There should be a way to specify that a function’s...

# language-proposals

[Marc Knaup]

There should be a way to specify that a function’s type parameter is contravariant.

Example:

inline fun <R> Any.toIntOrElse(elseBlock: () -> R): R where Int: R = …

should allow Int, Int?, Any and Any? as R.

[stantronic]

Is that not possible already?

[Marc Knaup]

I don’t know any way to achieve that 🤔

[Dico]

You mean the where Int : R part?

[Dico]

What is the point of this? What is the use case?

[Marc Knaup]

If R is nullable then the result of the function becomes nullable too.
Or if I want to return for example Number in the else block the result widens from Int to Number.
If you have Int the benefits of widening are limited, but they larger the hierarchy the more useful it becomes.

Nullable vs. non-nullable is main use-case though. I ran into that problem several times now.

[Dico]

I did not mean to also send that to channel, sorry

[Dico]

If you want to return Number instead of Int then the result should simply be Number.

[Dico]

Same with Any instead of Number

[Dico]

Perhaps you can do this:

fun <R, T> Any.toIntOrElse(elseBlock: () -> R): R
   where T : R, T : Int {...}

[Marc Knaup]

Nope, that won’t work either:

Type parameter cannot have any other bounds if it’s bounded by another type parameter

[Dico]

well, good try I guess