Effect Logger Multi-Writer

Effect equivalents of Go's slog.NewMultiHandler plus the scoped file logger pattern. The Go slog.NewMultiHandler and defer closeFunc() versions are in Foundations.

Pattern 1: Logger.zip

Direct slog.NewMultiHandler equivalent. Each logger fires for every log line.

// effect-utils/packages/@overeng/tui-react/src/effect/TuiLogger.ts:187-196
const layer = logToConsole === true
  ? Layer.merge(
      Logger.replace(Logger.defaultLogger, Logger.zip(Logger.defaultLogger, tuiLogger)),
      Logger.minimumLogLevel(minLevel),
    )
  : Layer.merge(
      Logger.replace(Logger.defaultLogger, tuiLogger),
      Logger.minimumLogLevel(minLevel),
    )

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Logger.zip(defaultLogger, tuiLogger) ↔ slog.New(slog.NewMultiHandler(handlerA, handlerB)). Output type is the zip of the two output types. Both loggers receive every message.

Pattern 2: Logger.layer([...])

Array-form API. mergeWithExisting: false replaces the default logger entirely instead of appending.

// t3code/apps/server/src/serverLogger.ts:8-16
export const ServerLoggerLive = Effect.gen(function* () {
  const config = yield* ServerConfig;
  const minimumLogLevelLayer = Layer.succeed(References.MinimumLogLevel, config.logLevel);
  const loggerLayer = Logger.layer([Logger.consolePretty(), Logger.tracerLogger], {
    mergeWithExisting: false,
  });

  return Layer.mergeAll(loggerLayer, minimumLogLevelLayer);
}).pipe(Layer.unwrap);

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Pattern 3: Scoped file logger with acquireRelease

Go closeFunc pattern, but structured by the Layer's scope. File handle is opened, attached to the logger, closed on Layer finalization.

// effect-utils/packages/@overeng/utils/src/node/FileLogger.ts:56-76
export const makeFileLogger = ({ logFilePath, threadName, colors }: MakeFileLoggerOptions) =>
  Layer.unwrapScoped(
    Effect.gen(function* () {
      yield* Effect.sync(() => fs.mkdirSync(path.dirname(logFilePath), { recursive: true }))

      const logFile = yield* Effect.acquireRelease(
        Effect.sync(() => fs.openSync(logFilePath, 'a', 0o666)),
        (fd) => Effect.sync(() => fs.closeSync(fd)),
      )

      return Logger.replace(
        Logger.defaultLogger,
        prettyLoggerTty({
          colors: colors ?? false,
          stderr: false,
          formatDate: (date) => `${defaultDateFormat(date)} ${threadName ?? ''}`,
          onLog: (str) => fs.writeSync(logFile, str),
        }),
      )
    }),
  )

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Effect.acquireRelease ties cleanup to the Layer's scope. Combine with Logger.zip (or Logger.layer([...])) when you want console + file:

const fileLogger = makeFileLogger({ logFilePath: '/var/log/app.log' });
const consoleLogger = Logger.replace(Logger.defaultLogger, Logger.prettyLogger());

yield* myProgram.pipe(Effect.provide(Logger.zip(consoleLogger, fileLogger)));

When to use which