Before we begin, make sure you have the following installed:

• JDK 1.8+
• SBT 1.2+

The source code for this chapter is available under our GitHub repository at: https://github.com/PacktPublishing/Learn-Scala-Programming/tree/master/Chapter02.

The type of something is a summation of the information the compiler owns about this something. In the most general case, we're talking about the type of a variable; the knowledge of the compiler includes the methods that are available on this variable and the classes that the variable extends. A very convenient feature of Scala is that it tries to use type inference where possible, freeing the developer from the need to define types explicitly.

Let's take a structured look at Scala's type system, starting with a short recap of its basics.

There are four ways to define a type in Scala:

• By using a literal to define a singleton type
• By using the type keyword to define...

Up until now, we have only talked about concrete types. Despite being quite simple, they already allow for the expression of a lot of properties of a program on the type level and they have these properties verified at compile time. Scala gives the developer even more freedom by allowing them to use types as parameters while defining methods, classes, or other types. In the next section, we will look at different ways to do this, starting with basic type parameters and type member definition, and continuing with type constraints and variance topics. We'll conclude our discussion with higher kinded types and type lambdas.

Type parameters are defined using square brackets...

We've already seen how simple types can be used to express domain constraints, as discussed in the Path-dependent types section. We implemented a lock that guaranteed at compile time that it is only possible to open and close it with the key created for this specific lock. We will conclude our study of type parameters and higher kinded types with two examples.

The first example will demonstrate an application of phantom types to create another version of the lock, which can guarantee the safety of state transitions at compile time without the use of inheritance.

The second example will show how self-recursive types can help to constrain possible subtyping.

The phantom...

The type system is one of the key components of the Scala language. It allows the developer to express expectations about the behavior of the program, which can then be checked at compile time. This reduces the number of tests needed to verify the correctness of the solution and the possibility of runtime errors.

Usually, strictly typed languages are associated with verbose code. Normally, this is not the case with Scala because of its powerful type inference mechanism.

Scala allows you to define very narrow types containing a single value as well as much wider types, even those represented as a combination of other types.

The type definition can be made more precise by using type constraints, type parameters, and variance.

We also looked at some examples of how the type system can be used to express domain constraints.

Needless to say, Scala's ecosystem is much richer...

1. Which type constraints can you name?
2. What implicit type constraints are added to a type if there are no type constraints defined on it by the developer?
3. Which operators can be used to refer to the nested type of some type?
4. Which type can be used as an infix type?
5. Why is the use of structural types discouraged in Scala?
6. What is expressed via variance?

• Mads Hartmann, and Ruslan Shevchenko, Professional Scala: You will learn how to write type-safe code concisely and expressively in an environment that lets you build for the JVM, browser, and more.

• Vikash Sharma, Learning Scala Programming: Learn how to write scalable and concurrent programs in Scala, a language that grows with you.