Xtend Elvis ?: operator doesn't work as expected

Question

I'm using Xtend to write an Android app, and I wanted to use the elvis operator to simplify the following (which works):

val c = if (projection != null) new MatrixCursor(projection) else new MatrixCursor(#[MediaStore$MediaColumns::DISPLAY_NAME, MediaStore$MediaColumns::SIZE])

By using elvis operator, I wrote:

val c = new MatrixCursor(projection ?: #[MediaStore$MediaColumns::DISPLAY_NAME, MediaStore$MediaColumns::SIZE])

which, as far as I understand, works in the same way.

However, I got this error in Eclipse: Type mismatch: cannot convert from Object to String[] What's wrong with it?

I'm using Xtend 2.4, the MatrixCursor constructor signature is MatrixCursor(String[]), and projection is defined explicitly as String[].

score 3 · Accepted Answer · answered Apr 17 '13 at 08:18

You are stumbling across a limitation of the type inference. The elvis operator is defined as something along these lines:

def <T> T elvis(T original, T placeholder) {
  ..
}

If order to bind the type variable T, both operand types are computed. Since the array literal #[..] is actually primarily a list literal, the binding for T is computed from String[] and List<String> rather than two String[]. Therefore the common supertype is Object so elvis is just an Object. You can convince the type system, that you want the second operand to be a String[] by explicitly casting it:

val c = new MatrixCursor(projection ?: #[DISPLAY_NAME, SIZE] as String[])

Thanks! Do you know whether this will be improved in the future, or is it by design? — Randy Sugianto 'Yuku', Apr 18 '13 at 02:55
We are likely going to improve on that one in the future. It's currently a limitation of the type inference. — Sebastian Zarnekow, Apr 18 '13 at 06:28

Xtend Elvis ?: operator doesn't work as expected

1 Answers1