Scala is a high-level programming language that combines object-oriented and functional programming paradigms. It runs on the Java Virtual Machine (JVM) and is designed to be concise, elegant, and type-safe.

In Scala, val is used to declare immutable variables, meaning their values cannot be changed once assigned. var is used to declare mutable variables, allowing their values to be reassigned.

For-comprehension in Scala is a syntactic sugar for chaining operations on monads such as Option, Either, and collections. It makes code more readable and expressive.

A trait in Scala is a unit of code reuse that encapsulates method and field definitions. Traits can be mixed into classes to add functionality, similar to interfaces in Java but more powerful.

Scala supports object-oriented programming with features like:

• Classes and objects
• Inheritance
• Polymorphism
• Encapsulation

Yes, Scala is widely used in big data and data engineering, especially with tools like Apache Spark.

Key features of Scala include:

• Functional programming support
• Object-oriented design
• Immutable collections
• Pattern matching
• Type inference
• Concurrency support

Scala runs on the JVM, just like Java, but offers more concise syntax and advanced features like pattern matching, type inference, and functional programming.

sbt (Simple Build Tool) is the de facto build tool for Scala. It is used for compiling code, running tests, managing dependencies, and packaging applications.

Scala was developed by Martin Odersky and first released in 2003. It was created to address some of the limitations of Java while providing a more concise and expressive syntax.

The JVM (Java Virtual Machine) is a virtual machine that runs Java bytecode. Scala is compiled to JVM bytecode, allowing it to run on any platform that supports the JVM and to interoperate with Java code and libraries.

A stream in Scala is a lazy list, meaning its elements are evaluated only when needed. This allows for the creation of infinite data structures and can improve performance by deferring computation.

Scala handles concurrency using libraries like Akka, which provides an actor-based model for building concurrent and distributed applications. This model simplifies the development of concurrent systems by avoiding shared mutable state.

Scala supports functional programming with features like:

• First-class functions: Functions can be assigned to variables, passed as parameters, or returned as results.
• Higher-order functions: Functions that take other functions as parameters or return them as results.
• Immutability: By default, Scala encourages the use of immutable variables (val), aligning with the functional programming principle of immutability.

Immutable collections in Scala are collections whose elements cannot be modified after they are created. This aligns with the functional programming principle of immutability and helps in writing safer and more predictable code.

Pattern matching in Scala is a powerful feature that allows you to match against values, types, and structures in a readable and expressive way. It is similar to a switch statement in other languages but more powerful.

In Scala, List is an immutable linked list, while Vector is an immutable indexed sequence. Vector provides faster random access and is generally more efficient for large collections.

Case classes in Scala are used to model immutable data. They automatically provide implementations for methods like equals, hashCode, and toString, and are commonly used in pattern matching.

Type inference in Scala allows the compiler to automatically deduce the type of a variable or expression, reducing the need for explicit type annotations and making the code more concise.

The Scala REPL (Read-Eval-Print Loop) is an interactive shell that allows you to evaluate Scala expressions and see the results immediately. It is useful for experimenting with code and learning the language.

A companion object in Scala is an object that is defined in the same file as a class and has the same name. It can access the private members of the class and is often used to define factory methods.

Tail recursion in Scala refers to a special kind of recursion where the recursive call is the last operation in the function. The Scala compiler can optimize tail-recursive functions to avoid stack overflow errors.

The App trait in Scala is a convenient way to create executable programs. By extending the App trait, the body of the object is automatically executed, eliminating the need for a main method.

Higher-order functions in Scala are functions that take other functions as parameters or return them as results. They are a key feature of functional programming and enable powerful abstractions.

A case class in Scala is a special type of class designed to simplify the creation of immutable data structures. They come with built-in functionalities like automatic implementation of equals, hashCode, and toString methods, making them ideal for pattern matching.