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Scala 2 Book migration - Control Structures
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‎_overviews/scala3-book/ca-context-bounds.md

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type: section
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description: This page demonstrates Context Bounds in Scala.
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languages: [ru, zh-cn]
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previous-page: ca-context-parameters
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next-page: ca-given-imports
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‎_overviews/scala3-book/ca-context-parameters.md

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type: section
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description: This page demonstrates how to declare context parameters, and how the compiler infers them at call-site.
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languages: [ru, zh-cn]
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previous-page: ca-extension-methods
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next-page: ca-context-bounds
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redirect_from: /scala3/book/ca-given-using-clauses.html

‎_overviews/scala3-book/ca-contextual-abstractions-intro.md

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type: chapter
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description: This chapter provides an introduction to the Scala 3 concept of Contextual Abstractions.
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languages: [ru, zh-cn]
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num: 60
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previous-page: types-others
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next-page: ca-extension-methods
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‎_overviews/scala3-book/ca-extension-methods.md

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type: section
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description: This page demonstrates how Extension Methods work in Scala 3.
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languages: [ru, zh-cn]
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previous-page: ca-contextual-abstractions-intro
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scala3: true

‎_overviews/scala3-book/ca-given-imports.md

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type: section
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description: This page demonstrates how 'given' import statements work in Scala 3.
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languages: [ru, zh-cn]
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previous-page: ca-context-bounds
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next-page: ca-type-classes
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scala3: true

‎_overviews/scala3-book/ca-implicit-conversions.md

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type: section
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description: This page demonstrates how to implement Implicit Conversions in Scala.
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previous-page: ca-multiversal-equality
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next-page: ca-summary
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‎_overviews/scala3-book/ca-multiversal-equality.md

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type: section
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description: This page demonstrates how to implement Multiversal Equality in Scala 3.
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languages: [ru, zh-cn]
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previous-page: ca-type-classes
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scala3: true

‎_overviews/scala3-book/ca-summary.md

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type: section
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description: This page provides a summary of the Contextual Abstractions lessons.
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languages: [ru, zh-cn]
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num: 68
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previous-page: ca-implicit-conversions
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next-page: concurrency
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‎_overviews/scala3-book/ca-type-classes.md

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type: section
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description: This page demonstrates how to create and use type classes.
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languages: [ru, zh-cn]
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previous-page: ca-given-imports
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next-page: ca-multiversal-equality
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redirect_from: /scala3/book/types-type-classes.html

‎_overviews/scala3-book/collections-classes.md

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type: section
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description: This page introduces the common Scala 3 collections types and some of their methods.
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languages: [ru, zh-cn]
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previous-page: collections-intro
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‎_overviews/scala3-book/collections-intro.md

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type: chapter
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description: This page provides and introduction to the common collections classes and their methods in Scala 3.
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languages: [ru, zh-cn]
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previous-page: packaging-imports
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‎_overviews/scala3-book/collections-methods.md

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type: section
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description: This page demonstrates the common methods on the Scala 3 collections classes.
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previous-page: collections-classes
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‎_overviews/scala3-book/collections-summary.md

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type: section
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description: This page provides a summary of the Collections chapter.
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languages: [ru, zh-cn]
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previous-page: collections-methods
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next-page: fp-intro
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‎_overviews/scala3-book/concurrency.md

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type: chapter
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description: This page discusses how Scala concurrency works, with an emphasis on Scala Futures.
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languages: [ru, zh-cn]
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previous-page: ca-summary
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‎_overviews/scala3-book/control-structures.md

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#### Binding matched patterns to variables
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You can bind the matched pattern to a variable to use type-specific behavior.
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You can bind the matched pattern to a variable to use type-specific behavior:
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{% tabs pattern-binding class=tabs-scala-version %}
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{% tab 'Scala 2' for=pattern-binding %}
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{% endtab %}
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{% endtabs %}
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{% comment %}
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TODO: Add in the new Scala 3 syntax shown on this page:
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http://dotty.epfl.ch/docs/reference/changed-features/match-syntax.html
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{% endcomment %}
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You can also write the code on the right side of the `=>` on multiple lines if you think is easier to read. Here’s one example:
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{% tabs control-structures-31 class=tabs-scala-version %}
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{% tab 'Scala 2' for=control-structures-31 %}
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```scala
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count match {
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case 1 =>
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println("one, a lonely number")
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case x if x == 2 || x == 3 =>
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println("two's company, three's a crowd")
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case x if x > 3 =>
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println("4+, that's a party")
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case _ =>
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println("i'm guessing your number is zero or less")
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}
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```
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{% endtab %}
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{% tab 'Scala 3' for=control-structures-31 %}
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```scala
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count match
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case 1 =>
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println("one, a lonely number")
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case x if x == 2 || x == 3 =>
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println("two's company, three's a crowd")
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case x if x > 3 =>
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println("4+, that's a party")
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case _ =>
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println("i'm guessing your number is zero or less")
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```
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{% endtab %}
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{% endtabs %}
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In Scala 3, `match` expressions can be chained:
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{% tabs 'control-structures-32' %}
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{% tab 'Scala 3 Only' %}
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```scala
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i match
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case odd: Int if odd % 2 == 1 => "odd"
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case even: Int if even % 2 == 0 => "even"
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case _ => "not an integer"
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match
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case "even" => true
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case _ => false
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```
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{% endtab %}
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{% endtabs %}
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## try/catch/finally
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In the following example, `openAndReadAFile` is a method that does what its name implies: it opens a file and reads the text in it, assigning the result to the mutable variable `text`:
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{% tabs control-structures-31 class=tabs-scala-version %}
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{% tab 'Scala 2' for=control-structures-31 %}
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{% tabs control-structures-33 class=tabs-scala-version %}
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{% tab 'Scala 2' for=control-structures-33 %}
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```
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{% tab 'Scala 3' for=control-structures-33 %}
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try

‎_overviews/scala3-book/fun-eta-expansion.md

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type: section
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description: This page discusses Eta-Expansion, the Scala technology that automatically and transparently converts methods into functions.
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languages: [ru, zh-cn]
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previous-page: fun-function-variables
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‎_overviews/scala3-book/fun-function-variables.md

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previous-page: fun-anonymous-functions
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‎_overviews/scala3-book/fun-hofs.md

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type: section
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description: This page demonstrates how to create and use higher-order functions in Scala.
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‎_overviews/scala3-book/fun-intro.md

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Where the previous chapter introduced Scala *methods*, this chapter digs into *functions*.
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The topics that are covered include anonymous functions, function variables, and higher-order functions (HOFs), including how to create your own HOFs.
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The topics that are covered include anonymous functions, partial functions, function variables, and higher-order functions (HOFs), including how to create your own HOFs.
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‎_overviews/scala3-book/fun-partial-functions.md

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---
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title: Partial Functions
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type: section
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description: This page shows how to use function variables in Scala.
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languages: [ru, zh-cn]
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num: 31
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previous-page: fun-function-variables
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next-page: fun-eta-expansion
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---
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A partial function is a function that may not be defined for all values of its argument type. In Scala, partial functions
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are unary functions implementing the `PartialFunction[A, B]` trait, where `A` is the argument type and `B` the result type.
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To define a partial function, use a `case` identical to those used in `match` expressions:
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{% tabs fun-partial-1 %}
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{% tab 'Scala 2 and 3' %}
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```scala
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{% endtab %}
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{% endtabs %}
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To check if a partial function is for an argument, use the `isDefinedAt` method:
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{% tab 'Scala 2 and 3' %}
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```scala
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```
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{% endtab %}
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{% endtabs %}
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Trying to apply a partial function to an argument not belonging to its domain results in `MatchError`:
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{% tabs fun-partial-3 %}
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{% tab 'Scala 2 and 3' %}
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```scala
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```
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{% endtab %}
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{% endtabs %}
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### Using partial functions
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A partial function can be passed as an argument to a method:
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{% tabs fun-partial-4 %}
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{% tab 'Scala 2 and 3' %}
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```scala
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```
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{% endtab %}
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{% endtabs %}
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You can define a default value for arguments not in domain with `applyOrElse`:
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{% tabs fun-partial-5 %}
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```scala
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```
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{% endtab %}
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{% endtabs %}
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Two partial function can be composed with `orElse`, the second function will be applied for arguments where the first
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one is not defined:
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{% tabs fun-partial-6 %}
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{% tab 'Scala 2 and 3' %}
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```scala
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val incrementedEvens: PartialFunction[Int, Int] = {
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}
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val res2 = List(1, 2, 3).collect(doubledOdds.orElse(incrementedEvens)) // List(2, 3, 6)
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```
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{% endtab %}
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{% endtabs %}

‎_overviews/scala3-book/fun-summary.md

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description: This page provides a summary of the previous 'Functions' sections.
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‎_overviews/scala3-book/fun-write-map-function.md

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description: This page demonstrates how to create and use higher-order functions in Scala.
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‎_overviews/scala3-book/fun-write-method-returns-function.md

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description: This page demonstrates how to create and use higher-order functions in Scala.
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‎_overviews/scala3-book/interacting-with-java.md

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description: This page demonstrates how Scala code can interact with Java, and how Java code can interact with Scala code.
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‎_overviews/scala3-book/packaging-imports.md

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description: A discussion of using packages and imports to organize your code, build related modules of code, control scope, and help prevent namespace collisions.
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‎_overviews/scala3-book/scala-for-java-devs.md

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type: chapter
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description: This page is for Java developers who are interested in learning about Scala 3.
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‎_overviews/scala3-book/scala-for-javascript-devs.md

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description: This chapter provides an introduction to Scala 3 for JavaScript developers
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