.lookup
. - Add
Data.Aeson.KeyMap.insertWith
function. - Use
unsafeDupablePerformIO
instead of incorrectaccursedUnutterablePerformIO
in creation of keys in TH serialisation. This fixes a bug in TH deriving, e.g. whenStrict
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 fromData.Aeson.KeyMap
module. - Export
mapWithKey
fromData.Aeson.KeyMap
module. - Export
ifromJSON
andiparse
fromData.Aeson.Types
. AddiparseEither
. - Add
MonadFix Parser
instance. - Make
Semigroup Series
slightly lazier - Add instances for
Generically
type
2.0.3.0
text-2.0
supportbytestring-0.11.2.0
support- Rewrite pure text literal unescaper.
- Add
QuickCheck
’sArbitrary
,CoArbitrary
andFunction
instances
2.0.2.0
- Add
IsList (KeyMap v)
instance. - Add
toMapText
andfromMapText
toData.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 unneededfast
flag. Also remove most ofINLINE
pragmas. In the effect,aeson
compiles almost twice as fast.To get
fast
compilation effect cabal-install users may specifyoptimization: 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 objectKeyMap
. -
Drop GHC-7 support
-
Remove Data.Aeson.Encode module
-
Double
andFloat
infinities are encoded as"+inf"
and"-inf"
. ChangeTo/FromJSONKey
instances to use"+inf"
and"-inf"
too. -
FromJSON ()
andFromJSON (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
andQuarterOfYear
(fromtime-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.*
andth-abstraction 0.4.*
, thanks to Oleg Grenrus.
1.5.4.0
- Add instances for
ToJSONKey
andFromJSONKey
toConst
, 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
anddata-fix
packages, thanks to Oleg Grenrus. - CPP cleanup, thanks to Oleg Grenrus.
- Instances for
dlist
’sData.DList.DNonEmpty.DNonEmpty
, thanks to Oleg Grenrus.
1.5.2.0
- Add
Ord Value
instance, thanks to Oleg Grenrus. - Export
rejectUnknownFields
fromData.Aeson
1.5.1.0
- Add instances for
these
, thanks to Oleg Grenrus.
1.5.0.0
- Fix bug in
rejectUnknownFields
not respectingfieldLabelModifier
, thanks to Markus Schirp. GFromJSON
members are no longer exported fromData.Aeson(.Types)
, if you are usinggParseJSON
consider switching togParseJSON'
, thanks to Oleg Grenrus.- Aeson no longer accepts unescaped control characters, thanks to Oleg Grenrus.
- Remove
CoerceText
since GHC >=7.8 hasCoercible
, thanks to Oleg Grenrus. - Rename the
GToJSON
class toGToJSON'
and expose it, thanks to Oleg Grenrus.
Closed tickets: http://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 ofRatio 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
toData.Aeson.Types
.parseFail = fail
but doesn’t require users to know aboutMonadFail
. 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
andformatRelativePath
functions to turn aJSONPath
into aString
, thanks to Robbie McMichael
1.4.5.0
-
Expose
(<?>)
,JSONPath
andJSONPathElement(..)
fromData.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
andToJSONKey
, 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 oftime-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.*
andhashable 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 otherwith*
combinators now also mention the JSON types they expect- New
unexpected
andprependFailure
combinators.
- Add
Contravariant
ToJSONKeyFunction
instance. Thanks to Oleg Grenrus. - Add
KeyValue
instance forObject
. 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
, andparseIndexedJSON
fromData.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 theaeson-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’sV1
, thanks to Will Yager - Added To/FromJSON instances for
primitive
’sArray
,SmallArray
,PrimArray
andUnliftedArray
, 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 thatData.Attoparsec.Number
is deprecated. - Remove deprecated
withNumber
, usewithScientific
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 usingunwrapUnaryRecords
, thanks to Xia Li-yao - Allow base-compat 0.10.*, thanks to Oleg Grenrus
1.3.0.0
Breaking changes:
GKeyValue
has been renamed toKeyValuePair
, thanks to Xia Li-yao- Removed unused
FromJSON
constraint inwithEmbeddedJson
, 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 asdecodeFile*
variants, thanks to Markus Hauck - Documentation fixes, thanks to Lennart Spitzner
- CPP cleanup, thanks to Ryan Scott
1.2.4.0
- Add
Ord
instance forJSONPathElement
, 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 theOption
newtype, thanks to Xia Li-yao.- Some documentation fixes, thanks to Jonathan Paug & Philippe Crama.
1.2.2.0
- Add
FromJSON
andToJSON
instances forDiffTime
, 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 forRecordToPair
, thanks to Xia Li-yao. - Everything related to
Options
is now exported fromData.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
andparserCatchError
combinators, thanks to Oleg Grenrus. -
Add
Generic
instance forValue
, 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 toOptions
. 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, usedefaultOptions
instead. -
The contents of
GToJSON
andGToEncoding
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
, andSumEncoding
. 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 toFalse
. 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)
andFromJSONKey (Identity a)
no longer require the unnecessaryFromJSON a
constraint. Thanks to Oleg Grenrus. -
Added
Data.Aeson.Encoding.pair'
which is a more general version ofData.Aeson.Encoding.pair
. Thanks to Andrew Martin. -
Day
s BCE are properly encoded and+
is now a valid prefix forDay
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 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