moonbitlang/jsonl

A tiny async reader for JSON Lines (newline-delimited JSON, also called NDJSON). Each line is one independent JSON value; this package turns such a stream into MoonBit Json values using the built-in moonbitlang/core/json parser.

It is native-only because it reads from moonbitlang/async/io streams.

API

• parse(text : String) -> Array[Json] raise — pure helper that decodes already-buffered text. Blank lines are skipped; the first malformed line raises.
• each(reader, visit) -> Unit raise (async) — stream values from any @io.Reader, invoking visit on each parsed value in order.
• read_all(reader) -> Array[Json] raise (async) — collect every value from a reader into an array.
• read_stdin() -> Array[Json] raise (async) — collect every value from standard input, so the caller need not import moonbitlang/async/stdio.

Buffered Input

The pure parse helper needs no IO, so it is handy for testing and for input you already hold as a string:

///|
test "parse buffered JSON Lines" {
  let values = @jsonl.parse(
    (
      #|{"event": "agent_step", "step": 1}
      #|{"event": "agent_finished", "answer": "DONE"}
    ),
  )
  assert_eq(values.length(), 2)
  assert_true(values[1] == { "event": "agent_finished", "answer": "DONE" })
}

Streaming From A Reader

each and read_all consume any moonbitlang/async/io reader — a pipe, a socket, @stdio.stdin, and so on — one line at a time:

///|
async test "stream JSON Lines from a pipe" {
  @async.with_task_group(group => {
    let (rd, wr) = @io.pipe()
    group.spawn_bg(() => {
      wr.write("{\"n\": 1}\n{\"n\": 2}\n")
      wr.close()
    })
    let values = @jsonl.read_all(rd)
    assert_eq(values.length(), 2)
  })
}

A common use is to consume a program's JSONL log over a pipe and assert on it with the typed Json values instead of an external tool such as jq.

License

Apache-2.0.