Winston for ReasonML

Usage no npm install needed!

<script type="module">
  import bsWinston from '';




This is adapter of winston for ReasonML. Because ReasonML is significantly different from js the API here is decidedly different from vanilla winston API. See the API section and examples below.


npm install bs-winston

And to bsconfig.json: "bs-dependencies": [..., "bs-winston", ...]


Initialize/Setup a logger e.g. in you write:

module Format = BsWinston.Format;
module Transport = BsWinston.Transport;

let logger =
    -> setLevel(Debug)
    -> addFormat(Format.createSimple())
    -> addTransport(Transport.createConsole(()))
    -> build);

Use the logger above in another module:

module L = BsWinston.Logger;
let logger = MyLogger.logger;

L.logInfoMsg(logger, "This is info message");
L.logWarnMsg(logger, "This messege is sent at warn level");

try (Js.Exn.raiseError("log this")) {
  | error => L.logErrorExn(logger, "Never fails to fail", error)

/* In a complicated case you may want to send extra json keys. It is possible: */
-> L.withMessage("the usual")
-> L.withJson("keyX", Js.Json.string("valueX"))
-> L.withJson("keyY", Js.Json.number(float_of_int(42)))
-> L.log;


Logger (module BsWinston.Logger)

  • Logger instance is of type BsWinston.Logger.t

  • Set level:

    • error: t => t
    • warn: t => t
    • info: t => t
    • verbose: t => t
    • debug: t => t
    • silly: t => t

    The functions do not mutate but rather create a new logger with the requested level.

  • Combinators:

    • withMessage: t => 'a => t

      Converts the message (2nd arg) into string (with Js.String.make) and combines it with the possible message in the logger (1st arg) (with one space " " in between to avoid clumping)

    • withExn: t => exn => t

      Converts the exception to Js.Json.t if it is Js.Exn.t or just stringifies other ocaml exceptions and sets the result to a field ("error" by default) which can be defined on logger options. (This function does not combine with itself i.e. latter usage overwrites the previous)

    • withJson: t => string => Js.Json.t => t

      Sets the json to a field named with the string parameter.

  • Output

    • log: t => unit

      Produces the side-effect of logging to the logger transport.

  • Convenience functions to make some common use cases less verbose:

    • logErrorMsg = (logger, msg) => logger -> error -> withMessage(msg) -> log
    • logErrorExn = (logger, msg, error) => logger -> error -> withMessage(msg) -> withExn(error) -> log
    • logWarnMsg = (logger, msg) => logger -> warn -> withMessage(msg) -> log
    • logInfoMsg = (logger, msg) => logger -> info -> withMessage(msg) -> log
    • logDebugMsg = (logger, msg) => logger -> debug -> withMessage(msg) -> log

Builder (module BsWinston.Builder)

Loggers can be build either with Builder functions or directly with Logger.create (which is sligthly more lower level).

  • create: unit => t Create a builder. ! This is without trasport and you need at least one transport to create a valid logger.
  • setLevel: t => level => t Logger wide max level to output.
  • setSilent: t => bool => t
  • addFormat: t => Format.t => t (See Formats) (The list will be auto-combined for winston)
  • addTransport: t => Transport.t => t (See Transports)
  • errorKey: t => string => t Set field for error content
  • build: t => Logger.t Create the logger instance.

Format (module BsWinston.Format)

Create winston formats with functions:

  • createAlign
  • createCli
  • createColorize
  • createJson
  • createLabel
  • createLogstash
  • createMetadata
  • createMs
  • createPadLevels
  • createPrettyPrint
  • createPrintf
  • createSimple
  • createSplat
  • createTimestamp
  • createUncolorize

See the src/Format.rei file for function argument details and the logform for further explanations.

Simple custom filters (without Symbol manipulation) can be created with:

  • fromMapFn: (Js.Dict.t(Js.Json.t) => Js.Dict.t(Js.Json.t)) => t

    The function given as argument takes the info object and returns manipulated info object which is then forwarded in the formatting chain.

Transport (module BsWinston.Transport)

Create winston transports with functions:

  • createConsole
  • createFile
  • createHttp
  • createStream

See the src/Transport.rei file for function argument details and the winston for further explanations.

Other transports: