Context Bounds

A context bound is a shorthand for expressing the common pattern of a context parameter that depends on a type parameter. Using a context bound, the maximum function of the last section can be written like this:

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

A bound like : Ord on a type parameter T of a method or class indicates a context parameter with Ord[T]. The context parameter(s) generated from context bounds come last in the definition of the containing method or class. For instance,

def f[T: C1 : C2, U: C3](x: T)(using y: U, z: V): R

would expand to

def f[T, U](x: T)(using y: U, z: V)(using C1[T], C2[T], C3[U]): R

Context bounds can be combined with subtype bounds. If both are present, subtype bounds come first, e.g.

def g[T <: B : C](x: T): R = ...


To ease migration, context bounds in Dotty map in Scala 3.0 to old-style implicit parameters for which arguments can be passed either with a (using ...) clause or with a normal application. From Scala 3.1 on, they will map to context parameters instead, as is described above.

If the source version is future-migration, any pairing of an evidence context parameter stemming from a context bound with a normal argument will give a migration warning. The warning indicates that a (using ...) clause is needed instead. The rewrite can be done automatically under -rewrite.


TypeParamBounds   ::=  [SubtypeBounds] {ContextBound}
ContextBound      ::=  ‘:’ Type