Jsonnet in trip2g

Values and comments

// A comment. JSON as is, plus trailing commas and unquoted keys.
{
  title: 'Hello',
  "tags": ['a', 'b'],
  draft: false,
  views: 3,
}
// → {"draft": false, "tags": ["a", "b"], "title": "Hello", "views": 3}

Strings take single or double quotes. Keys don't need quotes when they are plain words.

Variables

local names a value. It ends with ; and is visible in everything that follows:

local site = 'trip2g';
local year = 2026;
{ footer: site + ' © ' + year }
// → {"footer": "trip2g © 2026"}

+ joins strings, adds numbers, concatenates lists and merges objects. A number added to a string becomes text.

Fields of an object

local meta = { title: 'Post', tags: ['go'] };
{
  title: meta.title,
  hasTags: std.objectHas(meta, 'tags'),
  author: std.get(meta, 'author', 'unknown'),
}
// → {"author": "unknown", "hasTags": true, "title": "Post"}

meta.author fails when the field is missing. std.get(meta, 'author', default) returns the default instead, and std.objectHas asks whether the field is there. In patches and validators, use these two for fields a note may not have.

Conditions

local meta = { status: 'draft' };
{
  visible: if meta.status == 'published' then true else false,
  label: if meta.status == 'draft' then 'Draft' else '',
}
// → {"label": "Draft", "visible": false}

if is an expression with a value, like a ternary operator. Always write else: without it, a false condition gives null. A validator that returns null instead of a list gets a CRITICAL message.

Lists and comprehensions

local tags = ['go', 'Draft', 'web'];
{
  lower: [std.asciiLower(t) for t in tags],
  noDrafts: [t for t in tags if t != 'Draft'],
  count: std.length(tags),
}
// → {"count": 3, "lower": ["go", "draft", "web"], "noDrafts": ["go", "web"]}

[expression for x in list if condition] builds a new list. Validators are mostly written this way: one message for each element that breaks a rule.

Text

local name = 'Notes – 2026-10-18 – Cursor';
local parts = std.split(name, ' – ');
{
  parts: parts,
  msg: 'date %s, tool %s' % [parts[1], parts[2]],
  isNotes: std.startsWith(name, 'Notes'),
}
// → {"isNotes": true, "msg": "date 2026-10-18, tool Cursor",
//    "parts": ["Notes", "2026-10-18", "Cursor"]}

'…%s…' % value fills placeholders, like printf: %s for text, %d for a whole number. With several values, pass a list.

Functions

local err(msg) = { level: 'error', msg: msg };
local missing(meta, f) = !std.objectHas(meta, f);
local meta = { title: 'Post' };
[err('missing ' + f) for f in ['title', 'description'] if missing(meta, f)]
// → [{"level": "error", "msg": "missing description"}]

A function is a local with parameters. Small helpers like err keep a validator short.

Walking a list with state

When each element depends on the ones before it, for example "which section is this image in", use std.foldl. It walks the list and carries an accumulator along:

local nodes = [
  { type: 'heading', text: 'Intro' },
  { type: 'image' },
  { type: 'heading', text: 'Steps' },
];
std.foldl(
  function(acc, n)
    if n.type == 'heading' then acc + [{ title: n.text, images: 0 }]
    else acc[:std.length(acc) - 1]
      + [acc[std.length(acc) - 1] + { images: super.images + 1 }],
  nodes,
  [],
)
// → [{"images": 1, "title": "Intro"}, {"images": 0, "title": "Steps"}]

acc[:n] is a slice: all but the last section. obj + { images: super.images + 1 } copies the object with one field changed.