ca-context-bounds.md

title	type	description	languages	num	previous-page	next-page
Context Bounds	section	This page demonstrates Context Bounds in Scala.	zh-cn	61	ca-given-using-clauses	ca-given-imports

In many situations the name of a [context parameter]({% link _overviews/scala3-book/ca-given-using-clauses.md %}#using-clauses) doesn’t have to be mentioned explicitly, since it’s only used by the compiler in synthesized arguments for other context parameters. In that case you don’t have to define a parameter name, and can just provide the parameter type.

Background

For example, this maximum method takes a context parameter of type Ord, only to pass it on as an argument to max:

{% tabs context-bounds-max-named-param class=tabs-scala-version %}

{% tab 'Scala 2' %}

def maximum[A](xs: List[A])(implicit ord: Ord[A]): A =
  xs.reduceLeft(max(ord))

{% endtab %}

{% tab 'Scala 3' %}

def maximum[A](xs: List[A])(using ord: Ord[A]): A =
  xs.reduceLeft(max(using ord))

{% endtab %}

{% endtabs %}

Context bounds

Given that background, a context bound is a shorthand syntax for expressing the pattern of, “a context parameter applied to a type parameter.”

Using a context bound, the maximum method can be written like this:

{% tabs context-bounds-max-rewritten %}

{% tab 'Scala 2 and 3' %}

def maximum[A: Ord](xs: List[A]): A =
  xs.reduceLeft(max)

{% endtab %}

{% endtabs %}

A bound like : Ord on a type parameter A of a method or class indicates a context parameter with type Ord[A]. Under the hood, the compiler transforms this syntax into the one shown in the Background section.

For more information about context bounds, see the [“What are context bounds?”]({% link _overviews/FAQ/index.md %}#what-are-context-bounds) section of the Scala FAQ.