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Merge pull request #2659 from alvinj/merge-type-class-lessons
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‎_overviews/scala3-book/ca-context-bounds.md

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description: This page demonstrates Context Bounds in Scala 3.
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languages: [zh-cn]
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num: 61
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previous-page: types-type-classes
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previous-page: ca-given-using-clauses
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‎_overviews/scala3-book/ca-contextual-abstractions-intro.md

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@@ -5,7 +5,7 @@ description: This chapter provides an introduction to the Scala 3 concept of Con
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previous-page: types-others
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The design of Scala 3 focuses on **intent** rather than **mechanism**.
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Instead of offering one very powerful feature of implicits, Scala 3 offers several use-case oriented features:
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- **Retroactively extending classes**.
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In Scala 2, extension methods had to be encoded using implicit conversions or implicit classes.
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In contrast, in Scala 3 [extension methods][extension-methods] are now directly built into the language, leading to better error messages and improved type inference.
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- **Abstracting over contextual information**.
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[Using clauses][givens] allow programmers to abstract over information that is available in the calling context and should be passed implicitly.
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As an improvement over Scala 2 implicits, using clauses can be specified by type, freeing function signatures from term variable names that are never explicitly referred to.
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[Given instances][type-classes] allow programmers to define the _canonical value_ of a certain type.
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This makes programming with type-classes more straightforward without leaking implementation details.
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- **Retroactively extending classes**.
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In Scala 2, extension methods had to be encoded using implicit conversions or implicit classes.
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In contrast, in Scala 3 [extension methods][extension-methods] are now directly built into the language, leading to better error messages and improved type inference.
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- **Viewing one type as another**.
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Implicit conversion have been [redesigned][implicit-conversions] from the ground up as instances of a type-class `Conversion`.
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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: [zh-cn]
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num: 63
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previous-page: ca-given-imports
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next-page: ca-type-classes
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num: 59
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previous-page: ca-contextual-abstractions-intro
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‎_overviews/scala3-book/ca-given-imports.md

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@@ -5,7 +5,7 @@ description: This page demonstrates how 'given' import statements work in Scala
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‎_overviews/scala3-book/ca-given-using-clauses.md

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type: section
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description: This page demonstrates how to use 'given' instances and 'using' clauses in Scala 3.
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num: 60
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‎_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 3.
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‎_overviews/scala3-book/ca-multiversal-equality.md

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description: This page demonstrates how to implement Multiversal Equality in Scala 3.
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‎_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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‎_overviews/scala3-book/ca-type-classes.md

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title: Implementing Type Classes
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title: Type Classes
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type: section
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description: This page demonstrates how to create and use type classes in Scala 3.
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A _type class_ is an abstract, parameterized type that lets you add new behavior to any closed data type without using sub-typing.
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This is useful in multiple use-cases, for example:
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If you are coming from Java, you can think of type classes as something like [`java.util.Comparator[T]`][comparator].
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> The paper [“Type Classes as Objects and Implicits”][typeclasses-paper] (2010) by Oliveira et al. discusses the basic ideas behind type classes in Scala.
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> Even though the paper uses an older version of Scala, the ideas still hold to the current day.
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A type class is useful in multiple use-cases, for example:
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- Expressing how a type you don’t own---from the standard library or a third-party library---conforms to such behavior
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- Expressing such a behavior for multiple types without involving sub-typing relationships between those types
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For a real-world example of how type classes are used in Scala 3, see the `CanEqual` discussion in the [Multiversal Equality section][multiversal].
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[typeclasses-paper]: https://infoscience.epfl.ch/record/150280/files/TypeClasses.pdf
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[typeclasses-chapter]: {% link _overviews/scala3-book/ca-type-classes.md %}
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[comparator]: https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html
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[multiversal]: {% link _overviews/scala3-book/ca-multiversal-equality.md %}

‎_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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previous-page: ca-summary
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