DocumentationUpper Type Bounds
In Scala, type parameters and abstract types may be constrained by a type bound. Such type bounds limit the concrete values of the type variables and possibly reveal more information about the members of such types. An upper type bound T <: A declares that type variable T refers to a subtype of type A.
Here is an example which relies on an upper type bound for the implementation of the polymorphic method findSimilar:
trait Similar {
def isSimilar(x: Any): Boolean
}
case class MyInt(x: Int) extends Similar {
def isSimilar(m: Any): Boolean =
m.isInstanceOf[MyInt] &&
m.asInstanceOf[MyInt].x == x
}
object UpperBoundTest extends App {
def findSimilar[T <: Similar](e: T, xs: List[T]): Boolean =
if (xs.isEmpty) false
else if (e.isSimilar(xs.head)) true
else findSimilar[T](e, xs.tail)
val list: List[MyInt] = List(MyInt(1), MyInt(2), MyInt(3))
println(findSimilar[MyInt](MyInt(4), list))
println(findSimilar[MyInt](MyInt(2), list))
}
Without the upper type bound annotation it would not be possible to call method isSimilar in method findSimilar.
The usage of lower type bounds is discussed here.
Contents
- Introduction
- Abstract Types
- Annotations
- Classes
- Case Classes
- Compound Types
- Sequence Comprehensions
- Extractor Objects
- Generic Classes
- Implicit Parameters
- Inner Classes
- Mixin Class Composition
- Nested Functions
- Anonymous Function Syntax
- Currying
- Automatic Type-Dependent Closure Construction
- Operators
- Higher-order Functions
- Pattern Matching
- Polymorphic Methods
- Regular Expression Patterns
- Traits
- Upper Type Bounds
- Lower Type Bounds
- Explicitly Typed Self References
- Local Type Inference
- Unified Types
- Variances
- Views
- XML Processing
- Default Parameter values
- Named Parameters
