CODEWITHSUNDEEP
scalascala-2.11
pferrel
pferrel

Reputation: 5702

Using and extended type in an overriden method

I'm trying to create an abstract class that has a method that I want to take a param that is the same as or extends some base class. I thought it would be:

trait InputParams
abstract class A() {
  def input[T <: InputParams](datum: T) // abstract
  ...
}

case class ExtendedInputParams() extends InputParams
class B() extends A() {
  override def input[T <: InputParams](datum: T)...
  // the input(datum) method needs more to be able to treat datum as an
  // ExtendedInputParams, how do I do this?
  ...
}

in class B (I suspect A needs changes too) how do I define the datum type as ExtendedInputParamsfor (new B).input(some-datum-of-type-ExtendedInputParams) so the overriding input can treat datum as an ExtendedInputParams and still enforce that it is extending InputParams? I also want to enforce that the method overrides the abstract method in class A.

updated for clarity

Upvotes: 0

Views: 60

Answers (1)

som-snytt
som-snytt

Reputation: 39577

To narrow the constraint in the subclass:

scala> class P ; class Q extends P
defined class P
defined class Q

scala> class A { type T <: P ; def f[X <: T](a: X) = a.toString }
defined class A

scala> class B extends A { type T = Q ; override def f[X <: T](a: X) = a.toString * 2 }
defined class B

scala> (new B).f(new Q)
res0: String = Q@1ea930ebQ@1ea930eb

scala> (new B).f(new P)
<console>:14: error: inferred type arguments [P] do not conform to method f's type parameter bounds [A <: Q]
       (new B).f(new P)
               ^
<console>:14: error: type mismatch;
 found   : P
 required: A
       (new B).f(new P)
                 ^

scala> (new A { type T = P }).f(new P)
res3: String = P@4300e240

Reworded:

scala> class P ; class Q extends P { def q = 42 }
defined class P
defined class Q

scala> class X[T <: P] { def f[A <: T](a: A) = a.toString }
defined class X

scala> class Y[T <: Q] extends X[T] { override def f[A <: T](a: A) = a.q.toString }
defined class Y

scala> (new Y).f(new Q)
res0: String = 42

Upvotes: 1

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