Read Eval Print and Loop - REPL
Read Eval Print and Loop (REPL) is also know as a language shell. Many other languages have shells, like Lisp, Python, and Ruby for instance. The REPL is a simple environment to experiment the language in. It's possible to write very complex programs using REPL, but this is not the REPL goal. Using REPL does not invalidate the usage of an IDE like Eclipse or IntelliJ IDEA. REPL is ideal for testing simple commands and programs without having to spend much time configuring projects like you do with an IDE. The Scala REPL allows you to create a variable, functions, classes, and complex functions as well. There is a history of every command you perform; there is some level of autocomplete too. As a REPL user, you can print variable values and call functions.
Scala Hello World using the REPL
Let's get started. Go ahead, open your terminal, and type $ scala in order to open the Scala REPL. Once the REPL is open, you can just type "Hello World". By doing this, you perform two operations: eval and print. The Scala REPL will create a variable called res0, and store your String there. Then it will print the content of the res0 variable.