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Scala (programming language)

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</style><div role="note" class="hatnote navigation-not-searchable">For the music scale creation software, see <a href="Scala_(software)" title="Scala (software)">Scala (software)</a>.</div>

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</style><table class="infobox vevent"><tbody><tr><th colspan="2" class="infobox-above" style="background-color:#e0e0e0;">Scala</th></tr><tr><td colspan="2" class="infobox-image"></td></tr><tr><th scope="row" class="infobox-label"><a href="Programming_paradigm" title="Programming paradigm">Paradigm</a></th><td class="infobox-data"><a href="Multi-paradigm" class="mw-redirect" title="Multi-paradigm">Multi-paradigm</a>: <a href="Concurrent_programming" class="mw-redirect" title="Concurrent programming">concurrent</a>, <a href="Functional_programming" title="Functional programming">functional</a>, <a href="Imperative_programming" title="Imperative programming">imperative</a>, <a href="Object-oriented_programming" title="Object-oriented programming">object-oriented</a></td></tr><tr><th scope="row" class="infobox-label"><a href="Software_design" title="Software design">Designed by</a></th><td class="infobox-data"><a href="Martin_Odersky" title="Martin Odersky">Martin Odersky</a></td></tr><tr><th scope="row" class="infobox-label"><a href="Software_developer" class="mw-redirect" title="Software developer">Developer</a></th><td class="infobox-data organiser">Programming Methods Laboratory of <a href="%C3%89cole_Polytechnique_F%C3%A9d%C3%A9rale_de_Lausanne" title="École Polytechnique Fédérale de Lausanne">École Polytechnique Fédérale de Lausanne</a></td></tr><tr><th scope="row" class="infobox-label">First appeared</th><td class="infobox-data">20 January 2004 (2004-01-20)</td></tr><tr><td colspan="2" class="infobox-full-data"></td></tr><tr><th scope="row" class="infobox-label" style="white-space: nowrap;"><a href="Software_release_life_cycle" title="Software release life cycle">Stable release</a></th><td class="infobox-data"><div style="margin:0px;">3.7.1<a href="#cite_note-wikidata-727b6dee3f4e54ac7cbc00f60f3f1f04fe9224c5-v20-1">[1]</a>

/ 4 June 2025 (4 June 2025)</div></td></tr><tr style="display:none"><td colspan="2">

</td></tr><tr><th scope="row" class="infobox-label"><a href="Type_system" title="Type system">Typing discipline</a></th><td class="infobox-data"><a href="Type_inference" title="Type inference">Inferred</a>, <a href="Static_typing" class="mw-redirect" title="Static typing">static</a>, <a href="Strong_and_weak_typing" title="Strong and weak typing">strong</a>, <a href="Structural_type_system" title="Structural type system">structural</a></td></tr><tr><th scope="row" class="infobox-label">Implementation language</th><td class="infobox-data">Scala</td></tr><tr><th scope="row" class="infobox-label"><a href="Computing_platform" title="Computing platform">Platform</a></th><td class="infobox-data"><a href="Java_virtual_machine" title="Java virtual machine">Java virtual machine</a> (JVM) <a href="JavaScript" title="JavaScript">JavaScript</a> (<a rel="nofollow" class="external text" href="https://scala-js.org">Scala.js</a>) <a href="LLVM" title="LLVM">LLVM</a> (<a rel="nofollow" class="external text" href="http://scala-native.org">Scala Native</a>) (experimental)</td></tr><tr><th scope="row" class="infobox-label"><a href="Software_license" title="Software license">License</a></th><td class="infobox-data"><a href="Apache_License" title="Apache License">Apache</a> 2.0<a href="#cite_note-license-github-2">[2]</a></td></tr><tr><th scope="row" class="infobox-label"><a href="Filename_extension" title="Filename extension">Filename extensions</a></th><td class="infobox-data">.scala, .sc</td></tr><tr><th scope="row" class="infobox-label">Website</th><td class="infobox-data"><a rel="nofollow" class="external text" href="http://www.scala-lang.org">www.scala-lang.org</a></td></tr><tr><th colspan="2" class="infobox-header" style="background-color: #EEEEEE;">Influenced by</th></tr><tr><td colspan="2" class="infobox-full-data"><a href="Common_Lisp" title="Common Lisp">Common Lisp</a>,<a href="#cite_note-Scala_Macros-3">[3]</a> <a href="Eiffel_(programming_language)" title="Eiffel (programming language)">Eiffel</a>, <a href="Erlang_(programming_language)" title="Erlang (programming language)">Erlang</a>, <a href="Haskell" title="Haskell">Haskell</a>,<a href="#cite_note-4">[4]</a> <a href="Java_(programming_language)" title="Java (programming language)">Java</a>,<a href="#cite_note-popl06-5">[5]</a> <a href="OCaml" title="OCaml">OCaml</a>,<a href="#cite_note-popl06-5">[5]</a> <a href="Oz_(programming_language)" title="Oz (programming language)">Oz</a>, <a href="Pizza_(programming_language)" title="Pizza (programming language)">Pizza</a>,<a href="#cite_note-overview-6">[6]</a> <a href="Scheme_(programming_language)" title="Scheme (programming language)">Scheme</a>,<a href="#cite_note-popl06-5">[5]</a> <a href="Smalltalk" title="Smalltalk">Smalltalk</a>, <a href="Standard_ML" title="Standard ML">Standard ML</a><a href="#cite_note-popl06-5">[5]</a></td></tr><tr><th colspan="2" class="infobox-header" style="background-color: #EEEEEE;">Influenced</th></tr><tr><td colspan="2" class="infobox-full-data"><a href="Ceylon_(programming_language)" title="Ceylon (programming language)">Ceylon</a>, <a href="Chisel_(programming_language)" title="Chisel (programming language)">Chisel</a>, <a href="Fantom_(programming_language)" title="Fantom (programming language)">Fantom</a>, <a href="F_Sharp_(programming_language)" title="F Sharp (programming language)">F#</a>, <a href="C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a>, <a href="Kotlin_(programming_language)" title="Kotlin (programming language)">Kotlin</a>, <a href="Lasso_(programming_language)" title="Lasso (programming language)">Lasso</a>, <a href="Red_(programming_language)" title="Red (programming language)">Red</a>, <a href="Flix_(programming_language)" title="Flix (programming language)">Flix</a></td></tr><tr><td colspan="2" class="infobox-below hlist" style="border-top: 1px solid #aaa; padding-top: 3px;">

<ul><li> <a href="https://en.wikibooks.org/wiki/Scala" class="extiw external" title="wikibooks:Scala">Scala</a> at Wikibooks</li></ul>

</td></tr></tbody></table>

Scala (/ˈskɑːlɑː/ SKAH-lah)<a href="#cite_note-7">[7]</a><a href="#cite_note-8">[8]</a> is a <a href="Strong_and_weak_typing" title="Strong and weak typing">strongly</a> <a href="Statically_typed" class="mw-redirect" title="Statically typed">statically typed</a> <a href="High-level_programming_language" title="High-level programming language">high-level</a> <a href="General-purpose_programming_language" title="General-purpose programming language">general-purpose programming language</a> that supports both <a href="Object-oriented_programming" title="Object-oriented programming">object-oriented programming</a> and <a href="Functional_programming" title="Functional programming">functional programming</a>. Designed to be concise,<a href="#cite_note-9">[9]</a> many of Scala's design decisions are intended to address <a href="Criticism_of_Java" title="Criticism of Java">criticisms of Java</a>.<a href="#cite_note-overview-6">[6]</a>

Scala source code can be compiled to <a href="Java_bytecode" title="Java bytecode">Java bytecode</a> and run on a <a href="Java_virtual_machine" title="Java virtual machine">Java virtual machine</a> (JVM). Scala can also be transpiled to <a href="JavaScript" title="JavaScript">JavaScript</a> to run in a browser, or compiled directly to a native executable. When running on the JVM, Scala provides <a href="Language_interoperability" title="Language interoperability">language interoperability</a> with <a href="Java_(programming_language)" title="Java (programming language)">Java</a> so that libraries written in either language may be referenced directly in Scala or Java code.<a href="#cite_note-10">[10]</a> Like Java, Scala is <a href="Object-oriented_programming" title="Object-oriented programming">object-oriented</a>, and uses a <a href="Syntax_(programming_languages)" title="Syntax (programming languages)">syntax</a> termed <a href="List_of_programming_languages_by_type#Curly_bracket_languages" title="List of programming languages by type">curly-brace</a> which is similar to the language <a href="C_(programming_language)" title="C (programming language)">C</a>. Since Scala 3, there is also an option to use the <a href="Off-side_rule" title="Off-side rule">off-side rule</a> (indenting) to structure <a href="Block_(programming)" title="Block (programming)">blocks</a>, and its use is advised. <a href="Martin_Odersky" title="Martin Odersky">Martin Odersky</a> has said that this turned out to be the most productive change introduced in Scala 3.<a href="#cite_note-11">[11]</a>

Unlike Java, Scala has many features of <a href="Functional_programming" title="Functional programming">functional programming</a> languages (like <a href="Scheme_(programming_language)" title="Scheme (programming language)">Scheme</a>, <a href="Standard_ML" title="Standard ML">Standard ML</a>, and <a href="Haskell" title="Haskell">Haskell</a>), including <a href="Currying" title="Currying">currying</a>, <a href="Immutability" class="mw-redirect" title="Immutability">immutability</a>, <a href="Lazy_evaluation" title="Lazy evaluation">lazy evaluation</a>, and <a href="Pattern_matching" title="Pattern matching">pattern matching</a>. It also has an advanced type system supporting <a href="Algebraic_data_type" title="Algebraic data type">algebraic data types</a>, <a href="Covariance_and_contravariance_(computer_science)" title="Covariance and contravariance (computer science)">covariance and contravariance</a>, <a href="Higher-order_type_operator" class="mw-redirect" title="Higher-order type operator">higher-order types</a> (but not <a href="Parametric_polymorphism" title="Parametric polymorphism">higher-rank types</a>), <a href="Anonymous_type" title="Anonymous type">anonymous types</a>, <a href="Operator_overloading" title="Operator overloading">operator overloading</a>, <a href="Option_type" title="Option type">optional parameters</a>, <a href="Named_parameter" title="Named parameter">named parameters</a>, <a href="Raw_string" class="mw-redirect" title="Raw string">raw strings</a>, and an experimental exception-only version of algebraic effects that can be seen as a more powerful version of Java's <a href="Checked_exception" class="mw-redirect" title="Checked exception">checked exceptions</a>.<a href="#cite_note-12">[12]</a>

The name Scala is a portmanteau of scalable and language, signifying that it is designed to grow with the demands of its users.<a href="#cite_note-StepsInScala-13">[13]</a>

<div class="mw-heading mw-heading2"><h2 id="History">History</h2></div>

The design of Scala started in 2001 at the <a href="%C3%89cole_Polytechnique_F%C3%A9d%C3%A9rale_de_Lausanne" title="École Polytechnique Fédérale de Lausanne">École Polytechnique Fédérale de Lausanne</a> (EPFL) (in <a href="Lausanne" title="Lausanne">Lausanne</a>, <a href="Switzerland" title="Switzerland">Switzerland</a>) by <a href="Martin_Odersky" title="Martin Odersky">Martin Odersky</a>. It followed on from work on Funnel, a programming language combining ideas from functional programming and <a href="Petri_net" title="Petri net">Petri nets</a>.<a href="#cite_note-history-of-scala-14">[14]</a> Odersky formerly worked on <a href="Generic_Java" class="mw-redirect" title="Generic Java">Generic Java</a>, and <a href="Javac" title="Javac">javac</a>, Sun's Java compiler.<a href="#cite_note-history-of-scala-14">[14]</a>

After an internal release in late 2003, Scala was released publicly in early 2004 on the <a href="Java_platform" class="mw-redirect" title="Java platform">Java platform</a>,<a href="#cite_note-cacm-15">[15]</a><a href="#cite_note-overview-6">[6]</a><a href="#cite_note-history-of-scala-14">[14]</a><a href="#cite_note-spec-16">[16]</a> A second version (v2.0) followed in March 2006.<a href="#cite_note-overview-6">[6]</a>

On 17 January 2011, the Scala team won a five-year research grant of over €2.3 million from the <a href="European_Research_Council" title="European Research Council">European Research Council</a>.<a href="#cite_note-17">[17]</a> On 12 May 2011, Odersky and collaborators launched Typesafe Inc. (later renamed <a href="Lightbend_Inc." class="mw-redirect" title="Lightbend Inc.">Lightbend Inc.</a>), a company to provide commercial support, training, and services for Scala. Typesafe received a $3 million investment in 2011 from <a href="Greylock_Partners" title="Greylock Partners">Greylock Partners</a>.<a href="#cite_note-18">[18]</a><a href="#cite_note-19">[19]</a><a href="#cite_note-20">[20]</a><a href="#cite_note-21">[21]</a>

<div class="mw-heading mw-heading2"><h2 id="Platforms_and_license">Platforms and license</h2></div>

Scala runs on the <a href="Java_platform" class="mw-redirect" title="Java platform">Java platform</a> (<a href="Java_virtual_machine" title="Java virtual machine">Java virtual machine</a>) and is compatible with existing <a href="Java_(programming_language)" title="Java (programming language)">Java</a> programs.<a href="#cite_note-cacm-15">[15]</a> As <a href="Android_(operating_system)" title="Android (operating system)">Android</a> applications are typically written in Java and translated from Java bytecode into <a href="Dalvik_(software)" title="Dalvik (software)">Dalvik</a> bytecode (which may be further translated to native machine code during installation) when packaged, Scala's Java compatibility makes it well-suited to Android development, the more so when a functional approach is preferred.<a href="#cite_note-22">[22]</a>

The reference Scala software distribution, including compiler and libraries, is released under the <a href="Apache_license" class="mw-redirect" title="Apache license">Apache license</a>.<a href="#cite_note-Scala_2.12.8_is_now_available!-23">[23]</a>

<div class="mw-heading mw-heading3"><h3 id="Other_compilers_and_targets">Other compilers and targets</h3></div>

<a rel="nofollow" class="external text" href="https://www.scala-js.org/">Scala.js</a> is a Scala compiler that compiles to JavaScript, making it possible to write Scala programs that can run in web browsers or <a href="Node.js" title="Node.js">Node.js</a>.<a href="#cite_note-24">[24]</a> The compiler, in development since 2013, was announced as no longer experimental in 2015 (v0.6). Version v1.0.0-M1 was released in June 2018 and version 1.1.1 in September 2020.<a href="#cite_note-25">[25]</a>

Scala Native is a Scala <a href="Compiler" title="Compiler">compiler</a> that targets the <a href="LLVM" title="LLVM">LLVM</a> compiler infrastructure to create executable code that uses a lightweight managed runtime, which uses the <a href="Boehm_garbage_collector" title="Boehm garbage collector">Boehm garbage collector</a>. The project is led by Denys Shabalin and had its first release, 0.1, on 14 March 2017. Development of Scala Native began in 2015 with a goal of being faster than <a href="Just-in-time_compilation" title="Just-in-time compilation">just-in-time compilation</a> for the JVM by eliminating the initial runtime compilation of code and also providing the ability to call native routines directly.<a href="#cite_note-26">[26]</a><a href="#cite_note-27">[27]</a>

A reference Scala compiler targeting the <a href=".NET_Framework" title=".NET Framework">.NET Framework</a> and its <a href="Common_Language_Runtime" title="Common Language Runtime">Common Language Runtime</a> was released in June 2004,<a href="#cite_note-history-of-scala-14">[14]</a> but was officially dropped in 2012.<a href="#cite_note-28">[28]</a>

<div class="mw-heading mw-heading2"><h2 id="Examples">Examples</h2></div>

<div class="mw-heading mw-heading3"><h3 id=""Hello_World"_example">"Hello World" example</h3></div>

The <a href="Hello_World_program" class="mw-redirect" title="Hello World program">Hello World program</a> written in Scala 3 has this form:

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>@main def main() = println("Hello, World!")

</pre></div>

Unlike the <a href="Java_(programming_language)#Hello_world" title="Java (programming language)">stand-alone Hello World application for Java</a>, there is no class declaration and nothing is declared to be static.

When the program is stored in file HelloWorld.scala, the user compiles it with the command:

<pre>$ scalac HelloWorld.scala

</pre>

and runs it with

<pre>$ scala HelloWorld

</pre>

This is analogous to the process for compiling and running Java code. Indeed, Scala's compiling and executing model is identical to that of Java, making it compatible with Java build tools such as <a href="Apache_Ant" title="Apache Ant">Apache Ant</a>.

A shorter version of the "Hello World" Scala program is:

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>println("Hello, World!")

</pre></div>

Scala includes an interactive shell and scripting support.<a href="#cite_note-29">[29]</a> Saved in a file named <code>HelloWorld2.scala</code>, this can be run as a script using the command:

<pre>$ scala HelloWorld2.scala

</pre>

Commands can also be entered directly into the Scala interpreter, using the option <style data-mw-deduplicate="TemplateStyles:r886049734">

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</style>-e:

<pre>$ scala -e 'println("Hello, World!")'

</pre>

Expressions can be entered interactively in the <a href="REPL" class="mw-redirect" title="REPL">REPL</a>:

<div class="mw-highlight mw-highlight-lang-console mw-content-ltr" dir="ltr"><pre>$ scala

Welcome to Scala 2.12.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_131).

Type in expressions for evaluation. Or try :help.

scala> List(1, 2, 3).map(x => x * x)

res0: List[Int] = List(1, 4, 9)

scala>

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="Basic_example">Basic example</h3></div>

The following example shows the differences between Java and Scala syntax. The function mathFunction takes an integer, squares it, and then adds the cube root of that number to the natural log of that number, returning the result (i.e., <math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle n^{2/3}+\ln(n^{2})}">

<msup>

</mrow>

</mrow>

</msup>

<msup>

</mrow>

</msup>

</mstyle>

</mrow>

<annotation encoding="application/x-tex">{\displaystyle n^{2/3}+\ln(n^{2})}</annotation>

</semantics>

</math><img src="./c8b951dcd43293ba0ff1254fb0519d38b5aaa677.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:13.131ex; height:3.343ex;" alt="{\displaystyle n^{2/3}+\ln(n^{2})}" loading="lazy">):

<table>

<td><div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>// Java:

int mathFunction(int num) {

int numSquare = num*num;

return (int) (Math.cbrt(numSquare) +

Math.log(numSquare));

}

</pre></div>

</td></tr>

<tr>

<td><div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>// Scala: Direct conversion from Java

// no import needed; scala.math

// already imported as `math`

def mathFunction(num: Int): Int =

var numSquare: Int = num*num

return (math.cbrt(numSquare) + math.log(numSquare)).

asInstanceOf[Int]

</pre></div>

</td>

<td><div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>// Scala: More idiomatic

// Uses type inference, omits `return` statement,

// uses `toInt` method, declares numSquare immutable

import math.*

def mathFunction(num: Int) =

val numSquare = num*num

(cbrt(numSquare) + log(numSquare)).toInt

</pre></div>

</td></tr></tbody></table>

Some syntactic differences in this code are:

<ul><li>Scala does not require semicolons (;) to end statements.</li>

<li>Value types are capitalized (sentence case): <code>Int, Double, Boolean</code> instead of <code>int, double, boolean</code>.</li>

<li>Parameter and return types follow, as in <a href="Pascal_(programming_language)" title="Pascal (programming language)">Pascal</a>, rather than precede as in <a href="C_(programming_language)" title="C (programming language)">C</a>.</li>

<li>Methods must be preceded by <code>def</code>.</li>

<li>Local or class variables must be preceded by <code>val</code> (indicates an <a href="Immutable" class="mw-redirect" title="Immutable">immutable</a> variable) or <code>var</code> (indicates a <a href="Mutable" class="mw-redirect" title="Mutable">mutable</a> variable).</li>

<li>The <code>return</code> operator is unnecessary in a function (although allowed); the value of the last executed statement or expression is normally the function's value.</li>

<li>Instead of the Java cast operator <code>(Type) foo</code>, Scala uses <code>foo.asInstanceOf[Type]</code>, or a specialized function such as <code>toDouble</code> or <code>toInt</code>.</li>

<li>Function or method <code>foo()</code> can also be called as just <code>foo</code>; method <code>thread.send(signo)</code> can also be called as just <code>thread send signo</code>; and method <code>foo.toString()</code> can also be called as just <code>foo toString</code>.</li></ul>

These syntactic relaxations are designed to allow support for <a href="Domain-specific_language" title="Domain-specific language">domain-specific languages</a>.

Some other basic syntactic differences:

<ul><li>Array references are written like function calls, e.g. <code>array(i)</code> rather than <code>array[i]</code>. (Internally in Scala, the former expands into array.apply(i) which returns the reference)</li>

<li>Generic types are written as e.g. <code>List[String]</code> rather than Java's <code>List<String></code>.</li>

<li>Instead of the pseudo-type <code>void</code>, Scala has the actual <a href="Singleton_pattern" title="Singleton pattern">singleton class</a> <code>Unit</code> (see below).</li></ul>

<div class="mw-heading mw-heading3"><h3 id="Example_with_classes">Example with classes</h3></div>

The following example contrasts the definition of classes in Java and Scala.

<table>

<td><div class="mw-highlight mw-highlight-lang-java mw-content-ltr mw-highlight-lines" dir="ltr"><pre>// Java:

public class Point {

private double x, y;

public Point(double x, double y) {

this.x = x;

this.y = y;

}

public Point(double x, double y, boolean addToGrid) {

this(x, y);

if (addToGrid)

grid.addPoint(this);

}

public Point() {

this(0.0, 0.0);

}

private void addPoint(Point p) {

x += p.x;

y += p.y;

}

public double getX() {

return x;

}

public double getY() {

return y;

}

double distanceToPoint(Point other) {

return distanceBetweenPoints(x, y,

other.x, other.y);

}

private static Point grid = new Point();

static double distanceBetweenPoints(

double x1, double y1,

double x2, double y2) {

return Math.hypot(x1 - x2, y1 - y2);

}

}

</pre></div>

</td>

<td><div class="mw-highlight mw-highlight-lang-scala mw-content-ltr mw-highlight-lines" dir="ltr"><pre>// Scala

class Point(

var x: Double, var y: Double,

addToGrid: Boolean = false

):

import Point.*

def += (p: Point) =

x += p.x

y += p.y

if (addToGrid)

grid += this

def this() = this(0.0, 0.0)

def distanceToPoint(other: Point) =

distanceBetweenPoints(x, y, other.x, other.y)

end Point

object Point:

private val grid = new Point()

def distanceBetweenPoints(x1: Double, y1: Double,

x2: Double, y2: Double) =

math.hypot(x1 - x2, y1 - y2)

</pre></div>

</td></tr></tbody></table>

The code above shows some of the conceptual differences between Java and Scala's handling of classes:

<ul><li>Scala has no static variables or methods. Instead, it has singleton objects, which are essentially classes with only one instance. Singleton objects are declared using <code>object</code> instead of <code>class</code>. It is common to place static variables and methods in a singleton object with the same name as the class name, which is then known as a companion object.<a href="#cite_note-cacm-15">[15]</a> (The underlying class for the singleton object has a <code>$</code> appended. Hence, for <code>class Foo</code> with companion object <code>object Foo</code>, under the hood there's a class <code>Foo$</code> containing the companion object's code, and one object of this class is created, using the <a href="Singleton_pattern" title="Singleton pattern">singleton pattern</a>.)</li>

<li>In place of constructor parameters, Scala has class parameters, which are placed on the class, similar to parameters to a function. When declared with a <code>val</code> or <code>var</code> modifier, fields are also defined with the same name, and automatically initialized from the class parameters. (Under the hood, external access to public fields always goes through accessor (getter) and mutator (setter) methods, which are automatically created. The accessor function has the same name as the field, which is why it's unnecessary in the above example to explicitly declare accessor methods.) Note that alternative constructors can also be declared, as in Java. Code that would go into the <a href="Default_constructor" title="Default constructor">default constructor</a> (other than initializing the member variables) goes directly at class level.</li>

<li>In Scala it is possible to define operators by using symbols as method names. In place of <code>addPoint</code>, the Scala example defines <code>+=</code>, which is then invoked with <a href="Infix_notation" title="Infix notation">infix notation</a> as <code>grid += this</code>.</li>

<li>Default visibility in Scala is <code>public</code>.</li></ul>

<div class="mw-heading mw-heading2"><h2 id="Features_(with_reference_to_Java)">Features (with reference to Java)</h2></div>

Scala has the same compiling model as <a href="Java_(software_platform)" title="Java (software platform)">Java</a> and <a href="C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a>, namely separate compiling and <a href="Java_class_loader" title="Java class loader">dynamic class loading</a>, so that Scala code can call Java libraries.

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.<a href="#cite_note-cacm-15">[15]</a> In fact, Scala code can be <a href="Decompiled" class="mw-redirect" title="Decompiled">decompiled</a> to readable Java code, with the exception of certain constructor operations. To the <a href="Java_virtual_machine" title="Java virtual machine">Java virtual machine</a> (JVM), Scala code and Java code are indistinguishable. The only difference is one extra runtime library, <code>scala-library.jar</code>.<a href="#cite_note-30">[30]</a>

Scala adds a large number of features compared with Java, and has some fundamental differences in its underlying model of expressions and types, which make the language theoretically cleaner and eliminate several <a href="Corner_cases" class="mw-redirect" title="Corner cases">corner cases</a> in Java. From the Scala perspective, this is practically important because several added features in Scala are also available in C#.

<div class="mw-heading mw-heading3"><h3 id="Syntactic_flexibility">Syntactic flexibility</h3></div>

As mentioned above, Scala has a good deal of syntactic flexibility, compared with Java. The following are some examples:

<ul><li>Semicolons are unnecessary; lines are automatically joined if they begin or end with a token that cannot normally come in this position, or if there are unclosed parentheses or brackets.</li>

<li>Any method can be used as an <a href="Infix_operator" class="mw-redirect" title="Infix operator">infix operator</a>, e.g. <code>"%d apples".format(num)</code> and <code>"%d apples" format num</code> are equivalent. In fact, arithmetic operators like <code>+</code> and <code><<</code> are treated just like any other methods, since function names are allowed to consist of sequences of arbitrary symbols (with a few exceptions made for things like parens, brackets and braces that must be handled specially); the only special treatment that such symbol-named methods undergo concerns the handling of precedence.</li>

<li>Methods <code>apply</code> and <code>update</code> have syntactic short forms. <code>foo()</code>—where <code>foo</code> is a value (singleton object or class instance)—is short for <code>foo.apply()</code>, and <code>foo() = 42</code> is short for <code>foo.update(42)</code>. Similarly, <code>foo(42)</code> is short for <code>foo.apply(42)</code>, and <code>foo(4) = 2</code> is short for <code>foo.update(4, 2)</code>. This is used for collection classes and extends to many other cases, such as <a href="Software_transactional_memory" title="Software transactional memory">STM</a> cells.</li>

<li>Scala distinguishes between no-parens (<code>def foo = 42</code>) and empty-parens (<code>def foo() = 42</code>) methods. When calling an empty-parens method, the parentheses may be omitted, which is useful when calling into Java libraries that do not know this distinction, e.g., using <code>foo.toString</code> instead of <code>foo.toString()</code>. By convention, a method should be defined with empty-parens when it performs <a href="Side_effect_(computer_science)" title="Side effect (computer science)">side effects</a>.</li>

<li>Method names ending in colon (<code>:</code>) expect the argument on the left-hand-side and the receiver on the right-hand-side. For example, the <code>4 :: 2 :: Nil</code> is the same as <code>Nil.::(2).::(4)</code>, the first form corresponding visually to the result (a list with first element 4 and second element 2).</li>

<li>Class body variables can be transparently implemented as separate getter and setter methods. For <code>trait FooLike { var bar: Int }</code>, an implementation may be <code class="mw-highlight mw-highlight-lang-scala mw-content-ltr" style="" dir="ltr">object Foo extends FooLike { private var x = 0; def bar = x; def bar_=(value: Int) { x = value }} } }</code>. The call site will still be able to use a concise <code>foo.bar = 42</code>.</li>

<li>The use of curly braces instead of parentheses is allowed in method calls. This allows pure library implementations of new control structures.<a href="#cite_note-31">[31]</a> For example, <code>breakable { ... if (...) break() ... }</code> looks as if <code>breakable</code> was a language defined keyword, but really is just a method taking a <a href="Thunk" title="Thunk">thunk</a> argument. Methods that take thunks or functions often place these in a second parameter list, allowing to mix parentheses and curly braces syntax: <code>Vector.fill(4) { math.random }</code> is the same as <code>Vector.fill(4)(math.random)</code>. The curly braces variant allows the expression to span multiple lines.</li>

<li>For-expressions (explained further down) can accommodate any type that defines monadic methods such as <code>map</code>, <code>flatMap</code> and <code>filter</code>.</li></ul>

By themselves, these may seem like questionable choices, but collectively they serve the purpose of allowing <a href="Domain-specific_language" title="Domain-specific language">domain-specific languages</a> to be defined in Scala without needing to extend the compiler. For example, <a href="Erlang_(programming_language)" title="Erlang (programming language)">Erlang</a>'s special syntax for sending a message to an actor, i.e. <code>actor ! message</code> can be (and is) implemented in a Scala library without needing language extensions.

<div class="mw-heading mw-heading3"><h3 id="Unified_type_system">Unified type system</h3></div>

Java makes a sharp distinction between primitive types (e.g. <code>int</code> and <code>boolean</code>) and reference types (any <a href="Class_(computer_programming)" title="Class (computer programming)">class</a>). Only reference types are part of the inheritance scheme, deriving from <code>java.lang.Object</code>. In Scala, all types inherit from a top-level class <code>Any</code>, whose immediate children are <code>AnyVal</code> (value types, such as <code>Int</code> and <code>Boolean</code>) and <code>AnyRef</code> (reference types, as in Java). This means that the Java distinction between primitive types and boxed types (e.g. <code>int</code> vs. <code>Integer</code>) is not present in Scala; <a href="Boxing_(programming)" class="mw-redirect" title="Boxing (programming)">boxing</a> and unboxing is completely transparent to the user. Scala 2.10 allows for new value types to be defined by the user.

<div class="mw-heading mw-heading3"><h3 id="For-expressions">For-expressions</h3></div>

Instead of the Java "<a href="Foreach" class="mw-redirect" title="Foreach">foreach</a>" loops for looping through an iterator, Scala has <code>for</code>-expressions, which are similar to <a href="List_comprehension" title="List comprehension">list comprehensions</a> in languages such as <a href="Haskell" title="Haskell">Haskell</a>, or a combination of list comprehensions and <a href="Python_syntax_and_semantics#Generator_expressions" title="Python syntax and semantics">generator expressions</a> in <a href="Python_(programming_language)" title="Python (programming language)">Python</a>. For-expressions using the <code>yield</code> keyword allow a new <a href="Collection_(computer_science)" class="mw-redirect" title="Collection (computer science)">collection</a> to be generated by iterating over an existing one, returning a new collection of the same type. They are translated by the compiler into a series of <code>map</code>, <code>flatMap</code> and <code>filter</code> calls. Where <code>yield</code> is not used, the code approximates to an imperative-style loop, by translating to <code>foreach</code>.

A simple example is:

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>val s = for (x <- 1 to 25 if x*x > 50) yield 2*x

</pre></div>

The result of running it is the following vector:

<dl><dd><code>Vector(16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50)</code></dd></dl>

(Note that the expression <code>1 to 25</code> is not special syntax. The method <code>to</code> is rather defined in the standard Scala library as an extension method on integers, using a technique known as implicit conversions<a href="#cite_note-artima1-32">[32]</a> that allows new methods to be added to existing types.)

A more complex example of iterating over a map is:

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>// Given a map specifying Twitter users mentioned in a set of tweets,

// and number of times each user was mentioned, look up the users

// in a map of known politicians, and return a new map giving only the

// Democratic politicians (as objects, rather than strings).

val dem_mentions = for

(mention, times) <- mentions

account <- accounts.get(mention)

if account.party == "Democratic"

yield (account, times)

</pre></div>

Expression <code>(mention, times) <- mentions</code> is an example of <a href="Pattern_matching" title="Pattern matching">pattern matching</a> (see below). Iterating over a map returns a set of key-value <a href="Tuple_(computer_science)" class="mw-redirect" title="Tuple (computer science)">tuples</a>, and pattern-matching allows the tuples to easily be destructured into separate variables for the key and value. Similarly, the result of the comprehension also returns key-value tuples, which are automatically built back up into a map because the source object (from the variable <code>mentions</code>) is a map. Note that if <code>mentions</code> instead held a list, set, array or other collection of tuples, exactly the same code above would yield a new collection of the same type.

<div class="mw-heading mw-heading3"><h3 id="Functional_tendencies">Functional tendencies</h3></div>

While supporting all of the object-oriented features available in Java (and in fact, augmenting them in various ways), Scala also provides a large number of capabilities that are normally found only in <a href="Functional_programming" title="Functional programming">functional programming</a> languages. Together, these features allow Scala programs to be written in an almost completely functional style and also allow functional and object-oriented styles to be mixed.

Examples are:

<ul><li>No distinction between statements and expressions</li>

<li><a href="Type_inference" title="Type inference">Type inference</a></li>

<li><a href="Anonymous_function" title="Anonymous function">Anonymous functions</a> with capturing semantics (i.e., <a href="Closure_(computer_science)" class="mw-redirect" title="Closure (computer science)">closures</a>)</li>

<li>Immutable variables and objects</li>

<li><a href="Lazy_evaluation" title="Lazy evaluation">Lazy evaluation</a></li>

<li><a href="Delimited_continuation" title="Delimited continuation">Delimited continuations</a> (since 2.8)</li>

<li><a href="Higher-order_function" title="Higher-order function">Higher-order functions</a></li>

<li>Nested functions</li>

<li><a href="Currying" title="Currying">Currying</a></li>

<li><a href="Pattern_matching" title="Pattern matching">Pattern matching</a></li>

<li><a href="Algebraic_data_types" class="mw-redirect" title="Algebraic data types">Algebraic data types</a> (through case classes)</li>

<li><a href="Tuple_(computer_science)" class="mw-redirect" title="Tuple (computer science)">Tuples</a></li></ul>

<div class="mw-heading mw-heading4"><h4 id="Everything_is_an_expression">Everything is an expression</h4></div>

Unlike <a href="C_(programming_language)" title="C (programming language)">C</a> or <a href="Java_(programming_language)" title="Java (programming language)">Java</a>, but similar to languages such as <a href="Lisp_(programming_language)" title="Lisp (programming language)">Lisp</a>, Scala makes no distinction between statements and <a href="Expression_(computer_science)" title="Expression (computer science)">expressions</a>. All statements are in fact expressions that evaluate to some value. Functions that would be declared as returning <code>void</code> in C or Java, and statements like <code>while</code> that logically do not return a value, are in Scala considered to return the type <code>Unit</code>, which is a <a href="Singleton_(mathematics)" title="Singleton (mathematics)">singleton type</a>, with only one object of that type. Functions and operators that never return at all (e.g. the <code>throw</code> operator or a function that always exits <a href="Exception_handling" title="Exception handling">non-locally</a> using an exception) logically have return type <code>Nothing</code>, a special type containing no objects; that is, a <a href="Bottom_type" title="Bottom type">bottom type</a>, i.e. a subclass of every possible type. (This in turn makes type <code>Nothing</code> compatible with every type, allowing <a href="Type_inference" title="Type inference">type inference</a> to function correctly.)<a href="#cite_note-33">[33]</a>

Similarly, an <code>if-then-else</code> "statement" is actually an expression, which produces a value, i.e. the result of evaluating one of the two branches. This means that such a block of code can be inserted wherever an expression is desired, obviating the need for a <a href="Ternary_operator" class="mw-redirect" title="Ternary operator">ternary operator</a> in Scala:

<table>

<td><div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>// Java:

char hexDigit = (char)(x >= 10 ? x + 'A' - 10 : x + '0');

</pre></div>

</td>

<td><div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>// Scala:

val hexDigit = (if x >= 10 then x + 'A' - 10 else x + '0').toChar

</pre></div>

</td></tr></tbody></table>

For similar reasons, <code>return</code> statements are unnecessary in Scala, and in fact are discouraged. As in <a href="Lisp_(programming_language)" title="Lisp (programming language)">Lisp</a>, the last expression in a block of code is the value of that block of code, and if the block of code is the body of a function, it will be returned by the function.

To make it clear that all functions are expressions, even methods that return <code>Unit</code> are written with an equals sign

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>def printValue(x: String): Unit =

println("I ate a %s".format(x))

</pre></div>

or equivalently (with type inference, and omitting the unnecessary newline):

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>def printValue(x: String) = println("I ate a %s" format x)

</pre></div>

<div class="mw-heading mw-heading4"><h4 id="Type_inference">Type inference</h4></div>

Due to <a href="Type_inference" title="Type inference">type inference</a>, the type of variables, function return values, and many other expressions can typically be omitted, as the compiler can deduce it. Examples are <code>val x = "foo"</code> (for an immutable <a href="Constant_(programming)" class="mw-redirect" title="Constant (programming)">constant</a> or <a href="Immutable_object" title="Immutable object">immutable object</a>) or <code>var x = 1.5</code> (for a variable whose value can later be changed). Type inference in Scala is essentially local, in contrast to the more global <a href="Hindley-Milner" class="mw-redirect" title="Hindley-Milner">Hindley-Milner</a> algorithm used in Haskell, <a href="ML_(programming_language)" title="ML (programming language)">ML</a> and other more purely functional languages. This is done to facilitate object-oriented programming. The result is that certain types still need to be declared (most notably, function parameters, and the return types of <a href="Recursion_(computer_science)" title="Recursion (computer science)">recursive functions</a>), e.g.

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>def formatApples(x: Int) = "I ate %d apples".format(x)

</pre></div>

or (with a return type declared for a recursive function)

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>def factorial(x: Int): Int =

if x == 0 then 1

else x * factorial(x - 1)

</pre></div>

<div class="mw-heading mw-heading4"><h4 id="Anonymous_functions">Anonymous functions</h4></div>

In Scala, functions are objects, and a convenient syntax exists for specifying <a href="Anonymous_function" title="Anonymous function">anonymous functions</a>. An example is the expression <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">x => x < 2</code>, which specifies a function with one parameter, that compares its argument to see if it is less than 2. It is equivalent to the Lisp form <code class="mw-highlight mw-highlight-lang-lisp mw-content-ltr" style="" dir="ltr">(lambda (x) (< x 2))</code>. Note that neither the type of x nor the return type need be explicitly specified, and can generally be inferred by <a href="Type_inference" title="Type inference">type inference</a>; but they can be explicitly specified, e.g. as <code class="mw-highlight mw-highlight-lang-scala mw-content-ltr" style="" dir="ltr">(x: Int) => x < 2</code> or even <code class="mw-highlight mw-highlight-lang-scala mw-content-ltr" style="" dir="ltr">(x: Int) => (x < 2): Boolean</code>.

Anonymous functions behave as true <a href="Closure_(computer_science)" class="mw-redirect" title="Closure (computer science)">closures</a> in that they automatically capture any variables that are lexically available in the environment of the enclosing function. Those variables will be available even after the enclosing function returns, and unlike in the case of Java's anonymous inner classes do not need to be declared as final. (It is even possible to modify such variables if they are mutable, and the modified value will be available the next time the anonymous function is called.)

An even shorter form of anonymous function uses <a href="Free_variables_and_bound_variables" title="Free variables and bound variables">placeholder</a> variables: For example, the following:

can be written more concisely as

or even

<div class="mw-heading mw-heading4"><h4 id="Immutability">Immutability</h4></div>

Scala enforces a distinction between immutable and mutable variables. Mutable variables are declared using the <code>var</code> keyword and immutable values are declared using the <code>val</code> keyword.

A variable declared using the <code>val</code> keyword cannot be reassigned in the same way that a variable declared using the <code>final</code> keyword can't be reassigned in Java. <code>val</code>s are only shallowly immutable, that is, an object referenced by a val is not guaranteed to itself be immutable.

Immutable classes are encouraged by convention however, and the Scala standard library provides a rich set of immutable <a href="Collection_(abstract_data_type)" title="Collection (abstract data type)">collection</a> classes.

Scala provides mutable and immutable variants of most collection classes, and the immutable version is always used unless the mutable version is explicitly imported.<a href="#cite_note-34">[34]</a>

The immutable variants are <a href="Persistent_data_structure" title="Persistent data structure">persistent data structures</a> that always return an updated copy of an old object instead of updating the old object destructively in place.

An example of this is <a href="Persistent_data_structure#Linked_lists" title="Persistent data structure">immutable linked lists</a> where prepending an element to a list is done by returning a new list node consisting of the element and a reference to the list tail.

Appending an element to a list can only be done by prepending all elements in the old list to a new list with only the new element.

In the same way, inserting an element in the middle of a list will copy the first half of the list, but keep a reference to the second half of the list. This is called structural sharing.

This allows for very easy concurrency — no locks are needed as no shared objects are ever modified.<a href="#cite_note-35">[35]</a>

<div class="mw-heading mw-heading4"><h4 id="Lazy_(non-strict)_evaluation">Lazy (non-strict) evaluation</h4></div>

Evaluation is strict ("eager") by default. In other words, Scala evaluates expressions as soon as they are available, rather than as needed. However, it is possible to declare a variable non-strict ("lazy") with the <code>lazy</code> keyword, meaning that the code to produce the variable's value will not be evaluated until the first time the variable is referenced. Non-strict collections of various types also exist (such as the type <code>Stream</code>, a non-strict linked list), and any collection can be made non-strict with the <code>view</code> method. Non-strict collections provide a good semantic fit to things like server-produced data, where the evaluation of the code to generate later elements of a list (that in turn triggers a request to a server, possibly located somewhere else on the web) only happens when the elements are actually needed.

<div class="mw-heading mw-heading4"><h4 id="Tail_recursion">Tail recursion</h4></div>

Functional programming languages commonly provide <a href="Tail_call" title="Tail call">tail call</a> optimization to allow for extensive use of <a href="Recursion_(computer_science)" title="Recursion (computer science)">recursion</a> without <a href="Stack_overflow" title="Stack overflow">stack overflow</a> problems. Limitations in Java bytecode complicate tail call optimization on the JVM. In general, a function that calls itself with a tail call can be optimized, but mutually recursive functions cannot. <a href="Trampoline_(computing)" title="Trampoline (computing)">Trampolines</a> have been suggested as a workaround.<a href="#cite_note-36">[36]</a> Trampoline support has been provided by the Scala library with the object <code>scala.util.control.TailCalls</code> since Scala 2.8.0 (released 14 July 2010). A function may optionally be annotated with <code>@tailrec</code>, in which case it will not compile unless it is tail recursive.<a href="#cite_note-37">[37]</a>

An example of this optimization could be implemented using the <a href="Factorial" title="Factorial">factorial</a> definition. For instance, the recursive version of the factorial: <div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>def factorial(n: Int): Int =

if n == 0 then 1

else n * factorial(n - 1)

</pre></div>Could be optimized to the tail recursive version like this: <div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>@tailrec

def factorial(n: Int, accum: Int): Int =

else factorial(n - 1, n * accum)

</pre></div>However, this could compromise composability with other functions because of the new argument on its definition, so it is common to use <a href="Closure_(computer_programming)" title="Closure (computer programming)">closures</a> to preserve its original signature: <div class="mw-highlight mw-highlight-lang-scala mw-content-ltr mw-highlight-lines" dir="ltr"><pre>def factorial(n: Int): Int =

@tailrec

def loop(current: Int, accum: Int): Int =

if current == 0 then accum

else loop(current - 1, current * accum)

loop(n, 1) // Call to the closure using the base case

end factorial

</pre></div>This ensures tail call optimization and thus prevents a stack overflow error.

<div class="mw-heading mw-heading4"><h4 id="Case_classes_and_pattern_matching">Case classes and pattern matching</h4></div>

Scala has built-in support for <a href="Pattern_matching" title="Pattern matching">pattern matching</a>, which can be thought of as a more sophisticated, extensible version of a <a href="Switch_statement" title="Switch statement">switch statement</a>, where arbitrary data types can be matched (rather than just simple types like integers, Booleans and strings), including arbitrary nesting. A special type of class known as a case class is provided, which includes automatic support for pattern matching and can be used to model the <a href="Algebraic_data_type" title="Algebraic data type">algebraic data types</a> used in many functional programming languages. (From the perspective of Scala, a case class is simply a normal class for which the compiler automatically adds certain behaviors that could also be provided manually, e.g., definitions of methods providing for deep comparisons and hashing, and destructuring a case class on its constructor parameters during pattern matching.)

An example of a definition of the <a href="Quicksort" title="Quicksort">quicksort</a> algorithm using pattern matching is this:

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>def qsort(list: List[Int]): List[Int] = list match

case Nil => Nil

case pivot :: tail =>

val (smaller, rest) = tail.partition(_ < pivot)

qsort(smaller) ::: pivot :: qsort(rest)

</pre></div>

The idea here is that we partition a list into the elements less than a pivot and the elements not less, recursively sort each part, and paste the results together with the pivot in between. This uses the same <a href="Divide_and_conquer_algorithm" class="mw-redirect" title="Divide and conquer algorithm">divide-and-conquer</a> strategy of <a href="Mergesort" class="mw-redirect" title="Mergesort">mergesort</a> and other fast sorting algorithms.

The <code>match</code> operator is used to do pattern matching on the object stored in <code>list</code>. Each <code>case</code> expression is tried in turn to see if it will match, and the first match determines the result. In this case, <code>Nil</code> only matches the literal object <code>Nil</code>, but <code>pivot :: tail</code> matches a non-empty list, and simultaneously destructures the list according to the pattern given. In this case, the associated code will have access to a local variable named <code>pivot</code> holding the head of the list, and another variable <code>tail</code> holding the tail of the list. Note that these variables are read-only, and are semantically very similar to variable <a href="Name_binding" title="Name binding">bindings</a> established using the <a href="Let_expression" title="Let expression"><code>let</code></a> operator in Lisp and Scheme.

Pattern matching also happens in local variable declarations. In this case, the return value of the call to <code>tail.partition</code> is a <a href="Tuple_(computer_science)" class="mw-redirect" title="Tuple (computer science)">tuple</a> — in this case, two lists. (Tuples differ from other types of containers, e.g. lists, in that they are always of fixed size and the elements can be of differing types — although here they are both the same.) Pattern matching is the easiest way of fetching the two parts of the tuple.

The form <code>_ < pivot</code> is a declaration of an <a href="Anonymous_function" title="Anonymous function">anonymous function</a> with a placeholder variable; see the section above on anonymous functions.

The list operators <code>::</code> (which adds an element onto the beginning of a list, similar to <code>cons</code> in Lisp and Scheme) and <code>:::</code> (which appends two lists together, similar to <code>append</code> in Lisp and Scheme) both appear. Despite appearances, there is nothing "built-in" about either of these operators. As specified above, any string of symbols can serve as function name, and a method applied to an object can be written "infix"-style without the period or parentheses. The line above as written:

<dl><dd><dl><dd><code>qsort(smaller) ::: pivot :: qsort(rest)</code></dd></dl></dd></dl>

could also be written thus:

<dl><dd><dl><dd><code>qsort(rest).::(pivot).:::(qsort(smaller))</code></dd></dl></dd></dl>

in more standard method-call notation. (Methods that end with a colon are right-associative and bind to the object to the right.)

<div class="mw-heading mw-heading4"><h4 id="Partial_functions">Partial functions</h4></div>

In the pattern-matching example above, the body of the <code>match</code> operator is a <a href="Partial_function" title="Partial function">partial function</a>, which consists of a series of <code>case</code> expressions, with the first matching expression prevailing, similar to the body of a <a href="Switch_statement" title="Switch statement">switch statement</a>. Partial functions are also used in the exception-handling portion of a <code>try</code> statement:

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>try

...

catch

case nfe: NumberFormatException => { println(nfe); List(0) }

case _ => Nil

</pre></div>

Finally, a partial function can be used alone, and the result of calling it is equivalent to doing a <code>match</code> over it. For example, the prior code for <a href="Quicksort" title="Quicksort">quicksort</a> can be written thus:

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>val qsort: List[Int] => List[Int] =

</pre></div>

Here a read-only variable is declared whose type is a function from lists of integers to lists of integers, and bind it to a partial function. (Note that the single parameter of the partial function is never explicitly declared or named.) However, we can still call this variable exactly as if it were a normal function:

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>scala> qsort(List(6,2,5,9))

res32: List[Int] = List(2, 5, 6, 9)

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="Object-oriented_extensions">Object-oriented extensions</h3></div>

Scala is a pure <a href="Object-oriented_language" class="mw-redirect" title="Object-oriented language">object-oriented language</a> in the sense that every value is an <a href="Object_(computer_science)" title="Object (computer science)">object</a>. <a href="Data_type" title="Data type">Data types</a> and behaviors of objects are described by <a href="Class_(computer_science)" class="mw-redirect" title="Class (computer science)">classes</a> and <a href="Trait_(computer_science)" class="mw-redirect" title="Trait (computer science)">traits</a>. Class abstractions are extended by <a href="Subclass_(computer_science)" class="mw-redirect" title="Subclass (computer science)">subclassing</a> and by a flexible <a href="Mixin" title="Mixin">mixin</a>-based composition mechanism to avoid the problems of <a href="Multiple_inheritance" title="Multiple inheritance">multiple inheritance</a>.

Traits are Scala's replacement for Java's <a href="Protocol_(object-oriented_programming)" class="mw-redirect" title="Protocol (object-oriented programming)">interfaces</a>. Interfaces in Java versions under 8 are highly restricted, able only to contain abstract function declarations. This has led to criticism that providing convenience methods in interfaces is awkward (the same methods must be reimplemented in every implementation), and extending a published interface in a backwards-compatible way is impossible. Traits are similar to <a href="Mixin" title="Mixin">mixin</a> classes in that they have nearly all the power of a regular abstract class, lacking only class parameters (Scala's equivalent to Java's constructor parameters), since traits are always mixed in with a class. The <code>super</code> operator behaves specially in traits, allowing traits to be chained using composition in addition to inheritance. The following example is a simple window system:

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>abstract class Window:

// abstract

def draw()

class SimpleWindow extends Window:

def draw()

println("in SimpleWindow")

// draw a basic window

trait WindowDecoration extends Window

trait HorizontalScrollbarDecoration extends WindowDecoration:

// "abstract override" is needed here for "super()" to work because the parent

// function is abstract. If it were concrete, regular "override" would be enough.

abstract override def draw()

println("in HorizontalScrollbarDecoration")

super.draw()

// now draw a horizontal scrollbar

trait VerticalScrollbarDecoration extends WindowDecoration:

println("in VerticalScrollbarDecoration")

super.draw()

// now draw a vertical scrollbar

trait TitleDecoration extends WindowDecoration:

println("in TitleDecoration")

super.draw()

// now draw the title bar

</pre></div>

A variable may be declared thus:

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>val mywin = new SimpleWindow with VerticalScrollbarDecoration with HorizontalScrollbarDecoration with TitleDecoration

</pre></div>

The result of calling <code>mywin.draw()</code> is:

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>in TitleDecoration

in HorizontalScrollbarDecoration

in VerticalScrollbarDecoration

in SimpleWindow

</pre></div>

In other words, the call to <code>draw</code> first executed the code in <code>TitleDecoration</code> (the last trait mixed in), then (through the <code>super()</code> calls) threaded back through the other mixed-in traits and eventually to the code in <code>Window</code>, even though none of the traits inherited from one another. This is similar to the <a href="Decorator_pattern" title="Decorator pattern">decorator pattern</a>, but is more concise and less error-prone, as it doesn't require explicitly encapsulating the parent window, explicitly forwarding functions whose implementation isn't changed, or relying on run-time initialization of entity relationships. In other languages, a similar effect could be achieved at compile-time with a long linear chain of <a href="Inheritance_(object-oriented_programming)#Code_reuse" title="Inheritance (object-oriented programming)">implementation inheritance</a>, but with the disadvantage compared to Scala that one linear inheritance chain would have to be declared for each possible combination of the mix-ins.

<div class="mw-heading mw-heading3"><h3 id="Expressive_type_system">Expressive type system</h3></div>

Scala is equipped with an expressive static type system that mostly enforces the safe and coherent use of abstractions. The type system is, however, not <a href="Soundness" title="Soundness">sound</a>.<a href="#cite_note-unsound-38">[38]</a> In particular, the type system supports:

<ul><li><a href="Classes_(computer_science)" class="mw-redirect" title="Classes (computer science)">Classes</a> and <a href="Abstract_type" title="Abstract type">abstract types</a> as object members</li>

<li>Structural types</li>

<li>Path-dependent types</li>

<li>Compound types</li>

<li>Explicitly typed self references</li>

<li><a href="Generic_programming" title="Generic programming">Generic classes</a></li>

<li><a href="Polymorphism_(computer_science)" title="Polymorphism (computer science)">Polymorphic</a> methods</li>

<li>Upper and lower type bounds</li>

<li><a href="Variance_(computer_science)" class="mw-redirect" title="Variance (computer science)">Variance</a></li>

<li><a href="Annotation" title="Annotation">Annotation</a></li>

<li>Views</li></ul>

Scala is able to <a href="Type_inference" title="Type inference">infer types</a> by use. 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.

<div class="mw-heading mw-heading3"><h3 id="Type_enrichment">Type enrichment</h3></div>

A common technique in Scala, known as "enrich my library"<a href="#cite_note-39">[39]</a> (originally termed "<a href="Pimp_My_Ride" title="Pimp My Ride">pimp my library</a>" by Martin Odersky in 2006;<a href="#cite_note-artima1-32">[32]</a> concerns were raised about this phrasing due to its negative connotations<a href="#cite_note-40">[40]</a> and immaturity<a href="#cite_note-41">[41]</a>), allows new methods to be used as if they were added to existing types. This is similar to the C# concept of <a href="Extension_method" title="Extension method">extension methods</a> but more powerful, because the technique is not limited to adding methods and can, for instance, be used to implement new interfaces. In Scala, this technique involves declaring an <a href="Implicit_conversion" class="mw-redirect" title="Implicit conversion">implicit conversion</a> from the type "receiving" the method to a new type (typically, a class) that wraps the original type and provides the additional method. If a method cannot be found for a given type, the compiler automatically searches for any applicable implicit conversions to types that provide the method in question.

This technique allows new methods to be added to an existing class using an add-on library such that only code that imports the add-on library gets the new functionality, and all other code is unaffected.

The following example shows the enrichment of type <code>Int</code> with methods <code>isEven</code> and <code>isOdd</code>:

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>object MyExtensions:

extension (i: Int)

def isEven = i % 2 == 0

def isOdd = !i.isEven

import MyExtensions.* // bring implicit enrichment into scope

4.isEven // -> true

</pre></div>

Importing the members of <code>MyExtensions</code> brings the implicit conversion to extension class <code>IntPredicates</code> into scope.<a href="#cite_note-42">[42]</a>

<div class="mw-heading mw-heading2"><h2 id="Concurrency">Concurrency</h2></div>

Scala's standard library includes support for <a href="Futures_and_promises" title="Futures and promises">futures and promises</a>, in addition to the standard Java concurrency APIs. Originally, it also included support for the <a href="Actor_model" title="Actor model">actor model</a>, which is now available as a separate <a href="Source-available" class="mw-redirect" title="Source-available">source-available</a> platform <a href="Akka_(toolkit)" title="Akka (toolkit)">Akka</a><a href="#cite_note-AkkaAbout-43">[43]</a> licensed by <a href="Lightbend_Inc." class="mw-redirect" title="Lightbend Inc.">Lightbend Inc.</a> Akka actors may be <a href="Distributed_computing" title="Distributed computing">distributed</a> or combined with <a href="Software_transactional_memory" title="Software transactional memory">software transactional memory</a> (transactors). Alternative <a href="Communicating_sequential_processes" title="Communicating sequential processes">communicating sequential processes</a> (CSP) implementations for channel-based message passing are Communicating Scala Objects,<a href="#cite_note-CSO-44">[44]</a> or simply via <a href="JCSP" title="JCSP">JCSP</a>.

An Actor is like a thread instance with a mailbox. It can be created by <code>system.actorOf</code>, overriding the <code>receive</code> method to receive messages and using the <code>!</code> (exclamation point) method to send a message.<a href="#cite_note-45">[45]</a>

The following example shows an EchoServer that can receive messages and then print them.

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>val echoServer = actor(new Act:

become:

case msg => println("echo " + msg)

)

echoServer ! "hi"

</pre></div>

Scala also comes with built-in support for data-parallel programming in the form of Parallel Collections<a href="#cite_note-46">[46]</a> integrated into its Standard Library since version 2.9.0.

The following example shows how to use Parallel Collections to improve performance.<a href="#cite_note-47">[47]</a>

<div class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr"><pre>val urls = List("https://scala-lang.org", "https://github.com/scala/scala")

def fromURL(url: String) = scala.io.Source.fromURL(url)

.getLines().mkString("\n")

val t = System.currentTimeMillis()

urls.par.map(fromURL(_)) // par returns parallel implementation of a collection

println("time: " + (System.currentTimeMillis - t) + "ms")

</pre></div>

Besides futures and promises, actor support, and <a href="Data_parallelism" title="Data parallelism">data parallelism</a>, Scala also supports asynchronous programming with software transactional memory, and event streams.<a href="#cite_note-48">[48]</a>

<div class="mw-heading mw-heading2"><h2 id="Cluster_computing">Cluster computing</h2></div>

The most well-known open-source cluster-computing solution written in Scala is <a href="Apache_Spark" title="Apache Spark">Apache Spark</a>. Additionally, <a href="Apache_Kafka" title="Apache Kafka">Apache Kafka</a>, the <a href="Publish%E2%80%93subscribe" class="mw-redirect" title="Publish–subscribe">publish–subscribe</a> <a href="Message_queue" title="Message queue">message queue</a> popular with Spark and other stream processing technologies, is written in Scala.

<div class="mw-heading mw-heading2"><h2 id="Testing">Testing</h2></div>

There are several ways to test code in Scala. ScalaTest supports multiple testing styles and can integrate with Java-based testing frameworks.<a href="#cite_note-49">[49]</a> ScalaCheck is a library similar to Haskell's <a href="QuickCheck" title="QuickCheck">QuickCheck</a>.<a href="#cite_note-50">[50]</a> specs2 is a library for writing executable software specifications.<a href="#cite_note-51">[51]</a> ScalaMock provides support for testing high-order and curried functions.<a href="#cite_note-52">[52]</a> <a href="JUnit" title="JUnit">JUnit</a> and <a href="TestNG" title="TestNG">TestNG</a> are popular testing frameworks written in Java.

<div class="mw-heading mw-heading2"><h2 id="Versions">Versions</h2></div>

<th scope="col">Version

</th>

<th scope="col">Released

</th>

<th scope="col">Features

</th></tr>

<tr>

</th>

<td style="text-align:right;">8 December 2003

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">19 February 2004

</td>

<td>

<ul><li>scala.Enumeration</li>

<li>Scala license was changed to the revised BSD license</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">23 March 2004

</td>

<td>

<ul><li>Support for Java static inner classes</li>

<li>Library class improvements to Iterable, Array, xml.Elem, Buffer</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">9 June 2004

</td>

<td>

<ul><li>Views</li>

<li>XML literals (to "be dropped in the near future, to be replaced with XML string interpolation"<a href="#cite_note-54">[54]</a>)</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">16 September 2004

</td>

<td>

<ul><li>Support for Microsoft .NET</li>

<li>Method closures</li>

<li>Type syntax for parameterless methods changed from <code>[] T</code> to <code>=> T</code></li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">20 June 2005

</td>

<td>

<ul><li>Attributes</li>

<li><code>match</code> keyword replaces <code>match</code> method</li>

<li>Experimental support for runtime types</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">12 March 2006

</td>

<td>

<ul><li>Compiler completely rewritten in Scala</li>

<li>Experimental support for Java generics</li>

<li><code>implicit</code> and <code>requires</code> keywords</li>

<li><code>match</code> keyword only allowed infix operator</li>

<li><code>with</code> connective is only allowed following an <code>extends</code> clause</li>

<li>Newlines can be used as statement separators in place of semicolons</li>

<li>Regular expression match patterns restricted to sequence patterns only</li>

<li>For-comprehensions admit value and pattern definitions</li>

<li>Class parameters may be prefixed by val or var</li>

<li>Private visibility has qualifiers</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">17 March 2006

</td>

<td>

<ul><li>sbaz tool integrated in the Scala distribution</li>

<li><code>match</code> keyword replaces <code>match</code> method</li>

<li>Experimental support for runtime types</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">23 August 2006

</td>

<td>

<ul><li>Protected visibility has qualifiers</li>

<li>Private members of a class can be referenced from the companion module of the class and vice versa</li>

<li>Implicit lookup generalised</li>

<li>Typed pattern match tightened for singleton types</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">23 November 2006

</td>

<td>

<ul><li>Functions returning <code>Unit</code> don't have to explicitly state a return type</li>

<li>Type variables and types are distinguished between in pattern matching</li>

<li><code>All</code> and <code>AllRef</code> renamed to <code>Nothing</code> and <code>Null</code></li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">9 March 2007

</td>

<td>

<ul><li><code>private</code> and <code>protected</code> modifiers accept a <code>[this]</code> qualifier</li>

<li>Tuples can be written with round brackets</li>

<li>Primary constructor of a class can now be marked private or protected</li>

<li>Attributes changed to annotations with new syntax</li>

<li>Self aliases</li>

<li>Operators can be combined with assignment</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">2 May 2007

</td>

<td>

<ul><li>Type parameters and abstract type members can also abstract over type constructors</li>

<li>Fields of an object can be initialized before parent constructors are called</li>

<li>Syntax change for comprehensions</li>

<li>Implicit anonymous functions (with underscores for parameters)</li>

<li>Pattern matching of anonymous functions extended to support any arty</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">27 July 2007

</td>

<td>

<ul><li>Existential types</li>

<li>Lazy values</li>

<li>Structural types</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">7 February 2008

</td>

<td>

<ul><li>Java generic types supported by default</li>

<li>Case classes functionality extended</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">14 Jul 2010

</td>

<td>

<ul><li>Revision the common, uniform, and all-encompassing framework for collection types.</li>

<li>Type specialisation</li>

<li>Named and <a href="Default_argument" title="Default argument">default arguments</a></li>

<li>Package objects</li>

<li>Improved annotations</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">12 May 2011

</td>

<td>

<ul><li>Parallel collections</li>

<li>Thread safe <code>App</code> trait replaces <code>Application</code> trait</li>

<li><code>DelayedInit</code> trait</li>

<li>Java interop improvements</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">4 January 2013

</td>

<td>

<ul><li>Value classes<a href="#cite_note-65">[65]</a></li>

<li>Implicit classes<a href="#cite_note-66">[66]</a></li>

<li>String interpolation<a href="#cite_note-67">[67]</a></li>

<li>Futures and promises<a href="#cite_note-68">[68]</a></li>

<li>Dynamic and applyDynamic<a href="#cite_note-69">[69]</a></li>

<li>Dependent method types:

<ul><li><code class="mw-highlight mw-highlight-lang-scala mw-content-ltr" dir="ltr">def identity(x: AnyRef): x.type = x // the return type says we return exactly what we got</code></li></ul></li>

<li>New bytecode emitter based on ASM:

<ul><li>Can target JDK 1.5, 1.6 and 1.7</li>

<li>Emits 1.6 bytecode by default</li>

<li>Old 1.5 backend is deprecated</li></ul></li>

<li>A new pattern matcher: rewritten from scratch to generate more robust code (no more exponential blow-up)

<ul><li>code generation and analyses are now independent (the latter can be turned off with -Xno-patmat-analysis)</li></ul></li>

<li>Scaladoc improvements

<ul><li>Implicits (-implicits flag)</li>

<li>Diagrams (-diagrams flag, requires graphviz)</li>

<li>Groups (-groups)</li></ul></li>

<li>Modularized language features<a href="#cite_note-70">[70]</a></li>

<li>Parallel collections<a href="#cite_note-71">[71]</a> are now configurable with custom thread pools</li>

<li>Akka actors now part of the distribution

<ul><li>scala.actors have been deprecated and the akka implementation is now included in the distribution.</li></ul></li>

<li>Performance improvements

<ul><li>Faster inliner</li>

<li>Range#sum is now O(1)</li></ul></li>

<li>Update of ForkJoin library</li>

<li>Fixes in immutable TreeSet/TreeMap</li>

<li>Improvements to PartialFunctions</li>

<li>Addition of ??? and NotImplementedError</li>

<li>Addition of IsTraversableOnce + IsTraversableLike type classes for extension methods</li>

<li>Deprecations and cleanup</li>

<li>Floating point and octal literal syntax deprecation</li>

<li>Removed scala.dbc</li></ul>

Experimental features

<ul><li>Scala reflection<a href="#cite_note-72">[72]</a></li>

<li>Macros<a href="#cite_note-73">[73]</a></li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">6 June 2013

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">1 October 2013

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">18 March 2014

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">5 March 2015

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">21 April 2014

</td>

<td>

<ul><li>Collection performance improvements</li>

<li>Compiler performance improvements</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">20 May 2014

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">22 July 2014

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">31 October 2014

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">8 January 2015

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">5 March 2015

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">23 June 2015

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">8 March 2016

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">18 April 2017

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">13 November 2017

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">3 November 2016

</td>

<td>

<ul><li>Java 8 required</li>

<li>Java 8 bytecode generated</li>

<li>Java 8 SAM (<a href="Functional_interface" class="mw-redirect" title="Functional interface">Functional interface</a>) language support</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">5 December 2016

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">18 April 2017

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">26 July 2017

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">17 October 2017

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">15 March 2018

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">27 April 2018

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">27 September 2018

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">4 December 2018

</td>

<td>

<ul><li>First Scala 2.12 release with the license changed to Apache v2.0</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">5 August 2019

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">10 September 2019

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">16 March 2020

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">13 July 2020

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">12 January 2021

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">28 May 2021

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">14 Sep 2021

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">10 June 2022

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">16 September 2022

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">7 June 2023

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">25 February 2024

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">11 June 2019

</td>

<td>

<ul><li>Standard collections library redesigned</li>

<li>Literal types</li>

<li>Partial type unification</li>

<li>By-name implicits</li>

<li>Compiler optimizations</li></ul>

</td></tr>

<tr>

</th>

<td style="text-align:right;">18 September 2019

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">22 April 2020

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">25 June 2020

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">19 November 2020

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">22 February 2021

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">17 May 2021

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">1 November 2021

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">12 January 2022

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">21 September 2022

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">13 October 2022

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">7 June 2023

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">11 September 2023

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">26 February 2024

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">1 May 2024

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">25 Sep 2024

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">13 May 2021

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">31 July 2021

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">7 September 2021

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">21 October 2021

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">1 February 2022

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">12 April 2022

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">21 June 2022

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">5 September 2022

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">7 November 2022

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">30 January 2023

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">30 May 2023

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">7 September 2023

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">29 February 2024

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">29 February 2024

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">29 February 2024

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">29 March 2024

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr>

<tr>

</th>

<td style="text-align:right;">16 May 2024

</td>

<td data-sort-value="" style="vertical-align:middle; text-align:center" class="table-na">—

</td></tr></tbody></table>

<div class="mw-heading mw-heading2"><h2 id="Comparison_with_other_JVM_languages">Comparison with other JVM languages</h2></div>

Scala is often compared with <a href="Groovy_(programming_language)" class="mw-redirect" title="Groovy (programming language)">Groovy</a> and <a href="Clojure" title="Clojure">Clojure</a>, two other programming languages also using the JVM. Substantial differences between these languages exist in the type system, in the extent to which each language supports object-oriented and functional programming, and in the similarity of their syntax to that of Java.

Scala is <a href="Statically_typed" class="mw-redirect" title="Statically typed">statically typed</a>, while both Groovy and Clojure are <a href="Dynamically_typed" class="mw-redirect" title="Dynamically typed">dynamically typed</a>. This makes the type system more complex and difficult to understand but allows almost all<a href="#cite_note-unsound-38">[38]</a> type errors to be caught at compile-time and can result in significantly faster execution. By contrast, dynamic typing requires more testing to ensure program correctness, and thus is generally slower, to allow greater programming flexibility and simplicity. Regarding speed differences, current versions of Groovy and Clojure allow optional type annotations to help programs avoid the overhead of dynamic typing in cases where types are practically static. This overhead is further reduced when using recent versions of the JVM, which has been enhanced with an invoke dynamic instruction for methods that are defined with dynamically typed arguments. These advances reduce the speed gap between static and dynamic typing, although a statically typed language, like Scala, is still the preferred choice when execution efficiency is very important.

Regarding programming paradigms, Scala inherits the object-oriented model of Java and extends it in various ways. Groovy, while also strongly object-oriented, is more focused in reducing verbosity. In Clojure, object-oriented programming is deemphasised with functional programming being the main strength of the language. Scala also has many functional programming facilities, including features found in advanced functional languages like Haskell, and tries to be agnostic between the two paradigms, letting the developer choose between the two paradigms or, more frequently, some combination thereof.

Regarding syntax similarity with Java, Scala inherits much of Java's syntax, as is the case with Groovy. Clojure on the other hand follows the <a href="Lisp_(programming_language)" title="Lisp (programming language)">Lisp</a> syntax, which is different in both appearance and philosophy.

<div class="mw-heading mw-heading2"><h2 id="Adoption">Adoption</h2></div>

<div class="mw-heading mw-heading3"><h3 id="Language_rankings">Language rankings</h3></div>

Back in 2013, when Scala was in version 2.10, the <a href="ThoughtWorks" class="mw-redirect" title="ThoughtWorks">ThoughtWorks</a> Technology Radar, which is an opinion based biannual report of a group of senior technologists,<a href="#cite_note-139">[139]</a> recommended Scala adoption in its languages and frameworks category.<a href="#cite_note-140">[140]</a>

In July 2014, this assessment was made more specific and now refers to a “Scala, the good parts”, which is described as “To successfully use Scala, you need to research the language and have a very strong opinion on which parts are right for you, creating your own definition of Scala, the good parts.”.<a href="#cite_note-tw2-141">[141]</a>

In the 2018 edition of the State of <a href="Java_(programming_language)" title="Java (programming language)">Java</a> survey,<a href="#cite_note-142">[142]</a> which collected data from 5160 developers on various Java-related topics, Scala places third in terms of use of alternative languages on the <a href="JVM" class="mw-redirect" title="JVM">JVM</a>. Relative to the prior year's edition of the survey, Scala's use among alternative <a href="JVM_languages" class="mw-redirect" title="JVM languages">JVM languages</a> fell from 28.4% to 21.5%, overtaken by <a href="Kotlin_(programming_language)" title="Kotlin (programming language)">Kotlin</a>, which rose from 11.4% in 2017 to 28.8% in 2018.

The Popularity of Programming Language Index,<a href="#cite_note-pypl-143">[143]</a> which tracks searches for language tutorials, ranked Scala 15th in April 2018 with a small downward trend, and 17th in Jan 2021. This makes Scala the 3rd most popular JVM-based language after <a href="Java_(programming_language)" title="Java (programming language)">Java</a> and <a href="Kotlin_(programming_language)" title="Kotlin (programming language)">Kotlin</a>, ranked 12th.

The <a rel="nofollow" class="external text" href="https://redmonk.com/sogrady/category/programming-languages/">RedMonk Programming Language</a> Rankings, which establishes rankings based on the number of <a href="GitHub" title="GitHub">GitHub</a> projects and questions asked on <a href="Stack_Overflow" title="Stack Overflow">Stack Overflow</a>, in January 2021 ranked Scala 14th.<a href="#cite_note-redmonk2-144">[144]</a> Here, Scala was placed inside a second-tier group of languages–ahead of <a href="Go_(programming_language)" title="Go (programming language)">Go</a>, <a href="PowerShell" title="PowerShell">PowerShell</a>, and Haskell, and behind <a href="Swift_(programming_language)" title="Swift (programming language)">Swift</a>, <a href="Objective-C" title="Objective-C">Objective-C</a>, <a href="Microsoft_TypeScript" class="mw-redirect" title="Microsoft TypeScript">Typescript</a>, and <a href="R_(programming_language)" title="R (programming language)">R</a>.

The <a href="TIOBE_index" title="TIOBE index">TIOBE index</a><a href="#cite_note-tiobe1-145">[145]</a> of programming language popularity employs internet search engine rankings and similar publication counting to determine language popularity. In September 2021, it showed Scala in 31st place. In this ranking, Scala was ahead of Haskell (38th) and <a href="Erlang_(programming_language)" title="Erlang (programming language)">Erlang</a>, but below <a href="Go_(programming_language)" title="Go (programming language)">Go</a> (14th), <a href="Swift_(programming_language)" title="Swift (programming language)">Swift</a> (15th), and <a href="Perl" title="Perl">Perl</a> (19th).

As of 2022, JVM-based languages such as Clojure, Groovy, and Scala are highly ranked, but still significantly less popular than the original <a href="Java_(programming_language)" title="Java (programming language)">Java</a> language, which is usually ranked in the top three places.<a href="#cite_note-redmonk2-144">[144]</a><a href="#cite_note-tiobe1-145">[145]</a>

<div class="mw-heading mw-heading3"><h3 id="Companies">Companies</h3></div>

<ul><li>In April 2009, <a href="Twitter" title="Twitter">Twitter</a> announced that it had switched large portions of its backend from <a href="Ruby_(programming_language)" title="Ruby (programming language)">Ruby</a> to Scala and intended to convert the rest.<a href="#cite_note-146">[146]</a></li>

<li><a href="Tesla%2C_Inc." title="Tesla, Inc.">Tesla, Inc.</a> uses <a href="Akka_(toolkit)" title="Akka (toolkit)">Akka</a> with Scala in the backend of the Tesla Virtual Power Plant. Thereby, the <a href="Actor_model" title="Actor model">Actor model</a> is used for representing and operating devices that together with other components make up an instance of the virtual power plant, and <a href="Reactive_Streams" title="Reactive Streams">Reactive Streams</a> are used for data collection and data processing.<a href="#cite_note-147">[147]</a></li>

<li><a href="Apache_Kafka" title="Apache Kafka">Apache Kafka</a> is implemented in Scala with regards to most of its core and other critical parts. It is maintained and extended through the open source project and by the company Confluent.<a href="#cite_note-148">[148]</a></li>

<li><a href="Gilt_Groupe" title="Gilt Groupe">Gilt</a> uses Scala and <a href="Play_Framework" title="Play Framework">Play Framework</a>.<a href="#cite_note-149">[149]</a></li>

<li><a href="Foursquare_(service)" class="mw-redirect" title="Foursquare (service)">Foursquare</a> uses Scala and <a href="Lift_(web_framework)" title="Lift (web framework)">Lift</a>.<a href="#cite_note-150">[150]</a></li>

<li><a href="Coursera" title="Coursera">Coursera</a> uses Scala and <a href="Play_Framework" title="Play Framework">Play Framework</a>.<a href="#cite_note-151">[151]</a></li>

<li><a href="Apple_Inc." title="Apple Inc.">Apple Inc.</a> uses Scala in certain teams, along with Java and the Play framework.<a href="#cite_note-152">[152]</a><a href="#cite_note-153">[153]</a></li>

<li><a href="The_Guardian" title="The Guardian">The Guardian</a> newspaper's high-traffic website <a href="Guardian.co.uk" class="mw-redirect" title="Guardian.co.uk">guardian.co.uk</a><a href="#cite_note-154">[154]</a> announced in April 2011 that it was switching from Java to Scala.<a href="#cite_note-155">[155]</a><a href="#cite_note-156">[156]</a></li>

<li><a href="The_New_York_Times" title="The New York Times">The New York Times</a> revealed in 2014 that its internal content management system Blackbeard is built using Scala, <a href="Akka_(toolkit)" title="Akka (toolkit)">Akka</a>, and Play.<a href="#cite_note-157">[157]</a></li>

<li>The <a href="Huffington_Post" class="mw-redirect" title="Huffington Post">Huffington Post</a> newspaper started to employ Scala as part of its content delivery system Athena in 2013.<a href="#cite_note-158">[158]</a></li>

<li>Swiss bank <a href="UBS" title="UBS">UBS</a> approved Scala for general production use.<a href="#cite_note-159">[159]</a></li>

<li><a href="LinkedIn" title="LinkedIn">LinkedIn</a> uses the <a href="Scalatra" title="Scalatra">Scalatra</a> <a href="Microframework" title="Microframework">microframework</a> to power its Signal API.<a href="#cite_note-160">[160]</a></li>

<li><a href="Meetup" title="Meetup">Meetup</a> uses Unfiltered toolkit for real-time APIs.<a href="#cite_note-161">[161]</a></li>

<li><a href="Remember_the_Milk" title="Remember the Milk">Remember the Milk</a> uses Unfiltered toolkit, Scala and Akka for public API and real-time updates.<a href="#cite_note-162">[162]</a></li>

<li><a href="Verizon" title="Verizon">Verizon</a> seeking to make "a next-generation framework" using Scala.<a href="#cite_note-163">[163]</a></li>

<li><a href="Airbnb" title="Airbnb">Airbnb</a> develops open-source machine-learning software "Aerosolve", written in Java and Scala.<a href="#cite_note-164">[164]</a></li>

<li><a href="Zalando" title="Zalando">Zalando</a> moved its technology stack from Java to Scala and Play.<a href="#cite_note-165">[165]</a></li>

<li><a href="SoundCloud" title="SoundCloud">SoundCloud</a> uses Scala for its back-end, employing technologies such as Finagle (micro services),<a href="#cite_note-166">[166]</a> Scalding and Spark (data processing).<a href="#cite_note-167">[167]</a></li>

<li><a href="Databricks" title="Databricks">Databricks</a> uses Scala for the <a href="Apache_Spark" title="Apache Spark">Apache Spark</a> Big Data platform.</li>

<li><a href="Morgan_Stanley" title="Morgan Stanley">Morgan Stanley</a> uses Scala extensively in their finance and asset-related projects.<a href="#cite_note-168">[168]</a></li>

<li>There are teams within <a href="Google" title="Google">Google</a> and <a href="Alphabet_Inc." title="Alphabet Inc.">Alphabet Inc.</a> that use Scala, mostly due to acquisitions such as <a href="Firebase" title="Firebase">Firebase</a><a href="#cite_note-169">[169]</a> and Nest.<a href="#cite_note-170">[170]</a></li>

<li><a href="Walmart_Canada" title="Walmart Canada">Walmart Canada</a> uses Scala for their back-end platform.<a href="#cite_note-171">[171]</a></li>

<li><a href="Duolingo" title="Duolingo">Duolingo</a> uses Scala for their back-end module that generates lessons.<a href="#cite_note-172">[172]</a></li>

<li><a href="HMRC" class="mw-redirect" title="HMRC">HMRC</a> uses Scala for many UK Government tax applications.<a href="#cite_note-173">[173]</a></li>

<li><a href="M1_Finance" title="M1 Finance">M1 Finance</a> uses Scala for their back-end platform.<a href="#cite_note-174">[174]</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="Criticism">Criticism</h2></div>

In November 2011, <a href="Yammer" class="mw-redirect" title="Yammer">Yammer</a> moved away from Scala for reasons that included the learning curve for new team members and incompatibility from one version of the Scala compiler to the next.<a href="#cite_note-175">[175]</a> In March 2015, former VP of the Platform Engineering group at <a href="Twitter" title="Twitter">Twitter</a> <a href="Raffi_Krikorian" title="Raffi Krikorian">Raffi Krikorian</a>, stated that he would not have chosen Scala in 2011 due to its <a href="Learning_curve" title="Learning curve">learning curve</a>.<a href="#cite_note-176">[176]</a> The same month, <a href="LinkedIn" title="LinkedIn">LinkedIn</a> SVP <a href="Kevin_Scott_(computer_scientist)" title="Kevin Scott (computer scientist)">Kevin Scott</a> stated their decision to "minimize [their] dependence on Scala".<a href="#cite_note-177">[177]</a>

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<ul><li><a href="Sbt" class="mw-redirect" title="Sbt">sbt</a>, a widely used build tool for Scala projects</li>

<li><a href="Apache_Spark" title="Apache Spark">Spark</a> Framework is designed to handle, and process big-data and it solely supports Scala</li>

<li><a href="Neo4j" title="Neo4j">Neo4j</a> is a java spring framework supported by Scala with domain-specific functionality, analytical capabilities, graph algorithms, and many more</li>

<li><a href="Play_Framework" title="Play Framework">Play!</a>, an open-source Web application framework that supports Scala</li>

<li><a href="Akka_(toolkit)" title="Akka (toolkit)">Akka</a>, an open-source toolkit for building concurrent and distributed applications</li>

<li><a href="Chisel_(programming_language)" title="Chisel (programming language)">Chisel</a>, an open-source language built on Scala that is used for hardware design and generation.<a href="#cite_note-178">[178]</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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<li id="cite_note-spec-16"><a href="#cite_ref-spec_16-0">^</a> Martin Odersky, "The Scala Language Specification Version 2.7"

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<li id="cite_note-28"><a href="#cite_ref-28">^</a> <a rel="nofollow" class="external text" href="https://github.com/scala/scala/pull/1718">Expunged the .net backend. by paulp · Pull Request #1718 · scala/scala · GitHub</a>. Github.com (2012-12-05). Retrieved on 2013-11-02.

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<li id="cite_note-29"><a href="#cite_ref-29">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.scala-lang.org/old/node/166">"Getting Started with Scala"</a>. Scala-lang.org. 15 July 2008. Retrieved 31 July 2014.</cite>

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<li id="cite_note-31"><a href="#cite_ref-31">^</a> Scala's built-in control structures such as <code>if</code> or <code>while</code> cannot be re-implemented. There is a research project, Scala-Virtualized, that aimed at removing these restrictions: Adriaan Moors, Tiark Rompf, Philipp Haller and Martin Odersky. <a rel="nofollow" class="external text" href="https://dl.acm.org/citation.cfm?id=2103769">Scala-Virtualized</a>. Proceedings of the ACM SIGPLAN 2012 workshop on Partial evaluation and program manipulation, 117–120. July 2012.

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<li id="cite_note-33"><a href="#cite_ref-33">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://scala-lang.org/files/archive/spec/2.13/06-expressions.html">"Expressions | Scala 2.13"</a>. scala-lang.org. Retrieved 2021-05-24.</cite>

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<li id="cite_note-35"><a href="#cite_ref-35">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://docs.scala-lang.org/overviews/collections/concrete-immutable-collection-classes.html">"Collections - Concrete Immutable Collection Classes - Scala Documentation"</a>. Retrieved 4 July 2015.</cite>

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<li id="cite_note-36"><a href="#cite_ref-36">^</a> <cite id="CITEREFDougherty" class="citation web cs1">Dougherty, Rich. <a rel="nofollow" class="external text" href="http://blog.richdougherty.com/2009/04/tail-calls-tailrec-and-trampolines.html">"Rich Dougherty's blog"</a>. Retrieved 4 July 2015.</cite>

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<li id="cite_note-37"><a href="#cite_ref-37">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.scala-lang.org/api/current/scala/util/control/TailCalls$.html">"TailCalls - Scala Standard Library API (Scaladoc) 2.10.2 - scala.util.control.TailCalls"</a>. Scala-lang.org. Retrieved 2013-06-25.</cite>

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<li id="cite_note-39"><a href="#cite_ref-39">^</a> <cite id="CITEREFGiarrusso2013" class="citation conference cs1">Giarrusso, Paolo G. (2013). "Reify your collection queries for modularity and speed!". Proceedings of the 12th annual international conference on Aspect-oriented software development. ACM. <a href="ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<a rel="nofollow" class="external text" href="https://arxiv.org/abs/1210.6284">1210.6284</a>. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2012arXiv1210.6284G">2012arXiv1210.6284G</a>. <q>Also known as pimp-my-library pattern</q></cite>

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<li id="cite_note-40"><a href="#cite_ref-40">^</a> <cite id="CITEREFGilbert2011" class="citation web cs1">Gilbert, Clint (2011-11-15). <a rel="nofollow" class="external text" href="https://www.scala-lang.org/old/node/11504%3Fpage=3.html#comment-51270">"What is highest priority for Scala to succeed in corporate world (Should be in scala-debate?) ?"</a>. Scala-lang.org. Retrieved 2019-05-08.</cite>

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<li id="cite_note-41"><a href="#cite_ref-41">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://meta.stackexchange.com/questions/184514/should-we-enrich-or-pimp-scala-libraries">"Should we "enrich" or "pimp" Scala libraries?"</a>. stackexchange.com. 17 June 2013. Retrieved 15 April 2016.</cite>

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<li id="cite_note-175"><a href="#cite_ref-175">^</a> <cite id="CITEREFHale2011" class="citation web cs1">Hale, Coda (29 November 2011). <a rel="nofollow" class="external text" href="https://codahale.com/the-rest-of-the-story/">"The Rest of the Story"</a>. codahale.com. Retrieved 7 November 2013.</cite>

</li>

<li id="cite_note-176"><a href="#cite_ref-176">^</a> <cite class="citation audio-visual cs1">Krikorian, Raffi (17 March 2015). <a rel="nofollow" class="external text" href="https://techbus.safaribooksonline.com/video/software-engineering-and-development/9781491924563">O'Reilly Software Architecture Conference 2015 Complete Video Compilation: Re-Architecting on the Fly - Raffi Krikorian - Part 3</a> (video). O'Reilly Media. Event occurs at 4:57. Retrieved 8 March 2016. <q>What I would have done differently four years ago is use Java and not used Scala as part of this rewrite. [...] it would take an engineer two months before they're fully productive and writing Scala code.</q></cite>

</li>

<li id="cite_note-177"><a href="#cite_ref-177">^</a> <cite id="CITEREFScott2015" class="citation web cs1">Scott, Kevin (11 Mar 2015). <a rel="nofollow" class="external text" href="https://www.quora.com/Is-LinkedIn-getting-rid-of-Scala">"Is LinkedIn getting rid of Scala?"</a>. quora.com. Retrieved 25 January 2016.</cite>

</li>

<li id="cite_note-178"><a href="#cite_ref-178">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://aspire.eecs.berkeley.edu/projects/chisel-constructing-hardware-in-a-scala-embedded-language/">"Chisel: Constructing Hardware in a Scala Embedded Language"</a>. UC Berkeley APSIRE. Retrieved 27 May 2020.</cite>

</li>

</ol></div>

<div class="mw-heading mw-heading2"><h2 id="Further_reading">Further reading</h2></div>

/* start https://en.wikipedia.org/ */

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<div class="side-box-text plainlist">Wikibooks has a book on the topic of: <a href="https://en.wikibooks.org/wiki/Scala" class="extiw external" title="wikibooks:Scala">Scala</a></div></div>

</div>

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<ul><li><cite id="CITEREFOderskySpoonVenners2019" class="citation book cs1">Odersky, Martin; Spoon, Lex; Venners, Bill (15 December 2019). <a rel="nofollow" class="external text" href="https://www.artima.com/shop/programming_in_scala_4ed">Programming in Scala: A Comprehensive Step-by-step Guide</a> (4th ed.). Artima Inc. p. 896. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0-9815316-1-8</bdi>.</cite></li>

<li><cite id="CITEREFHorstmann2016" class="citation book cs1">Horstmann, Cay (15 December 2016). <a rel="nofollow" class="external text" href="https://www.informit.com/title/9780134540566">Scala for the Impatient</a> (2nd ed.). <a href="Addison-Wesley_Professional" class="mw-redirect" title="Addison-Wesley Professional">Addison-Wesley Professional</a>. p. 384. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0-134-54056-6</bdi>.</cite></li>

<li><cite id="CITEREFWamplerPayne2014" class="citation book cs1">Wampler, Dean; Payne, Alex (14 December 2014). <a rel="nofollow" class="external text" href="https://oreilly.com/catalog/9781491949856/">Programming Scala: Scalability = Functional Programming + Objects</a> (2nd ed.). <a href="O'Reilly_Media" title="O'Reilly Media">O'Reilly Media</a>. p. 583. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-1-491-94985-6</bdi>.</cite></li>

<li><cite id="CITEREFSuereth2011" class="citation book cs1">Suereth, Joshua D. (Spring 2011). <a rel="nofollow" class="external text" href="https://archive.org/details/scaladepth00suer_381">Scala in Depth</a>. <a href="Manning_Publications" title="Manning Publications">Manning Publications</a>. p. <a rel="nofollow" class="external text" href="https://archive.org/details/scaladepth00suer_381/page/n245">225</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-1-935182-70-2</bdi>.</cite></li>

<li><cite id="CITEREFMeredith2011" class="citation book cs1">Meredith, Gregory (2011). <a rel="nofollow" class="external text" href="https://github.com/leithaus/XTrace/blob/monadic/src/main/book/content/monadic.pdf">Monadic Design Patterns for the Web</a> (PDF) (1st ed.). p. 300.</cite></li>

<li><cite id="CITEREFOderskySpoonVenners2008" class="citation book cs1">Odersky, Martin; Spoon, Lex; Venners, Bill (10 December 2008). <a rel="nofollow" class="external text" href="https://www.artima.com/pins1ed">Programming in Scala, eBook</a> (1st ed.). Artima Inc.</cite></li></ul>

</div>

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</style><div id="Java_(software_platform)419" style="font-size:114%;margin:0 4em"><a href="Java_(software_platform)" title="Java (software platform)">Java (software platform)</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%">Platforms</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<li><a href="Java_Platform%2C_Micro_Edition" title="Java Platform, Micro Edition">Java ME</a> (Micro)</li>

<li><a href="Java_Platform%2C_Standard_Edition" title="Java Platform, Standard Edition">Java SE</a> (Standard)</li>

<li><a href="Jakarta_EE" title="Jakarta EE">Jakarta EE</a> (Enterprise)</li>

<li><a href="Android_software_development#SDK" title="Android software development">Android SDK</a></li>

<li><a href="GraalVM" title="GraalVM">GraalVM</a></li></ul>

</div></td><td class="noviewer navbox-image" rowspan="5" style="width:1px;padding:0 0 0 2px"><div></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Technologies</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%">Oracle</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Squawk_virtual_machine" title="Squawk virtual machine">Squawk</a></li>

<li><a href="Java_Development_Kit" title="Java Development Kit">Java Development Kit</a></li>

<li><a href="OpenJDK" title="OpenJDK">OpenJDK</a></li>

<li><a href="Java_virtual_machine" title="Java virtual machine">Java virtual machine</a></li>

<li><a href="JavaFX" title="JavaFX">JavaFX</a></li>

<li><a href="Maxine_Virtual_Machine" title="Maxine Virtual Machine">Maxine VM</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Platform</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Java_applet" title="Java applet">Applets</a></li>

<li><a href="Jakarta_Servlet" title="Jakarta Servlet">Servlets</a></li>

<li><a href="MIDlet" title="MIDlet">MIDlets</a></li>

<li><a href="Java_Web_Start" title="Java Web Start">Web Start</a> (JNLP)</li>

<li><a href="Java_Platform_Module_System" title="Java Platform Module System">Modules</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Major third-party</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Blackdown_Java" title="Blackdown Java">Blackdown</a></li>

<li><a href="Eclipse_(software)" title="Eclipse (software)">Eclipse</a></li>

<li><a href="GNU_Classpath" title="GNU Classpath">GNU Classpath</a></li>

<li><a href="Apache_Harmony" title="Apache Harmony">Harmony</a></li>

<li><a href="Hibernate_(framework)" title="Hibernate (framework)">Hibernate</a></li>

<li><a href="IcedTea" title="IcedTea">IcedTea</a></li>

<li><a href="Jazelle" title="Jazelle">Jazelle</a></li>

<li><a href="Spring_Framework" title="Spring Framework">Spring</a></li>

<li><a href="Apache_Struts" title="Apache Struts">Struts</a></li>

<li><a href="Oracle_TopLink" title="Oracle TopLink">TopLink</a></li>

<li><a href="WildFly" title="WildFly">WildFly</a></li></ul>

</div></td></tr></tbody></table><div></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">History</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Java_version_history" title="Java version history">Java version history</a></li>

<li><a href="Java_Community_Process" title="Java Community Process">Java Community Process</a></li>

<li><a href="Sun_Microsystems" title="Sun Microsystems">Sun Microsystems</a></li>

<li><a href="Free_Java_implementations" title="Free Java implementations">Free Java implementations</a></li>

<li>Slogan: <a href="Write_once%2C_run_anywhere" title="Write once, run anywhere">Write once, run anywhere</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="List_of_JVM_languages" title="List of JVM languages">JVM languages</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<li><a href="BeanShell" title="BeanShell">BeanShell</a></li>

<li><a href="Clojure" title="Clojure">Clojure</a></li>

<li><a href="Groovy_(programming_language)" class="mw-redirect" title="Groovy (programming language)">Groovy</a></li>

<li><a href="JRuby" title="JRuby">JRuby</a></li>

<li><a href="Jython" title="Jython">Jython</a></li>

<li><a href="Kotlin_(programming_language)" title="Kotlin (programming language)">Kotlin</a></li>

<li><a href="Processing" title="Processing">Processing</a></li>

<li><a href="Rhino_(JavaScript_engine)" title="Rhino (JavaScript engine)">Rhino</a></li>

<li><a href="Oxygene_(programming_language)" title="Oxygene (programming language)">Oxygene</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Community</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%">Conferences</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="JavaOne" title="JavaOne">JavaOne</a></li>

<li><a href="Devoxx" title="Devoxx">Devoxx</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Organizations</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Apache_Software_Foundation" class="mw-redirect" title="Apache Software Foundation">Apache Software Foundation</a></li>

<li><a href="Eclipse_Foundation" title="Eclipse Foundation">Eclipse Foundation</a></li>

<li><a href="Java_Community_Process" title="Java Community Process">Java Community Process</a></li>

<li><a href="Oracle_Corporation" title="Oracle Corporation">Oracle Corporation</a></li>

<li><a href="Sun_Microsystems" title="Sun Microsystems">Sun Microsystems</a>, <a href="Sun_Microsystems_Laboratories" class="mw-redirect" title="Sun Microsystems Laboratories">Sun Microsystems Laboratories</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">People</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="James_Gosling" title="James Gosling">James Gosling</a></li>

<li><a href="Arthur_van_Hoff" title="Arthur van Hoff">Arthur van Hoff</a></li>

<li><a href="Urs_H%C3%B6lzle" title="Urs Hölzle">Urs Hölzle</a></li>

<li>Patrick Naughton</li></ul>

</div></td></tr></tbody></table><div></div></td></tr><tr><td class="navbox-abovebelow" colspan="3"><div> Category • <a href="Portal%3AComputer_programming" title="Portal:Computer programming">Computer programming portal</a></div></td></tr></tbody></table></div>

<div class="navbox-styles"></div><div role="navigation" class="navbox" aria-labelledby="Common_Language_Infrastructure106" style="padding:3px"><table class="nowraplinks hlist mw-collapsible autocollapse navbox-inner" style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="col" class="navbox-title" colspan="2"><div id="Common_Language_Infrastructure106" style="font-size:114%;margin:0 4em"><a href="Common_Language_Infrastructure" title="Common Language Infrastructure">Common Language Infrastructure</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%">Architecture</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Application_domain" title="Application domain">Application domain</a></li>

<li><a href="Code_Access_Security" title="Code Access Security">Code Access Security</a></li>

<li><a href="Common_Intermediate_Language" title="Common Intermediate Language">Common Intermediate Language</a>

<ul><li><a href="List_of_CIL_instructions" title="List of CIL instructions">instructions</a></li></ul></li>

<li><a href="Common_Type_System" title="Common Type System">Common Type System</a></li>

<li><a href="Platform_Invocation_Services" title="Platform Invocation Services">Platform Invocation Services</a></li>

<li><a href="Virtual_Execution_System" title="Virtual Execution System">Virtual Execution System</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Components</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Assembly_(CLI)" title="Assembly (CLI)">Assembly</a></li>

<li><a href="Delegate_(CLI)" title="Delegate (CLI)">Delegate</a></li>

<li><a href="Global_Assembly_Cache" title="Global Assembly Cache">Global Assembly Cache</a></li>

<li><a href="Manifest_(CLI)" title="Manifest (CLI)">Manifest</a></li>

<li><a href="Metadata_(CLI)" title="Metadata (CLI)">Metadata</a></li>

<li><a href="Standard_Libraries_(CLI)" title="Standard Libraries (CLI)">Standard Libraries</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Implementations</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%">Microsoft</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<li><a href=".NET_Framework" title=".NET Framework">.NET Framework</a></li>

<li><a href=".NET_Compact_Framework" title=".NET Compact Framework">.NET Compact Framework</a></li>

<li><a href=".NET_Micro_Framework" title=".NET Micro Framework">.NET Micro Framework</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Other</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<li><a href="DotGNU" title="DotGNU">DotGNU</a></li></ul>

</div></td></tr></tbody></table><div></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="List_of_CLI_languages" title="List of CLI languages">Languages</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%">Major</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<li><a href="Visual_Basic_(.NET)" title="Visual Basic (.NET)">Visual Basic</a></li>

<li><a href="PowerShell" title="PowerShell">PowerShell</a></li></ul>

<li><a href="Clojure" title="Clojure">Clojure</a></li>

<li><a href="Cobra_(programming_language)" title="Cobra (programming language)">Cobra</a></li>

<li><a href="IronScheme" title="IronScheme">IronScheme</a></li>

<li><a href="IronPython" title="IronPython">IronPython</a></li>

<li><a href="IronRuby" title="IronRuby">IronRuby</a></li>

<li><a href="JScript_.NET" title="JScript .NET">JScript .NET</a></li>

<li><a href="Nemerle" title="Nemerle">Nemerle</a></li>

<li><a href="Oxygene_(programming_language)" title="Oxygene (programming language)">Oxygene</a></li>

<li><a href="Phalanger_(compiler)" title="Phalanger (compiler)">Phalanger</a></li>

<li><a href="Microsoft_Small_Basic" title="Microsoft Small Basic">Small Basic</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Comparison</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<li><a href="Comparison_of_C_Sharp_and_Visual_Basic_.NET" title="Comparison of C Sharp and Visual Basic .NET">C# and Visual Basic .NET</a></li>

<li><a href="Comparison_of_Visual_Basic_and_Visual_Basic_.NET" title="Comparison of Visual Basic and Visual Basic .NET">Visual Basic and Visual Basic .NET</a></li></ul>

</div></td></tr></tbody></table><div></div></td></tr></tbody></table></div>

<div class="navbox-styles"></div><div role="navigation" class="navbox" aria-labelledby="Programming_languages625" style="padding:3px"><table class="nowraplinks hlist mw-collapsible expanded navbox-inner" style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="col" class="navbox-title" colspan="2"><div id="Programming_languages625" style="font-size:114%;margin:0 4em"><a href="Programming_language" title="Programming language">Programming languages</a></div></th></tr><tr><td class="navbox-abovebelow" colspan="2"><div>

<ul><li><a href="Comparison_of_programming_languages" title="Comparison of programming languages">Comparison</a></li>

<li><a href="Timeline_of_programming_languages" title="Timeline of programming languages">Timeline</a></li>

<li><a href="History_of_programming_languages" title="History of programming languages">History</a></li></ul>

</div></td></tr><tr><td colspan="2" class="navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<li><a href="ALGOL" title="ALGOL">ALGOL</a>

<ul><li><a href="Simula" title="Simula">Simula</a></li></ul></li>

<li><a href="Assembly_language" title="Assembly language">Assembly</a></li>

<li><a href="BASIC" title="BASIC">BASIC</a>

<ul><li><a href="Visual_Basic" title="Visual Basic">Visual Basic</a>

<ul><li><a href="Visual_Basic_(classic)" title="Visual Basic (classic)">classic</a></li>

<li><a href="COBOL" title="COBOL">COBOL</a></li>

<li><a href="Erlang_(programming_language)" title="Erlang (programming language)">Erlang</a>

<ul><li><a href="Elixir_(programming_language)" title="Elixir (programming language)">Elixir</a></li></ul></li>

<li><a href="Forth_(programming_language)" title="Forth (programming language)">Forth</a></li>

<li><a href="Fortran" title="Fortran">Fortran</a></li>

<li><a href="Haskell" title="Haskell">Haskell</a></li>

<li><a href="JavaScript" title="JavaScript">JavaScript</a></li>

<li><a href="Julia_(programming_language)" title="Julia (programming language)">Julia</a></li>

<li><a href="Kotlin_(programming_language)" title="Kotlin (programming language)">Kotlin</a></li>

<li><a href="MATLAB" title="MATLAB">MATLAB</a></li>

<ul><li><a href="OCaml" title="OCaml">OCaml</a></li></ul></li></ul></li>

<li><a href="Pascal_(programming_language)" title="Pascal (programming language)">Pascal</a>

<ul><li><a href="Object_Pascal" title="Object Pascal">Object Pascal</a></li></ul></li>

<li><a href="Prolog" title="Prolog">Prolog</a></li>

<li><a href="Python_(programming_language)" title="Python (programming language)">Python</a></li>

<li><a href="Scratch_(programming_language)" title="Scratch (programming language)">Scratch</a></li>

<li><a href="Shell_script" title="Shell script">Shell</a></li>

<li><a href="Smalltalk" title="Smalltalk">Smalltalk</a></li>

<li><a href="Swift_(programming_language)" title="Swift (programming language)">Swift</a></li>

</div></td></tr><tr><td class="navbox-abovebelow" colspan="2"><div>

<ul><li> Lists: <a href="List_of_programming_languages" title="List of programming languages">Alphabetical</a></li>

<li><a href="List_of_programming_languages_by_type" title="List of programming languages by type">Categorical</a></li>

<li><a href="Generational_list_of_programming_languages" title="Generational list of programming languages">Generational</a></li>

<li><a href="Non-English-based_programming_languages" title="Non-English-based programming languages">Non-English-based</a></li>

<li> Category</li></ul>

</div></td></tr></tbody></table></div>

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