aeson: Fast JSON parsing and generation

[ bsd3, json, library, text, web ] [ Propose Tags ]

A JSON parsing and generation library optimized for ease of use and high performance.

Parsing performance with GHC 6.12.3 on a late 2010 MacBook Pro (2.66GHz Core i7), for mostly-English tweets from Twitter's JSON search API:

  • English, 854 bytes: 29029 msg/sec (23.6 MB/sec)

  • English, 6.4 KB: 6407 msg/sec (40.3 MB/sec)

  • English, 31.2 KB: 1265 msg/sec (38.8 MB/sec)

  • English, 61.5 KB: 585 msg/sec (35.2 MB/sec)

Handling heavily-escaped text is a little more work. Here is parsing performance with Japanese tweets, where much of the text is entirely Unicode-escaped:

  • Japanese, 14.6 KB: 2227 msg/sec (31.9 MB/sec)

  • Japanese, 44.1 KB: 671 msg/sec (29.6 MB/sec)

Encoding performance on the same machine and data:

  • English, 854 bytes: 43439 msg/sec (35.4 MB/sec)

  • English, 6.4 KB: 7127 msg/sec (44.8 MB/sec)

  • Engish, 61.5 KB: 765 msg/sec (46.0 MB/sec)

  • Japanese, 14.6 KB: 4727 msg/sec (67.5 MB/sec)

  • Japanese, 44.1 KB: 1505 msg/sec (64.8 MB/sec)

With GHC 7.0.2, the story is mixed: parsing is 20-40% slower than GHC 6.12.3, while encoding performance ranges from about the same to twice as fast (on numeric data).

(A note on naming: in Greek mythology, Aeson was the father of Jason.)

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Versions [RSS],,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, (info)
Dependencies attoparsec (>=, base (>=4 && <5), blaze-builder (>=, bytestring, containers, deepseq (<1.2), ghc-prim, hashable, integer (>=0.1 && <0.2), integer-gmp, mtl, old-locale, syb, text (>=, time (>=1.1.3), unordered-containers (>=, vector (>=0.7) [details]
License BSD-3-Clause
Copyright Copyright 2011 MailRank, Inc.
Author Bryan O'Sullivan <>
Maintainer Bryan O'Sullivan <>
Revised Revision 2 made by phadej at 2021-10-20T14:58:37Z
Category Text, Web, JSON
Home page
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Source repo head: git clone
head: hg clone
Uploaded by BryanOSullivan at 2011-04-05T17:06:13Z
Distributions Arch:, Debian:, Fedora:, FreeBSD:, LTSHaskell:, NixOS:, Stackage:, openSUSE:
Reverse Dependencies 2272 direct, 5467 indirect [details]
Downloads 586271 total (1066 in the last 30 days)
Rating 3.0 (votes: 40) [estimated by Bayesian average]
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Readme for aeson-

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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://

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,