Observing Results

Add logging, tracing, and metrics without changing success or error values.

Observation methods return the original Result unchanged.

Observe success

const result = parsePayload(input).tap((payload) => {
  metrics.increment("payload.parsed");
  logger.debug("Parsed payload", { id: payload.id });
});

Use tapAsync for an async side effect.

Observe errors

const result = parsePayload(input).tapError((error) => {
  metrics.increment("payload.parse_failed", { tag: error._tag });
});

Use tapErrorAsync for an async side effect.

Observe either branch

const result = loadUser(id).tapBoth({
  ok: (user) => logger.info("Loaded user", { userId: user.id }),
  err: (error) => logger.warn("Could not load user", { tag: error._tag }),
});

tapBothAsync takes two Promise-returning handlers.

Static and data-last forms

Result.tapError(result, reportError);

const withErrorReporting = Result.tapError((error: AppError) => reportError(error));
const observed = withErrorReporting(result);

Safety contract

A throwing or rejected observer becomes Panic. Observation should not be able to quietly change the Result’s error type. If telemetry failure is expected and recoverable, model that telemetry operation separately rather than hiding it in tap.