Is it possible to constrain a Scala generic type to only types that have overloaded a specific operator?
Example:
def foo[T](...):T = {...}
foo(...) * .70
Searching Google has yet to produce a solution. I considered restricting T to be of type Numeric (or scala's equivalent), but then I want to allow users to define custom types and specify their own behaviors for these operators.
Use a typeclass.
trait Multiply[A] {
def multiply(x: A, y: Double): A
}
object Multiply {
def apply[A](implicit instance: Multiple[A]): Multiply[A] = instance
trait Ops[A] {
def typeClassInstance: Multiply[A]
def self: A
def *(y: Double): A = typeClassInstance.multiply(self, y)
}
trait ToMultiplyOps {
implicit def toMultiplyOps[A](target: A)(implicit tc: Multiply[A]): Ops[A] = new Ops[A] {
val self = target
val typeClassInstance = tc
}
}
trait AllOps[A] extends Ops[A] {
def typeClassInstance: Multiply[A]
}
object ops {
implicit def toAllMultiplyOps[A](target: A)(implicit tc: Multiply[A]): AllOps[A] = new AllOps[A] {
val self = target
val typeClassInstance = tc
}
}
}
(Note that this is equivalent to what's generated by Michael Pilquist's simulacrum library, so you don't really need all this boilerplate)
Then your users can do this:
case class Foo(x: Int)
object Foo {
implicit val multiplyFoo: Multiply[Foo] = new Multiply[Foo] {
def multiply(foo: Foo, y: Double): Foo = ???
}
}
Foo(42) * 3.1415
Yes, there are so-called structural types. You can define them as follows:
def foo[T <: {def yourOperator:Unit}(...) : T = {...}
where yourOperator is the name of the method definition and Unit the return type. In this example T will accept every type which is declaring a method called yourOperator with Unit as return type.
This could be used for example to handle a resource which is Closeable. Imagine the following code.
def operatorOnResource[T <: {def close : Unit}](resource: T) : Unit = {
//do stuff with the resource
resource.close
}
Please note that this example is also kind of a risk, because T has only constraints on its structure. Hence, even if T satisfies the structural needs the semantic of the close could be different.
