module std.obs

std.obs — observability (§15.5, II.1): structured logs (D-3110), spans and metrics (D-5240..D-5259; docs/observability.md).

log.info("order {id} paid {amount}") writes one line to stderr and keeps every interpolated value as a separate field named after its expression (id, amount). The message is only built when the level is enabled. secret<T> values show as *** everywhere. The level comes from --log-level l or OXL_LOG (trace debug info warn error, default info, III.6); the format is text (default) or JSON lines (--log-format json or OXL_LOG_FORMAT=json):

oxl

2026-11-03T15:04:05.250Z INFO  order 42 paid 10.5 id=42 amount=10.5
{"ts":"2026-11-03T15:04:05.250Z","level":"info","msg":"order 42 paid 10.5","fields":{"id":"42","amount":"10.5"}}

Intrinsics: compiler/src/codegen/CGLog.cpp over runtime/src/obs/log.cpp.

Example:

oxl✓ checked in CI

use std.obs { log, metric, span }

fn main() io {
  let served = metric.counter("requests_total", "requests served")
  span.with("handle") {
    let id = 7
    log.info("request {id} handled")
    served.inc()
  }
  print("{served.value()}")
}

Output

1.0

Types

data `Counter`	A value that only grows (requests served, errors).
data `Gauge`	A value that goes up and down (queue length, connections).
data `Histogram`	A distribution (latencies, sizes) counted into buckets.
enum `Level`	Severity of a log record, lowest first.
data `log`	Namespace of the logging functions
data `metric`	Namespace of the metric constructors.
data `Span`	An open span of the calling task.
data `span`	Namespace of the span functions

module std.obs

Types

See also