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PlatformLogger

Effect’s logging system generally writes messages to the console by default. However, you might prefer to store logs in a file for easier debugging or archiving. The PlatformLogger.toFile function creates a logger that sends log messages to a file on disk.

toFile

Creates a new logger from an existing string-based logger, writing its output to the specified file.

If you include a batchWindow duration when calling toFile, logs are batched for that period before being written. This can reduce overhead if your application produces many log entries. Without a batchWindow, logs are written as they arrive.

Note that toFile returns an Effect that may fail with a PlatformError if the file cannot be opened or written to. Be sure to handle this possibility if you need to react to file I/O issues.

Example (Directing Logs to a File)

This logger requires a FileSystem implementation to open and write to the file. For Node.js, you can use NodeFileSystem.layer.

import { PlatformLogger } from "@effect/platform"
import { NodeFileSystem } from "@effect/platform-node"
import { Effect, Layer, Logger } from "effect"


// Create a string-based logger (logfmtLogger in this case)
const myStringLogger = Logger.logfmtLogger


// Apply toFile to write logs to "/tmp/log.txt"
const fileLogger = myStringLogger.pipe(PlatformLogger.toFile("/tmp/log.txt"))


// Replace the default logger, providing NodeFileSystem
// to access the file system
const LoggerLive = Logger.replaceScoped(Logger.defaultLogger, fileLogger).pipe(
  Layer.provide(NodeFileSystem.layer),
)


const program = Effect.log("Hello")


// Run the program, writing logs to /tmp/log.txt
Effect.runFork(program.pipe(Effect.provide(LoggerLive)))
/*
Logs will be written to "/tmp/log.txt" in the logfmt format,
and won't appear on the console.
*/

In the following example, logs are written to both the console and a file. The console uses the pretty logger, while the file uses the logfmt format.

Example (Directing Logs to Both a File and the Console)

import { PlatformLogger } from "@effect/platform"
import { NodeFileSystem } from "@effect/platform-node"
import { Effect, Layer, Logger } from "effect"


const fileLogger = Logger.logfmtLogger.pipe(PlatformLogger.toFile("/tmp/log.txt"))


// Combine the pretty logger for console output with the file logger
const bothLoggers = Effect.map(fileLogger, (fileLogger) =>
  Logger.zip(Logger.prettyLoggerDefault, fileLogger),
)


const LoggerLive = Logger.replaceScoped(Logger.defaultLogger, bothLoggers).pipe(
  Layer.provide(NodeFileSystem.layer),
)


const program = Effect.log("Hello")


// Run the program, writing logs to both the console (pretty format)
// and "/tmp/log.txt" (logfmt)
Effect.runFork(program.pipe(Effect.provide(LoggerLive)))