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Programming language: Elixir
License: MIT License
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Rotor is a build system for Elixir projects. Use it to compile things, run commands or do anything that needs to be run when files change.

Wreckers don't call for backup, they call for cleanup ~!

Define your rotor watch groups in config/rotors.exs in your project and they'll be loaded when your app starts


  • Works with any web framework or even plain mix projects
  • Easy to use
  • Extendable with simple functions
  • Can be configured to run commands or code or go to the moon.


  • Add rotor as a dependency to your mix.exs
  • Define watch groups in config/rotors.exs
  • Run Rotor.start in your IEx console to run the rotors

Example 1: Reload Elixir modules whenever they change

# config/rotors.exs

use Rotor.Config

paths = ["lib/**/*"]

Rotor.define :ex_modules, paths, fn(changed, _all)->

Make changes to any file in the lib dir of your project and watch it reload in your console

Example 2: Compile CoffeeScript files whenever they change

# config/rotors.exs

use Rotor.Config

paths = ["assets/libs/*.coffee", "assets/*.coffee"]
Rotor.define :coffee_assets, paths, fn(changed_files, all_files)->
  |> coffee
  |> concat
  |> write_to("priv/static/assets/app.js")

touch a file that's in the path provided and watch the rotor function being run.

The above example uses the coffee_rotor.

NOTE: Rotor is not a replacement for mix. It is intended to be used as your sidekick during development.


What is a watch group?

A set of paths you want to watch is called a Watch group". Each watch group has the following:

  • name
  • a list of paths to watch
  • rotor function - a function that is run everytime any of the files in the paths changes. It should accept 2 arguments
    • changed_files - a list of maps, each containing info about a changed file
    • all_files - a list of maps, each containing info about all files that matched the path

Where to define watch groups?

config/rotors.exs is prefered. But if you want to define them elsewhere feel free. Take a look at examples

How to run them?

Run Rotor.start in your IEx console to run the rotors.

You can also automate this by adding Rotor.start somewhere in your code. But be careful ~!

How to define watch groups?

# With default options
Rotor.define(name, files, rotor_function)

# With options
Rotor.define(name, files, rotor_function, options)

The rotor function is passed info about the list of files that match the paths specified. The rotor function calls other little functions called rotors, that run certain tasks.

paths = ["assets/javascripts/libs/*.js", "assets/javascripts/*.js"]
Rotor.define :javascripts, paths, fn(changed_files, all_files)->
  |> concat
  |> write_to("priv/static/assets/app.js")

The fourth argument is options. It accepts a map. The following are valid options:

  • manual - defaults to false. If set to true, paths will only be polled when Rotor.run/1 or Rotor.run_async/1 is called.
  • interval - defaults to 2500 milliseconds (2.5 seconds). This is the interval at which files are checked for changes.

Manually running watch group's rotor function

If you want files to be polled only when you say so (and not at intervals). Then pass the manual option as true when adding a group. Then use one of the following functions to trigger a poll.

  • Rotor.run(group_name) - will poll paths and run the Rotor function synchronously
  • Rotor.run_async(group_name) - will poll paths and run the Rotor function asynchronously


Rotor ships with a few simple helper functions in the Rotor.BasicRotors module.

  • read_files(files) - reads contents of files, and returns files with a property called contents
  • copy_files(files, destination_dir) - copies files to destination_dir
  • concat(files) - concats contents of files and returns a string
  • write_to(contents, output_path) - writes the contents to the file path specified in output path
  • reload_modules(files) - reloads the modules in the list of files passed

You can also write your own. Check the "Writing custom rotors" section below.

Other stuff

  • To remove a watch group
  • To list all watch groups
  • To run a watch group's rotor function forcefully


paths = ["assets/stylesheets/libs/*.css", "assets/stylesheets/*.css"]
Rotor.define :stylesheets, paths, fn(changed_files, all_files)->
  |> concat
  |> write_to("app.css")

paths = ["assets/images/*", "assets/fonts/*"]
Rotor.define :images_and_fonts, paths, fn(changed_files, all_files)->
  copy_files(files, "priv/static/assets")

Writing custom rotors

Rotors are just functions that accept data and do something.

Checkout coffee_rotor, which provides a rotor to compile CoffeeScript files.


Copyright © 2014, Akash Manohar J, under the MIT License

Inspired by gulp

*Note that all licence references and agreements mentioned in the rotor README section above are relevant to that project's source code only.