The Union Type Quiz

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

 0. Please give your typing approach an intuitive name:
    
Consistent, Unified Subsumption


 1. Please describe the guiding principle of your typing approach in one sentence:
    
Union is chosen over widening only when all types participating in the subsumption would be widened[1].

[1] https://groups.google.com/d/msg/scala-language/B6f-iubR9Mk/XFjQFsiQbpYJ


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

    ( ) Yes, even if they are lossy
    ( ) 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]?

    ( ) Always
    ( ) Never
    ( ) Sometimes: ______________________________
    (X) Other:     Need both.


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

    ( ) Always
    ( ) Never
    ( ) Sometimes: ______________________________
    (X) Other:    For GC and runtime performance yes, but it must have same semantics as Option[T]
in that it was be logically unboxed before use, c.f. the plans to make extractors nearly as
performant as case classes.


 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, to get T then use a cast. Note if this expression is the result type of a function
declared to return T, then the subsumption is to T as the result type, per my rule in question 1.
    ( ) Other: ______________________________


 9. Does your answer change if
     - trait T is sealed?
       ( ) Yes, because: __________________________________________________
       (X) No, because:  of my rule in question 1
     - A and/or B are classes?
       ( ) Yes, because: __________________________________________________
       (X) No, because:  of my rule in question 1
     - A/B/T are type constructors?
       ( ) Yes, because: __________________________________________________
       (X) No, because:  of my rule in question 1


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

    ( ) Option[Int]
    (X) Some[Int]|None, ditto question 8. Note that once we have unions, it would make sense to deprecate
Option and instead write the Option[Int] type as Some[Int]|None. Thus casts won't be necessary.
    ( ) Other: ______________________________


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

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


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

    ( ) Always
    ( ) Never
    ( ) Only if the element types enumerate all subclasses/subobjects of a sealed supertype
    (X) Other: my rule in question 1


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

    ( ) Always
    (X) Never, except where my rule in question 1 allows it.
    ( ) 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?

Structural types subsume to any nominal (i.e. named) types they are conjuncted `with`,
else to AnyRef.

We can't extract unnamed types from the union in a pattern match. Unions of nominal types
is ambitious enough for current Scala type system. There is talk about a future Scala type
system that would be more ambitious.


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?

    ( ) Yes
    (X) 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?

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


18. Will your design break existing, valid code?

    ( ) Yes
    (X) Yes, but it doesn't matter because: as far as I can see, it is the only way we
can have a consistent, unified rule for subsumption. Unfortunate it wasn't done earlier,
yet hopefully there orders-of-magnitude more Scala code to be written especially if we
closing gaping holes such as the lack of unions.
    ( ) No, because: ______________________________
    ( ) Maybe?


19. Describe how null will work with union types:

A union may have a null value.


20. Will your design make a difference whether a type has been inferred and has been specified explicitly?
 
No, always adhere to the rule I provided in question 1.