CODEWITHSUNDEEP
scalatypesclass-hierarchystructural-typing
Alexandre Martins
Alexandre Martins

Reputation: 624

How to do structural typing in scala that ALSO only accepts subclasses?

Instead of describing the problem in words, let me just show you a Scala Interpreter session that shows what I want to do.

    scala> class A extends Parent{
         | def name = "Alex"
         | }
    defined class A

    scala> class B extends Parent{
         | def name = "Bernardo"
         | }
    defined class B

    scala> def addFamilyName[T <: Parent](fn:String, c:T{def name():String}) = c.name + " " + fn
    addFamilyName: [T <: Parent](fn: String, c: T{def name(): String})java.lang.String

    scala> addFamilyName( "Martins", new A())
    <console>:11: error: type mismatch;
     found   : A
     required: ?{def name(): String}
           addFamilyName( "Martins", new A())
                             ^

So basically I want to define a type in a parameter that is both a subclass of a certain type and also contains a method with the signature def name():String.

NOTE: I'm trying to do it this way because my class hierarchy is already way to complicated. Given this, I prefer to not add a ParentWithName abstract class or trait if it can be avoided.

Upvotes: 2

Views: 262

Answers (1)

Owen
Owen

Reputation: 39366

Believe it or not, the issue is in the parentheses in the method signature. This works:

def addFamilyName[T <: Parent](fn:String, c:T{def name:String}) =
    c.name + " " + fn

Though I should add you don't actually need a type parameter. This is just as good:

def addFamilyName(fn:String, c:Parent{def name:String}) =
    c.name + " " + fn

Upvotes: 6

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