XO0438: log argument performs an effect

A call to log.debug, log.info, log.warn or log.error (or a Logger’s) does not evaluate its arguments when the record is below the configured level. An argument that reads a file, calls the network or has another effect of its own therefore runs only sometimes, which Go’s log/slog does not do. Do the work before the call (let n = f()) when it must always run, or keep the argument pure. See std.md 6 and decision 0558.

Example

fn main(os: Os) uses fs.read, log {
  log.info("size", {"n": os.fs.read_file("x").len()})
}

Fix

fn main(os: Os) uses fs.read, log {
  let n = os.fs.read_file("x").len()
  log.info("size", {"n": n})
}

Run xo explain XO0438 for this text in a terminal, or see section 10.6 of the specification.