The Scala REPL is a powerful feature that makes it more straightforward and concise to write Scala code on the Scala shell. REPL stands for Read-Eval-Print-Loop also called the Interactive Interpreter. This means it is a program for:

1. Reading the expressions you type in.
2. Evaluating the expression in step 1 using the Scala compiler.
3. Printing out the result of the evaluation in step 2.
4. Waiting (looping) for you to enter further expressions.

From the figure, it is evident that there is no magic, the variables are inferred automatically to the best types they deem fit at compile time. If you look even more carefully, when I tried to declare:

 i:Int = "hello"

Then, the Scala shell throws an error saying the following:

<console>:11: error: type mismatch;
  found   : String("hello")
  required: Int
        val i:Int = "hello"
                    ^

According to Odersky, "Mapping a character to the character map over a RichString should again yield a RichString, as in the following interaction with the Scala REP". The preceding statement can be proved using the following line of code:

scala> "abc" map (x => (x + 1).toChar) 
res0: String = bcd

However, if someone applies a method from Char to Int to a String, then what happens? In that case, Scala converts them, as a vector of integer also called immutable is a feature of Scala collection, as shown in Figure 9. We will look at the details on Scala collection API in Chapter 4, Collections APIs.

"abc" map (x => (x + 1)) 
res1: scala.collection.immutable.IndexedSeq[Int] = Vector(98, 99, 100)

Both static and instance methods of objects are also available. For example, if you declare x as a string hello and then try to access both the static and instance methods of objects x, they are available. In the Scala shell, type x then . and <tab> and then you will find the available methods:

scala> val x = "hello"
x: java.lang.String = hello
scala> x.re<tab>
reduce             reduceRight         replaceAll            reverse
reduceLeft         reduceRightOption   replaceAllLiterally   reverseIterator
reduceLeftOption   regionMatches       replaceFirst          reverseMap
reduceOption       replace             repr
scala> 

Since this is all accomplished on the fly via reflection, even anonymous classes you've only just defined are equally accessible:

scala> val x = new AnyRef{def helloWord = "Hello, world!"}
x: AnyRef{def helloWord: String} = $anon$1@58065f0c
 scala> x.helloWord
    def helloWord: String
 scala> x.helloWord
 warning: there was one feature warning; re-run with -feature for details
 res0: String = Hello, world!

The preceding two examples can be shown on the Scala shell, as follows:

- Odersky