X means "preferred" x means "I could live with that, too"

gistfile1.txt

The Union Type Quiz
===================

 0. Please give your typing approach an intuitive name:
    User-friendly, backward-compatible union types


 1. Please describe the guiding principle of your typing approach in one sentence:
    Union types are used for more precise typing, with some special coding for sealed hierarchies.
    It is backward compatible with existing code because the inferred union types are always a
    subtype of the formerly inferred common supertype (LUB).
    The main focus is user-friendliness, reducing the amount of surprising or infinite types the
    compiler infers.


 2. Should there be predefined implicit widening conversions between numbers?

    ( ) Yes, even if they are lossy
    (x) Yes, but only non-lossy ones
    (X) No
    ( ) Other: ______________________________


 3. What should be the inferred type of the expression
      List(1, 1.0)?

    ( ) List[Double]
    ( ) List[AnyVal]
    (X) List[Int|Double]
    ( ) Other: ______________________________


 4. What should be the inferred type of the expression
      List(1: java.lang.Integer, 1.0: java.lang.Double)?

    ( ) List[Number with Comparable[_ >: Double with Integer <: Number with Comparable[_ >: Double with Integer <: Number ...]]]
    (X) List[java.lang.Integer|java.lang.Double]
    ( ) Other: ______________________________


 5. Do you prefer a union type A|B to Either[A, B]?

    (X) Always
    ( ) Never
    ( ) Sometimes: ______________________________
    ( ) Other:     ______________________________

    Why?
    It doesn't wrap its elements, provides the API of
    the common supertype of A and B.


 6. Do prefer a nullable type T? to e. g. Option[T]?

    ( ) Always
    (X) Never
    ( ) Sometimes: ______________________________
    ( ) Other:     ______________________________

    Why?
    Option might not always be the right abstraction.


 7. Explain why your answer to A|B vs. Either[A, B] is consistent and intuitive with your answer to T? vs. Option[T]:
    Union types are more fundamental than nullable types.
    Compared to nullable types, union types provide already a useful API without
    wrapping it into a different type like Option.


 8. Given the following declarations ...

      trait T
      object A extends T
      object B extends T

    ... what should be the inferred type of
      if (true) A else B

    ( ) T
    (X) A|B
    ( ) Other: ______________________________


 9. Does your answer change if
     - trait T is sealed?
       (X) Yes, because: The common supertype should be inferred.
       ( ) No, because:  __________________________________________________
     - A and/or B are classes?
       ( ) Yes, because: __________________________________________________
       (X) No, because:  __________________________________________________
     - A/B/T are type constructors?
       ( ) Yes, because: __________________________________________________
       ( ) No, because:  __________________________________________________
       (X) No idea.


10. What should be the inferred type of
      if (true) Some(1) else None?

    (X) Option[Int]
    ( ) Some[Int]|None
    ( ) Other: ______________________________


11. What should be the inferred type of
      if (true) Nil else List(1).asInstanceOf[::[Int]]?

    (X) List[Int]
    ( ) Nil|::[Int]
    ( ) Other: ______________________________


12. When should a nominal type be preferred to a more precise union type?

    ( ) Always
    ( ) Never
    (X) Only if the element types enumerate all subclasses/subobjects of a sealed supertype
    ( ) Other: ______________________________


13. When should a more precise structural type be preferred to a union type?

    (x) Always
    ( ) Never
    (X) Only if the structural type is specified explicitly
    ( ) Other: ______________________________


14. Given the following declarations ...

      trait T { def foo: Int }
      class A extends T { def foo: Int = ???; def bar: Int = ??? }
      class B extends T { def foo: Int = ???; def bar: Int = ???; def bippy: Int = ??? }

    ... which members are allowed to be called on an instance aOrB of type A|B?

    [X] aOrB.toString
    [X] aOrB.foo
    [ ] aOrB.bar
    [ ] aOrB.bippy


15. How will inference of union types interact with structural types?
    No idea. Is there any interesting interaction?

16. Given the following definitions ...

      val x: AnyRef { def foo: Int } = null
      val y: AnyRef { def foo: Int } = null

    ... should it be allowed to call xOrY.foo?

    (X) Yes
    ( ) No
    ( ) Other: ______________________________


17. Given the following definitions ...

      val x = new AnyRef { def foo: Int = 23}
      val y = new AnyRef { def foo: Int = 42}

    ... should it be allowed to call xOrY.foo?

    (X) Yes
    ( ) No
    ( ) Other: ______________________________


18. Will your choice from above break existing, valid code?

    ( ) Yes
    ( ) Yes, but it doesn't matter because: ______________________________
    (X) No, because: Inferring union types will create only more precise types than before, never less precise code.
    ( ) Maybe?


19. Describe how null will work with union types:
    Have not thought about whether there is anything special to consider.


20. Will your design make a difference whether a type has been inferred and has been specified explicitly?
    No.