std/log

Records with a level, a time, a message, and fields of keys and values, written as text or as JSON Lines to standard error, standard output, and files that rotate by size or by time.

import "std/log"

Logger log = logger()                        // INFO and up, to standard error
log_to_file(log, "app.log", format: "json", max_bytes: 10000000, keep: 3)
log.info("server started", {"port": "8080"})
Logger request = log.with_field("request", "42")
request.warn("slow answer", {"ms": "950"})
// 2026-11-06T16:45:07.123Z WARN  slow answer request=42 ms=950

Function or method	Result
`logger(level, format)`	a `Logger` of records at `level` and up, `LogLevel.INFO` unless given, to standard error as `"text"` or `"json"`
`quiet_logger(level)`	a `Logger` that writes nowhere until outputs are added
`log_to_stdout(l, format)`	adds standard output
`log_to_file(l, path, format:, max_bytes:, rotate_every:, keep:, level:)`	adds a file, and gives its `LogOutput`; `max_bytes` rotates it before it passes that size, `rotate_every` when the `"hour"` or `"day"` changes, `keep` is how many old files stay, and `level` the least level it takes
`l.trace`, `l.debug`, `l.info`, `l.warn`, `l.error` `(message, fields)`	a record, with fields from a `map[str, str]`
`l.with_field(key, value)`	a logger that writes where `l` does, with the field in each record
`l.enabled(level)`	whether a record at `level` would be written
`log_on_loop(l)`, `l.flush()`	keep the records and write them after the event loop's current callback, so a callback never waits for a file; `flush` writes what is kept

A text record is the time in UTC, the level, the message, and key=value for each field, in quotes when it has spaces; a JSON record has time, level, message, and the fields. A file rotated by size becomes path.1, the one before path.2, and so on; one rotated by time becomes path.2026-11-06, or path.2026-11-06T13 by the hour.