aeson
Fast JSON parsing and encoding
Version on this page: | 1.0.2.1@rev:1 |
LTS Haskell 22.39: | 2.1.2.1@rev:4 |
Stackage Nightly 2024-10-31: | 2.2.3.0@rev:2 |
Latest on Hackage: | 2.2.3.0@rev:2 |
aeson-1.0.2.1@sha256:cf848d5d07a3e6962d7a274d452c772bc1c413a0f9f2d5f112fdde4556a7d7f1,5796
Module documentation for 1.0.2.1
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/bos/aeson.git
There’s also a Mercurial mirror:
hg clone http://bitbucket.org/bos/aeson
(You can create and contribute changes using either git or Mercurial.)
Authors
This library is written and maintained by Bryan O’Sullivan, [email protected].
Changes
For the latest version of this document, please see https://github.com/bos/aeson/blob/master/changelog.md.
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 fromData.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
andToJSONKey
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
, andFromJSON2
. -
Added
Data.Aeson.Encoding
with functions to safely writeToJSON
instances usingtoEncoding
.
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 cancabal 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
andFromJSON
classes. In the same wayRead
andShow
handle lists specifically. This removes need for overlapping instances to handleString
. -
Added a new
sumEncoding
optionUntaggedValue
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 yieldedError in $.x.a[2]: failed to parse field" x: failed to parse field a: expected Int, encountered Boolean
and now yieldsError 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 thebytestring-builder
compatibility package. -
Export
decodeWith
,decodeStrictWith
,eitherDecodeWith
, andeitherDecodeStrictWith
fromData.Aeson.Parser
. This allows decoding using explicit parsers instead of usingFromJSON
instances. -
Un-orphan internal instances to render them in haddocks.
Other changes:
-
Integral
FromJSON
instances now only accept integral values. E.g. parsing3.14
toInt
fails instead of succeeding with the value3
. -
Over/underflows are now caught for bounded numeric types.
-
Remove the
contents
field encoding withallNullaryToStringTag = 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
withunwrapUnaryRecords = True
would produce an invalid encoding:"unwrap\":""
. -
ToJSON
instances usinggenericToEncoding
andomitNothingFields
no longer produce invalid JSON. -
Added instances for
DList
,Compose
,Product
,Sum
.
0.11.2.0
- Enable
PolyKinds
to generalizeProxy
,Tagged
, andConst
instances. - Add
unsafeToEncoding
inData.Aeson.Types
, use with care!
0.11.1.4
- Fix build with
base >= 4.8
andunordered-containers < 0.2.6
.
0.11.1.3
- Fix build on TH-less GHCs
0.11.1.2
- Fix build with
base < 4.8
andunordered-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
andRight
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
andFromJSON
instances except for[a]
are no longerOVERLAPPABLE
. Mark your instance asOVERLAPPING
if it overlaps any of the other aeson instances. -
All
ToJSON
andFromJSON
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 forZonedTime
andUTCTime
. This is one of the formats allowed by ISO 8601. -
Added
ToJSON
andFromJSON
instances for theVersion
,Ordering
, andNatural
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 thantoJSON
. (You must write or code-gen atoEncoding
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-precisionInteger
type. -
When using
toEncoding
,[Char]
can be encoded without a conversion toText
. 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 intermediateValue
.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
usestoJSON
. You will not see any performance improvement unless you write an implementation oftoEncoding
, which can be very simple:instance ToJSON Coord where toEncoding = genericToEncoding defaultOptions
(Behind the scenes, the
encode
function usestoEncoding
now, so if you implementtoEncoding
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
andToJSON
instances for types in data families. -
If you use Template Haskell or generics, and used to use the
camelTo
function to rename fields, the newcamelTo2
function is smarter. For example,camelTo
will renameCamelAPICase
tocamelapi_case
(ugh!), whilecamelTo2
will map it tocamel_api_case
(yay!). -
New
ToJSON
andFromJSON
instances for the following time-related types:Day
,LocalTime
. -
FromJSON
UTCTime
parser accepts the same values as forZonedTime
, but converts any time zone offset into a UTC time. -
The
Result
type is now an instance ofFoldable
andTraversable
. -
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 theResult
type lacked an implementation offail
(oops). This has been corrected. -
Semantics of
(.:?)
operator are changed. It’s doesn’t anymore accept presentNull
value. -
Added
(Foldable t, ToJSON a) => ToJSON (t a)
overlappable instance. You might seeNo 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
andjson'
parsers are now synonyms forvalue
andvalue'
, in conformance with the looser semantics of RFC 7159. -
Renamed
encodeToByteStringBuilder
to the more compactencodeToBuilder
.
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 forUTCTime
is rendered with higher (picosecond) resolution. -
The
value
parser now correctly handles leading whitespace. -
New instances of
ToJSON
andFromJSON
forData.Sequence
andData.Functor.Identity
. TheValue
type now has aRead
instance. -
ZonedTime
parser ordering now favours the standardJSON
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
andFromJSON
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, andbytestring
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
andToJSON
instances forTree
andScientific
. -
Fixed the
ToJSON
instances forUTCTime
andZonedTime
.
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
, andTimeZone
- The
-
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 improvesaeson
’s JSON encoding performance by 33% relative toaeson
0.4.As part of achieving this improvement, an API change was necessary. The
fromValue
function in theData.Aeson.Encode
module now uses thetext
package’sBuilder
type instead of theblaze-builder
package’sBuilder
type.
0.3 to 0.4
-
The new
decode
function complements the longstandingencode
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
andFromJSON
instances. To see how to use GHC generics, refer toexamples/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 toexamples/GenericSYB.hs
. -
-
We switched the intermediate representation of JSON objects from
Data.Map
toData.HashMap
which has improved type conversion performance. -
Instances of
ToJSON
andFromJSON
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.