Getting Started

Installing Effekt

Currently, the Effekt language is implemented in Scala and compiles to JavaScript. To use the Effekt compiler and run the resulting programs, you need to have


The recommended way to install Effekt is by running:

npm install -g

This will download the compiler and install the effekt command in your path.

Alternatively, you can also download a specific release on the release page on Github. And then install it with

npm install -g <PATH_TO_FILE>/effekt.tgz

You can find the installation location by running

npm root -g

For example, on Mac OSX the effekt binary will be placed in:


You can always remove the effekt installation by:

npm remove -g effekt


The Effekt “binary” is actually just an executable jar-file. NPM will also install scripts ( on Linux) and (effekt.cmd on Windows) to start the jar-file with java -jar.

Your first Effekt Program

Now let’s get started. We create a file called hello.effekt and fill it with the following contents:

def main() = {
  println("Hello world!")

On this page, you can execute the above example, by clicking “run” on the right:


Feel free to edit the above program and try to run it again.

You can also install Effekt locally and then execute the example by compiling and running it with effekt.

$ effekt hello.effekt
Hello world!

You can find example programs in the examples folder.

Compiling Programs with Effekt

Running the above command, you can notice that this created a ./out directory containing the compiled JavaScript files. We can also run the program (without compiling it again) by directly using node:

$ node
Welcome to Node.js v13.6.0.
Type ".help" for more information.
> hello = require('./out/hello.js')
{ main: [Function: main] }
> hello.main().run()
Hello world!

To only compile Effekt sources to JavaScript without running them, provide the --compile flag. For example:

effekt --compile hello.effekt

Again, this will generate JavaScript files in ./out. This output folder can be configured by providing arguments --out ./otherdir.

The recommended workflow is to open your files in VSCode and then run them using the Effekt REPL. The Effekt REPL is started by just invoking effekt (without any arguments):

$ effekt

  _____     ______  __  __     _    _
 (_____)   |  ____|/ _|/ _|   | |  | |
   ___     | |__  | |_| |_ ___| | _| |_
  (___)    |  __| |  _|  _/ _ \ |/ / __|
  _____    | |____| | | ||  __/   <| |_
 (_____)   |______|_| |_| \___|_|\_\\__|

 Welcome to the Effekt interpreter. Enter a top-level definition, or
 an expression to evaluate.

 To print the available commands, enter :help

> import hello
> :status
import hello

> main()
Hello world!

Here, you can define functions, import modules and enter expressions to evaluate. Executing commands in the REPL compiles the corresponding Effekt-programs to Javascript (you can find them in ./out/) and runs them with Node.js.

If you change the contents of the hello.effekt file to

module hello

def main() = {

and simply enter main() in the open REPL, you can notice that all necessary dependencies are automatically recompiled:

> main()

Setting Up VSCode

Effekt comes with a basic language server implementation (LSP). The effekt-vscode repository The extension is currently not published, so you will have to install it manually by downloading a vsix file. You can download the extension of the latest release on Github:

In VSCode select

  1. Preferences / Extensions in the menu,
  2. ... on the top right corner of the extensions menu, and
  3. Install from VSIX ... selecting the vsix-file.


After installing the vsix file, you might be faced with the error

“Couldn’t start client Effekt Language Server”

and need to set the path to the effekt binary.

For Mac OS users this probably works out of the box after installing effekt with npm (that is, once the effekt command is in your path). For Windows users, you might need to double check the path (it defaults to effekt.cmd, which can be found in %APPDATA%/Roaming/npm). Linux users should point VSCode to the shell script, located in the npm install path.

You might need to restart VSCode after changing the settings.

With this setup the extension should start the server when an Effekt file is opened.

Workspace Folder

For the language services to work, you need to open the project’s root-folder in VSCode. In the case of this artifact, the supplementary material folder is the project-folder. Similarly, we recommend to start the effekt command ( on Linux) in a terminal in the project’s root folder.

Effekt on Windows

Generally, we recommend using cmd at the moment. If you want to use Effekt with Microsoft’s PowerShell, there is a few things you need to look out for.

  1. The Effekt executable is unsigned at the moment. So running it in PowerShell raises an error. You can work around this issue by disabling the check for the current sessions. Please be aware that this is a rather drastic work around and should be used with care.
    Set-ExecutionPolicy -Scope Process -ExecutionPolicy Bypass

    This only works for the current session and needs to be repeated every time you start a new terminal.

  2. Ansi colors are not enabled out of the box. The easiest way is to set VirtualTerminalLevel in the registry to true. As always, be careful when tempering with the registry and only do it when you know what you are doing!

Effekt on Linux

Instead of using the effekt command, on linux for now the shell-script has to be used. It is installed alongside effekt using npm install ....

  1. Open the settings of VSCode
  2. Search for effekt
  3. Edit the Effekt: Executable property
  4. Enter the path to, e.g. /usr/bin/
  5. Restart VSCode