Replace `where` syntax current ```class Foo lt A B gt val a A val b B where A Bar A Baz``` proposed ```class Foo lt A Bar amp Baz B gt val a A val b B ``` not like ` amp ` is getting any use in Kotlin anyhow

Replace `where` syntax current ```class Foo lt A B gt val a kotlinlang #language-proposals

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Hullaballoonatic

07/02/2019, 9:52 PM

Replace

where

syntax current:

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class Foo<A, B>(val a: A, val b: B) where A: Bar, A: Baz

proposed:

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class Foo<A: Bar & Baz, B>(val a: A, val b: B)

not like

is getting any use in Kotlin anyhow...

➕ 4

Also, a shoot-for-the-moon proposal: Adding

or

functionality, similar to how it is used in javascript. A quick example of how that would be really nice:

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fun <N: Byte | Int | Float | Double> areaByMetres(length: N, width: N) = Area(length.toDouble().metre, width.toDouble().metre)

To allow for

areaByMetres

to accept any number type that can be safely cast up to Double for either of its params would take 8 more declarations of the function. If we were doing this for volume, then it would take 27 more. Yikes! Of course allowing for multiple possible types has a lot more ramifications, and I imagine would be very difficult to add to a strongly typed language, and is arguably no longer strongly typed.

kevinmost

07/02/2019, 9:57 PM

I think the expectation is that you'd use sealed classes for unions like that. In this situation, of course, the sealed classes require an extra allocation. Maybe value types on the JVM will save us one day?

karelpeeters

07/02/2019, 9:58 PM

There's a lot of discussion (including comments by the Kotlin devs) here, you might be interested: https://discuss.kotlinlang.org/t/union-types/77

Hullaballoonatic

07/02/2019, 9:58 PM

thank you