Groovy

Groovy comes with a compiler called groovyc. For each script, groovyc generates a class that extends groovy.lang.Script, which contains a main method so that Java can execute it. The name of the compiled class matches the name of the script being compiled.

For example, with this HelloWorld.groovy script:

println "Hello World"

That becomes something like this code:

class HelloWorld extends Script {
    public static void main(String[] args) {
        println "Hello World"
    }
}

Scala

Scala comes with a compiler called scalac. I don't know how it works.

For example, with the same HelloWorld.scala script:

println("Hello World")

The code is not valid for scalac, because the compiler expected class or object definition, but works in Scala REPL interpreter. How is possible? Is it wrapped in a class before execution?