I’ve been building Jetro, a Rust JSON query language inspired by functional language paradigm.

The project started about three years ago, but recently went through a serious design overhaul. The newer direction is planner-driven, and focused on being expressive without becoming overwhelming.

It aims to cover much of the everyday usefulness of jq, but with a smaller, more approachable query language and a Rust embeddable engine. Jetro uses simd-json as its primary JSON parser and tries hard not to do unnecessary work on top.

One fun part is demand propagation:

    use jetro::Jetro;

    let j = Jetro::from_bytes(bytes)?;
    let out = j.collect(
      "$.orders
        .map({ id: @.id, total: @.items.map(@.price * @.qty).sum() })
        .filter(@.total > 100)
        .map(@.id)
        .first()"
    )?;

    parse/materialize all orders
    compute totals for every order
    filter all shaped orders
    extract all ids
    then take the first one

    need one id
    <- need one matching order
    <- pull orders one at a time
    <- compute total for the current candidate
    <- if it matches, emit the id and stop

The same idea works for richer object-shaping queries too:

    let out = j.collect(
      "{
        errors: $.events
          .drop_while(@.level != 'error')
          .filter(@.service == 'checkout')
          .map(match @ with {
            { level: 'error', message: msg, timestamp: ts } -> {
              kind: 'error',
              ts: ts,
              msg: msg
            },
            { level: 'warn', message: msg } -> {
              kind: 'warning',
              msg: msg
            },
            _ -> {
              kind: 'other'
            }
          })
          .take(20),
    
        slow_orders: $.orders
          .filter(@.latency_ms > 500)
          .map({ id: @.id, latency: @.latency_ms })
          .take(10),
    
        first_vip: $.customers
          .filter(@.tier == 'vip')
          .map({ id: @.id, region: @.region })
          .first()
      }"
    )?;

Would love get some feedbacks, or ideas for what would make this useful to you.

Thanks

Jetro CLI: https://github.com/mitghi/jetrocli Book: https://github.com/mitghi/jetro-book