bs-log

Logging implementation for ReasonML / BuckleScript.

Features

• Zero runtime in production builds.
• Multiple logging levels.
• Customizable verbosity via environment variable.
• [@log] helper.
• ReasonReact integration.
• Custom loggers.
• Logging in libraries.

Installation

Get the package:

# yarn
yarn add bs-log
# or npm
npm install --save bs-log

Then add it to bsconfig.json:

"bs-dependencies": [
  "bs-log"
],
"ppx-flags": ["bs-log/ppx"]

PPX is highly recommended but optional (read details below).

Usage

There are 4 log levels:

• debug
• info
• warn
• error

You can log message of specific level using either PPX or common functions:

/* ppx */
[%log.info "Info level message"];

/* non-ppx */
BrowserLogger.info(__MODULE__, "Info level message");

If you use PPX, value of __MODULE__ variable will be injected automatically.

Additional data

You can add data to log entry like this:

/* ppx */
[%log.info "Info level message"; ("Foo", 42)];
[%log.info
  "Info level message";
  ("Foo", {"x": 42});
  ("Bar", [1, 2, 3]);
];

/* non-ppx */
BrowserLogger.infoWithData(__MODULE__, "Info level message", ("Foo", 42));
BrowserLogger.infoWithData2(
  __MODULE__,
  "Info level message",
  ("Foo", {"x": 42}),
  ("Bar", [1, 2, 3]),
);

Currently, logger can accept up to 7 additional entries.

Verbosity customization

You can set maximum log level via environment variable BS_LOG. This feature is available only when you use PPX.

Let's say you want to log only warnings and errors. To make it happen, run your build like this:

BS_LOG=warn bsb -clean-world -make-world

Available BS_LOG values:

• *: log everything
• debug: basically, the same as *
• info: log everything except debug level messages
• warn: log warn & error messages
• error: log error messages only
• off: don't log anything

If BS_LOG is set to off, nothing will be logged and none of the log entries will appear in your JS assets.

In case if BS_LOG environment variable is not set, log level warn will be used.

Also, see Usage in libraries.

PPX vs non-PPX

PPX gives you ability to customize maximum log level of your build and eliminates unwanted log entries from production builds. Also, it enables ReasonReact integration. If for some reason you want to use non-PPX api, then you have to handle elimination of log entries yourself on post-compilation stage.

Default logger compiles log entries to console.* method calls so those are discardable via UglifyJS/TerserJS or Babel plugin.

[@log] helper

This helper can be placed in front of any switch expression with constructor patterns and it will inject debug expressions into each branch.

let _ = x => [@log] switch (x) {
  | A => "A"
  | B(b) => b
}

Without a @log helper, an equivalent would be:

let _ = x => switch (x) {
  | A =>
    [%log.debug "A"];
    "A";
  | B(b) =>
    [%log.debug "B with payload"; ("b", b)];
    b;
}

You can pass optional custom namespace to helper like this: [@log "MyNamespace"].

[@log] helper works only for switch expressions with constructor patterns, for now. Let us know in the issues if you need to handle more cases.

ReasonReact integration

Using [@log] helper, you can log dispatched actions in your components.

Annotate reducer function like this:

let reducer = (state, action) => [@log] switch (action) {
  ...
}

These entries are logged on the debug level so none of those will appear in production builds.

Custom loggers

bs-log ships with 2 loggers:

• BrowserLogger (default)
• NodeLogger

And you can easily plug your own.

For example, in development, you want to log everything to console using default logger, but in production, you want to disable console logging and send error level events to bug tracker.

To implement your own logger, you need to create a module (e.g. BugTracker.re) and set the following environment variables for production build.

BS_LOG=error
BS_LOGGER=BugTracker

Considering that you want to log only error level messages, you need to create functions only for errors logging.

/* BugTracker.re */

let error = (__module__, event) =>
  RemoteBugTracker.notify(event ++ " in " ++ __module__);

let errorWithData = (__module__, event, (label, data)) =>
  RemoteBugTracker.notify(
    event ++ " in " ++ __module__,
    [|(label, data)|], /* dummy example of passing additional data */
  );

let errorWithData2 = (
  __module__,
  event,
  (label1, data1),
  (label2, data2),
) =>
  RemoteBugTracker.notify(
    event ++ " in " ++ __module__,
    [|
      (label1, data1),
      (label2, data2),
    |],
  );

/* Up to 7 */

You don't have to re-implement all functions from default logger, only the ones you actually use. Don't worry to forget to implement something. If later on, you will attempt to use unimplemented method it will be compile time error.

Usage in libraries

I you are developing a library and want to use bs-log during development process, you can do so without spamming output of consumers of your library.

bs-log/ppx accepts --lib flag:

"ppx-flags": [
  ["bs-log/ppx", "--lib=my-lib"]
]

Once this flag is passed, you need to provide special value of BS_LOG to log your entries:

BS_LOG=my-lib=* bsb -make-world

If consumers of your lib would like to see log output from your lib, they can do so too by extending a value of BS_LOG variable:

BS_LOG=*,my-lib=error bsb -make-world

Few more examples to illustrate how it works:

# log everything from application code only
BS_LOG=* bsb -make-world

# log everything from application code
# log errors from `my-lib`
BS_LOG=*,my-lib=error bsb -make-world

# log everything from application code
# log errors from `my-lib-1`
# log warnings and errors from `my-lib-2`
BS_LOG=*,my-lib-1=error,my-lib-2=warn bsb -make-world

Caveats

Logging is disabled after file save
If you run bsb via editor integration, make sure editor picked up BS_LOG variable. E.g. if you use Atom run it like this:

BS_LOG=info atom .

If your editor is telling you, variables used in ppx are unused, you can either:

1. prefix such variables with _
2. or open editor with BS_LOG variable set to appropriate level.

Changing value of BS_LOG/BS_LOGGER doesn't make any effect
When you change a value of BS_LOG and/or BS_LOGGER, -clean-world before the next build.

Developing

Repo consists of 2 parts:

• BuckleScript loggers: dependencies are managed by yarn
• Dune ppx: dependencies are managed by esy

Clone repo and install deps:

esy install
yarn install

Build loggers and ppx:

yarn run build
esy build

To explore generated output, extend bsconfig.json:

"sources": [
  "src",
  {
    "dir": "test",
    "type" : "dev"
  }
],
"ppx-flags": [
  "./_build/default/bin/bin.exe"
]

And rebuild BuckleScript project:

BS_LOG=* yarn run build