| Paradigm(s) | Multi-paradigm: functional, object-oriented, imperative |
|---|---|
| Appeared in | 2003 |
| Designed by | Martin Odersky |
| Developer | Programming Methods Laboratory of EPFL |
| Stable release | 2.9.1 (August 29, 2011[1]) |
| Typing discipline | static, strong, inferred, structural |
| Influenced by | Java, Pizza,[2] Haskell, Erlang, Standard ML, Objective Caml, Smalltalk, Scheme |
| Influenced | Fantom, Ceylon |
| Platform | JVM, CLR |
| License | BSD |
| Website | www.scala-lang.org |
| Scala at Wikibooks | |
Scala ( /ˈskɑːlə/ skah-lə) is a multi-paradigm programming language designed to integrate features of object-oriented programming and functional programming.[2] The name Scala is a portmanteau of "scalable" and "language", signifying that it is designed to grow with the demands of its users. James Strachan, the creator of Groovy, described Scala as a possible successor to Java.[3]
Contents |
The design of Scala started in 2001 at the École Polytechnique Fédérale de Lausanne (EPFL) by Martin Odersky, following on from work on Funnel, a programming language combining ideas from functional programming and Petri nets.[4] Odersky had previously worked on Generic Java and javac, Sun's Java compiler.[4]
Scala was released late 2003 / early 2004 on the Java platform, and on the .NET platform in June 2004.[2][4][5] A second version of the language, v2.0, was released in March 2006.[2]
On 17 January 2011 the Scala team won a 5 year research grant of over €2.3 million from the European Research Council.[6] On 12 May 2011, Odersky and collaborators launched Typesafe, a company to provide commercial support, training, and services for Scala. Typesafe received $3 million investment from Greylock Partners.[7][8][9][10]
Scala runs on the Java platform (Java Virtual Machine) and is compatible with existing Java programs. It also runs on Android smartphones.[11] An alternative implementation exists for the .NET platform.[12][13]
Scala has the same compilation model as Java and C# (separate compilation, dynamic class loading), so Scala code can call Java libraries (or .NET libraries in the .NET implementation).
Scala's operational characteristics are the same as Java's. The Scala compiler generates byte code that is nearly identical to that generated by the Java compiler. In fact, Scala code can be decompiled to readable Java code, with the exception of certain constructor operations. To the JVM, Scala code and Java code are indistinguishable. The only difference is a single extra runtime library, scala-library.jar.[14]
The Scala software distribution, including compiler and libraries, is released under a BSD license.[15]
Lift is a free web application framework that aims to deliver benefits similar to Ruby on Rails. The use of Scala means that any existing Java library and Web container can be used in running Lift applications.
In April 2009 Twitter announced they had switched large portions of their backend from Ruby to Scala and intended to convert the rest.[16] In addition, Foursquare uses Scala and Lift.[17]
GridGain provides Scala-based DSL for cloud computing.[18]
In April 2011, The Guardian newspaper's website guardian.co.uk announced that it was switching from Java to Scala,[19] starting with the Content API for selecting and collecting news content.[20] The website is one of the highest-traffic English-language news websites, and according to its editor has the second largest online readership of any English-language newspaper in the world, after the New York Times.[21]
Swiss bank UBS approved Scala for general production usage.[22]
Scala supports functional programming. The language provides a lightweight syntax for defining anonymous functions, supports higher-order functions, allows functions to be nested, and supports currying. Using the keyword lazy defers the initialization of a value until this value is used. Delimited continuations are supported since version 2.8.
Scala has case classes and built-in support for pattern matching. These features can be used to model the algebraic data types used in many functional programming languages.
Tail call optimization is not supported completely, because the JVM lacks tail call support. In simple cases, the Scala compiler can optimize tail calls into loops.[23]
An implementation of a sorting algorithm (similar to quicksort) in functional style:
def qsort: List[Int] => List[Int] = { case Nil => Nil case pivot :: tail => val (smaller, rest) = tail.partition(_ < pivot) qsort(smaller) ::: pivot :: qsort(rest) }
Scala is a pure object-oriented language in the sense that every value is an object. Data types and behaviors of objects are described by classes and traits. Class abstractions are extended by subclassing and by a flexible mixin-based composition mechanism to avoid the problems of multiple inheritance.
Scala is equipped with an expressive static type system that enforces the safe and coherent use of abstractions. In particular, the type system supports:
Scala is able to infer types by usage. This makes most static type declarations optional. Static types need not be explicitly declared unless a compiler error indicates the need. In practice, some static type declarations are included for the sake of code clarity. Lack of explicit type declarations gives Scala the appearance of a dynamically typed language.
The design of Scala acknowledges the fact that, in practice, the development of domain-specific applications often requires domain-specific language extensions. Scala provides a novel combination of language mechanisms that make it easy to smoothly add new language constructs in the form of libraries:
A joint use of both features facilitates the definition of new statements without extending the syntax and without using macro-like meta-programming facilities.
Scala has in its standard library support for the actor model, in addition to the standard Java concurrency APIs. An alternative CSP implementation for channel-based message passing is Communicating Scala Objects.[24]
Here is the classic Hello world program written in Scala:
object HelloWorld extends App { println("Hello, world!") }
Unlike the stand-alone Hello World application for Java, there is no class declaration and nothing is declared to be static; a singleton object created with the object keyword is used instead.
With the program saved in a file named HelloWorld.scala, it can be compiled from the command line:
$ scalac HelloWorld.scala
To run it:
$ scala -classpath . HelloWorld
This is analogous to the process for compiling and running Java code. Indeed, Scala's compilation and execution model is identical to that of Java, making it compatible with Java build tools such as Ant.
A shorter version of the "Hello world" Scala program is:
println("Hello, world!")
Saved in a file named HelloWorld2.scala, this can be run as a script without prior compilation using:
$ scala HelloWorld2.scala
Commands can also be fed directly into the Scala interpreter, using the option -e, and escaping the quotes using \:
$ scala -e "println(\"Hello, World!\")"
There are several ways to test code in Scala: