Scala is a general-purpose programming language principally targeting the Java Virtual Machine. Designed to express common programming patterns in a concise, elegant, and type-safe way, it fuses both imperative-programming and functional-programming styles. Its key features are:

• Static typing
• Advanced type system with type inference and declaration-site variance
• Function types (including anonymous) which support lexical closures
• Pattern-matching
• Implicit parameters and conversions which support the typeclass and enrich-my-library patterns
• Mixin composition
• Full interoperability with Java
• Powerful concurrency constructs
• Advanced language constructs such as delimited continuations and an experimental macro system

For more information, see the official Scala Introduction and Scala Documentation.

To search for Scala symbols such as => in Stack Overflow, you can use symbolhound search.

To search Scala documentation, you can use scalex.

A community list of available libraries is available at Awesome Scala

There is Scala Library Index by Scala Center: Scaladex.

Free Scala programming books and guides

• Programming in Scala, First Edition
• Scala By Example (PDF)
• A Scala Tutorial for Java programmers
• Scala for Java refugees
• Scala School
• Scala Tutorials
• Scala Tour
• Scala for the Impatient (first part available for free at http://typesafe.com/resources/free-books)
• Coursera Course - Functional Programming in Scala
• Scala Exercises
• Underscore books
• The Neophyte's Guide to Scala

Stack Overflow Scala Tutorial

0. Introduction to Scala

• What's so great about Scala?

0. Variables/values

• Difference between val and var
• val-mutable versus var-immutable
• How to assign vals correctly?
• Multiple variable declarations
• Lazy vals
  • What do they do?
  • How are they implemented?
• Identifier rules

0. Methods

• Equal sign in methods
• Operator notation and the rules
• Multiple ways to define a method/function (leave parentheses in method calls)
• Unary operators
• Multiple parameter lists
• Right-associative method names
• Side effect free methods
  • Missing parentheses at methods
  • Why no i++ in Scala?
• How to mix punctuation with alphanumeric characters in method names?
• Shortcut methods (+=, -=, *=, ...)
• List of "magic method" names (apply, unapply/unapplySeq, update)
• Named arguments / optional parameters
• Type inference in return type
• Difference between ## and hashCode

0. Literals, statements and blocks

• Difference between block and statement
• Weak conformance
• Symbols
• Difference between braces and parentheses
• Dereference keyword (backtick usage)
• Local definitions
• Uses of an underscore
• Identifier rules
• Punctuation (operator) overview

0. Loops/recursion

• Loop
  • Simple loop
  • Break out of a loop
  • How to call a function x times
• Tail recursion
  • What is tail recursion?
  • How to write tail recursive code?
  • Performance
  • Optimization rules
• Trampolining

0. Data structures / Collections

• Collections design tutorial
• Collection standard practice
• Immutable Collections
  • List
  • Vector
  • Range
  • Tuple
  • Map
  • Set
• Mutable Collections
  • Array and multi-dimensional Arrays
  • Buffer
  • How to use mutable Collections?
  • Mutating a mutable collection using map?
• Lazy Collections
  • Why are they needed?
  • View
  • Iterator
  • Stream
  • Streams vs Views vs Iterators
  • Return a lazy collection
• Parallel Collections
  • How do they work?
  • Is it possible to change their behavior?
• Conversions
  • Casting and ascription
  • Java Collection to Scala Collection and vice versa
  • Convert to another Collections
  • breakout
• Memory footprint
• How are Scala collections able to return the correct collection type from an operation?

0. For-comprehension

• Explanation
• nested for-expressions
• Use for inside expressions

0. Enumeration

• Explanation

0. Pattern matching

• Explanation
• Value binding (x @ X) / type binding (x: X)
• How to do a multi-match
• Guards
• How to match variables or values?
• How is pattern match implemented under the hood?
• Exhaustive pattern
• Pattern match in for-expressions
• Ignore cases / no default value
• Conjunction
• Pattern match PartialFunctions
• Match Regex
• Pattern matching with more than one match

0. Classes, objects and types

• Difference between class and object
• Why is object a Singleton?
• Why are singleton objects more object-oriented?
• Companion objects
• Difference between class and type
• What's the difference between a class with a companion object and a class and object with the same name?
• Static initializer
• Constructor overloading
• Static data in non-objects
• How to get the static/runtime type of a class
• Type projection (A#B)

0. Packages, imports and visibility identifiers

• Imports
  • Multiple imports / import renames / hide imports
  • Shorthand imports
  • Local imports
• Packages
  • Nested packages / multiple package definitions
  • Package objects
• Visibility
  • Explanation
  • Private constructors
  • Private variables

0. Inheritance

• Explanation
• Early initializer

0. Extractors

• Explanation (Example: conjunctions)
• Infix notation for type parameters (X[A, B] => A X B)

0. Case classes

• Explanation
• case classes vs. enums
• What are the disadvantages to declaring Scala case classes
• Compiler generated code for case classes

0. Parameterized types

• How to declare them?
• Upper / lower bounds
• Unify numerics
• How to get around type erasure?
• Abstract Types vs Generics
• Variances

0. Traits

• Usage
• Mixin multiple traits
• Traits vs abstract classes
• What does trait A <: B mean?
• Linearization
• Mixin a trait with overridden behavior
• Dynamic mixins
• How are they implemented under the hood?
• How to access one of multiple traits of superclass?

0. Self references

• Unify types
• Self reference naming
• Multiple self references
• Difference between self references and subtyping

0. Error handling

• Exceptions
  • Explanation
  • Stand-alone try block
  • Ignore an exception
  • Try-catch-generalization
• Option
  • Explanation
• Either
  • Explanation
  • How to use them?
• What to use?

0. Type handling

• Context ([A : B]) and view bounds ([A <% B])
• Types
  • Non-Nullable type
  • Existential type
  • Structural type ("duck typing")
  • Path-dependent type
  • Union types (type disjunction)
  • Monad type
  • Compound type in structural type
  • Compound type in self references
  • Compound type in parameterized type
• Advantages of Scala's type system

0. Annotations

• What are they?
• How to use them?

0. Functions/Function literals

• Explanation
• Functions vs methods
• Pass functions
• Currying
• PartialFunction
• Placeholder syntax and their replacement rules
• Placeholder syntax with right-associative methods
• Difference between => Type, () => Type and Unit => Type
• Functional Quicksort
• Keyword return in higher order functions and performance problem with that
• Function composition
• Closures in Scala

0. Type safety

• Manifest
  • Explanation
  • How do they work?
  • Are Manifests possible in constructors?
• Generalized Type Constraints (<:<, <%<, =:=)
• Path-Dependent Types
• Enforce type difference
• Enforce no type bounds

0. Implicits

• Implicit definitions / parameters
• Implicit conversions / Monkey patching
• Identifier implicitly
• Where does Scala look for implicits?
• Should implicits used?
• Style of implicits
• Convert multiple values with one implicit conversion
• Chaining implicits

0. Reflection

• What is a TypeTag and how do I use it?
• How to work with Reflection?

0. Enrich-my-library pattern (formerly known as pimp-my-library)

• Explanation
• CanBuildFrom
• Returning original types while extending the Collection API

0. Concurrency overview
1. Actors

• Differences between Actor implementations
• Design Patterns
• Worst practices

0. Use Java from Scala and vice versa

• Iterate over Java collection
• Migrate Java to Scala
• Transparent conversions useful?
• Using Scala traits with implemented methods in Java
• How to work with javap for Scala/Java interop?
• Scala signatures
• How does abstract override work internally?

0. XML literals

• Explanation

0. Scala Swing

• Explanation
• Examples

0. Type Programming

• Overview

0. Functional Scala

• Is immutability expensive?
• Is Scala functional programming slower than traditional coding?
• OOP in a purely FP context?
• Continuations
  • Explanation
  • How to activate them?
• Design patterns for functional-oo hybrid languages?
• Type programming
• Higher kinded types
• Type lambdas (SomeType[({type λ[α] = Either[A, α]})#λ])
• Virtual Classes
• forall in Scala
• Understanding for comprehension in Scala

Further learning

0. Learning Resources

• How can I learn more about Scala's type relationships?
• Scala Cheat Sheet
• Good examples of idiomatic scala code?
• Solidifying Scala knowledge

0. REPL

• How does it work behind the scenes?

0. Working with scalac and scala

• Show syntax trees
• Meaning of !# (bang-pound) in a shell script?

0. Operator precedence

• Specification rules
• Actual scalac precedence implementation
• Understanding precedence rules in practice

0. Scala style

• Scala Style Guide
• Elements of Scala Style
• Effective Scala
• Strategic Scala Style

0. Functional Programming Principles in Scala, a functional programming course on Coursera taught by Martin Odersky, the creator of Scala.
1. Principles of Reactive Programming, a reactive functional programming course on Coursera taught by Martin Odersky, Erik Meijer, Roland Kuhn.
2. Parallel Programming, a parallel programming course on Coursera taught by Viktor Kuncak and Aleksandar Prokopec.