JSON Schema library for Haskell
Haskell Python Nix
Latest commit db91ca2 Jan 14, 2017 @seagreen
+ Report details of `"required"` validator failure.
+ Bump `directory`.


hjsonschema logo

A Haskell implementation of the most commonly used JSON Schema specification (Draft 4).

Hackage / GitHub / Travis CI


  • As of 2017 json-schema-org has begun releasing new drafts of the standard. Once this work stabilizes hjsonschema will add coverage for the latest draft.

  • Requires pcre (pkgs.pcre in Nixpkgs).

  • Schemas with circular references can cause infinite loops. hjsonschema does loop detection but it may not be solid yet -- please open an issue if you find a situation where it fails.


See here.


Run all tests:

stack test

Run only local tests:

stack test hjsonschema:local

Run remote tests (makes GETs to json-schema.org, also temporarily starts an HTTP server on port 1234):

stack test hjsonschema:remote



  • Be a correct and fast implementation of the spec.

  • Be a useful reference for implementers in other languages. Haskell's high level nature, expressive type system and referential transparency suit this purpose well.

Good Parts

  • Passes all the required tests in the language agnostic test suite.

  • Very modular, which should make it easy to support future versions of the specification.

Bad Parts

  • Uses the pcre-heavy regular expression library for the "pattern" validator. It should use a library based on the ECMA 262 regex dialect, which the spec requires.

  • Currently doesn't support the optional "format" validators.


  • JSON-Schema-Test-Suite is vendored from commit # aabcb3427745ade7a0b4d49ff016ad7eda8b898b here.

  • src/draft4.json is from commit # f3d5aeb5ffbe9d9a5a0ceb761dc47c7c4c2efa68 here.


TJ Weigel created the logo.

Tim Baumann wrote aeson-schema, on which hjsonschema's test code and its implementation of SchemaGraph were based.

Julian Berman maintains the fantastic language agnostic test suite.