Creating a Scala REPL

Written by Tushar Mathur

The best way to learn a new language is to create a simple file for that programming language and keep running it in watch mode. I personally use watch and VSCode when I want to quickly prototype something. watch helps me run that code continuously as I type and provides me with instant feedback on the terminal. I have found this technique really helpful in the past specially while learning something new.

Here are the steps to setup a basic scala REPL on mac.

Create a .scala File

Create a file named main.scala

// main.scala

object Main extends App {
  println("Customary Hello World!")
}

Install Watch

watch is a great OSX utility that keeps running a command every few seconds.

brew install watch

Watch & Run

Setup the watch command to keep running your code in the background 

Execute:

watch scala main.scala

Compile Only:

watch scalac main.scala

Using SBT

[SBT] provides a an alternative way to watch and run scala code.

sbt ~run

This way the main.scala is only executed when the file actually changes. The disadvantage of this approach is that you can not pass a random file path.

Also checkout repl.it if you don’t want to setup all this stuff locally. Unfortunately at the time of writing this blog, scala wasn’t supported.