@marblejs/core defines a pluggable Logger dependency registered by default to the context of every server factory. Having that you can inspect things like, registered context dependencies, mapped HTTP routes, request/response logs, I/O event traces, etc.


The interface allows you to define the log tag, type, main message and an optional level, which if not defined defaults to LoggerLevel.INFO.

tag: 'event_bus',
type: 'saveOfferDocument$',
message: 'Saving offer document...',
level: LoggerLevel.INFO
enum LoggerLevel { INFO, WARN, ERROR, DEBUG, VERBOSE };
interface LoggerOptions = {
tag: string;
type: string;
message: string;
level?: LoggerLevel;

Marble.js logger is an instance of IO monad which represents a synchronous effectful computation. In order to execute the logger you have to "run the IO action".

Logger :: LoggerOptions -> IO<void>;
import { LoggerTag, LoggerToken, LoggerLevel, use, r } from '@marblejs/core';
import { requestValidator$, t } from '@marblejs/middleware-io';
import { tap } from 'rxjs/operators';
import { UserDto } from './user.dto';
const foo$ = r.pipe(
r.useEffect((req$, ctx) => {
const logger = useContext(LoggerToken)(ctx.ask);
const logMagic = logger({
tag: LoggerTag.HTTP
level: LoggerLevel.INFO,
type: 'foo$',
message: 'Here the magic happens...',
return req$.pipe(
// ..

Custom logger

You can override the binding and map the stdout IO operations to a different destination (e.g. file or external service) when needed.

import * as O from 'fp-ts/lib/Option';
import { pipe } from 'fp-ts/lib/pipeable';
import { Logger, LoggerLevel, createReader } from '@marblejs/core';
import { CustomLogger } from './customLogger';
export const CustomLoggerReader = createReader<Logger>(() => opts => {
const tag = opts.tag;
const level = opts.level ?? LoggerLevel.INFO;
const message = `${opts.type} ${opts.message}`;
const log = pipe(
[LoggerLevel.ERROR]: CustomLogger.error,
[LoggerLevel.INFO]: CustomLogger.log,
[LoggerLevel.WARN]: CustomLogger.warn,
O.getOrElse(() => CustomLogger.log),
return () => log({ tag, message });

Then in your server definition you have to override the binding.

import { bindTo, createServer, LoggerToken } from '@marblejs/core';
import { CustomeLoggerReader } from './logger.reader.ts';
const server = createServer({
// ...
dependencies: [