Fun fact: Unlike Int, UInt doesn't extend Number, but sometimes it does (lemmy.world)

submitted 1 year ago by SillyBanana@lemmy.world to c/kotlin@programming.dev

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I bumped into this interesting behavior around unsigned types. At first I was confused why UInt doesn't extend Number, but as I was experimenting, it got weirder. It sometimes is a Number (see case c below) and sometimes isn't (see case d)?!

val a: Int = 12
a is Number // true

val b: UInt = 12u
b is Number // doesn't compile, `Incompatible types: Number and UInt`

val c: UInt = 12u
(c as Any) is Number // true, Idea says "No cast needed"

val d: Any = 12u
d is Number // false

I guess this is partly legacy of Java, which doesn't have unsigned types, and partly effect of UInt being value class. I'm curious if someone has a deeper explanation.

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[-] daijoubu@lemmy.world 2 points 1 year ago

For your example d I'm getting true. At least when using JVM. When switching to Wasm, both c and d result in false.

So it seems that in Kotlin UInt is not a Number, but at runtime on the JVM it is.