Low-level JSON parsing and pretty-printing library for OCaml
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Yojson: low-level JSON library for OCaml

This library is for manipulating the json AST directly. For mapping between OCaml types and json, we recommend atdgen.

Library documentation

Currently at https://mjambon.github.io/mjambon2016/yojson-doc/Yojson.html

The link above is for the ocamldoc-generated of some past version of yojson. Suggestions on how to keep this documentation updated automatically for each minor release are welcome.

Design goals

  • reducing inter-package dependencies by the use of polymorphic variants for the JSON tree type

  • allowing variants of the JSON tree type to be shipped by the library itself or to be easily created as extensions of the library

  • allowing type-aware serializers/deserializers such as json-static to read and write directly without going through a JSON tree, for efficiency purposes. This requires making readers and writers of JSON atoms (int, string, etc.) to be exported and composable.

  • providing a few non-standard, optional extensions of JSON. These extensions will include:

    • optional quotes around "simple" field/constructor names
    • a syntax for tuples (at least 2 elements): (x, y)
    • a syntax for variants (0 or 1 arg only): <Foo> <Bar:"abc">

Other choices already in json-wheel

  • distinction between ints and floats (optional)

  • Getting rid of the UTF-X encoding constraint that prevents from exchanging binary data:

    • encoding is ASCII except for the contents of string literals
    • string literals may represent arbitrary sequence of bytes
    • \uABCD escapes in string literals expand to UTF-8


  • no dependency on ocamlnet for UTF-8