fson.fedem.eu is live

FSON has its own site starting today, at fson.fedem.eu. Like its siblings cparse and callonce, FSON has lived so far as an internal dependency and a set of Markdown files. Giving it a public home is a step towards treating it as something other people can pick up.

What FSON is

A superset of JSON — and, since v0.9, a pragmatic superset of JSON5 — for files that people edit by hand:

// service configuration
{
  database: {
    host: "db.internal.example",   // production endpoint
    --user: "root",                /* disabled credential, kept for reference */
    "connection timeout": 2.50,
  },
  retries: [1, 2, 5,],
  --debug: false
}

Comments that nest, bare identifier keys, a -- prefix that switches a member off without deleting it, %include for shared defaults, and |-block multi-line strings. Every JSON document is already valid FSON, and almost every JSON5 one — two narrow exceptions aside.

Three things to read

• The notation. The grammar in full, one page per production, each with its railroad diagram. This is the reason the site exists — FSON is a notation, and a notation deserves to be written down properly.
• The library. fson is the C++ implementation: load an FSON file, query it with dotted paths and typed getters, modify it, and save it back without losing a single comment. Comments and their positions, member order, key form, -- markers and the exact spelling of every number all survive the cycle.
• The tools. fson-check validates and reformats FSON and converts it to strict JSON or JSON5. And the editor tooling — the FSON VS Code extension and the highlight.js grammar — is out under the new Fedem publisher on the Visual Studio Marketplace. The highlight.js grammar is what colours the fson ` blocks in this very post.

What is not here yet

• The Marketplace listing. The extensions are packaged and the publisher (Fedem) is registered; the first vsce publish is imminent. Until then, grab the .vsix from Downloads.

Update (2026-09-08): fson is now MIT-licensed (was

Proprietary) and v1.0.0 is cut — source archives are on the

Downloads page. See the 1.0.0 post.

The family

fson.fedem.eu is the third of the fedem library-showcase sites, sharing a layout, a navbar switcher and a contact form with cparse (the parser core FSON is built on) and callonce.