Fast JSON parsing and encoding

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BSD-3-Clause licensed by Bryan O'Sullivan
Maintained by Adam Bergmark
This version can be pinned in stack with:aeson-,5796

Welcome to aeson Hackage Build Status

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://

There’s also a Mercurial mirror:

  • hg clone

(You can create and contribute changes using either git or Mercurial.)


This library is written and maintained by Bryan O’Sullivan, [email protected].


For the latest version of this document, please see

  • 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.

  • 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.

  • Decoding performance has been significantly improved (see 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.

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:
    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.

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

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

  • Fix build on TH-less GHCs

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

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

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

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.


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

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

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

Bug fixes:

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

  • Fixed JSONPath messages for Seq to include indices.

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

  • Fix missing quotes surrounding time encodings.

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


  • Various updates to support GHC 8.

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.


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

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

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

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

    instance ToJSON Coord where
      toEncoding = genericToEncoding defaultOptions

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

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

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

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

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

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

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

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

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

  • GHC 7.2 and older are no longer supported.

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

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

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

  • A stray export of encodeToBuilder got away!

  • 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.

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

  • 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.

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

  • Support time-1.5.

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

  • 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 Supporting the older versions had become increasingly difficult, to the point where it was no longer worth it.

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

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

  • eitherDecode, decodeStrictWith: fixed bugs.

  • Added FromJSON and ToJSON instances for Tree and Scientific.

  • Fixed the ToJSON instances for UTCTime and ZonedTime.

0.6 series

  • Much improved documentation.

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

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

  • Added ToJSON/FromJSON instances for:

    • The Fixed class
    • ISO-8601 dates: UTCTime, ZonedTime, and TimeZone
  • Added accessor functions for inspecting Values.

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

0.5 to 0.6

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

    In the generic APIs of versions 0.4 and 0.5, fields whose names began with a "_" character would have this character removed. This no longer occurs, as it was both buggy and surprising (

  • Fixed a bug in generic decoding of nullary constructors (

0.4 to 0.5

  • When used with the UTF-8 encoding performance improvements introduced in version 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 (

  • 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 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.