Drop val from data classes

Language Design
1

I think that data classes should allow fields to be declared without needing val or var, in which case it would default to val.

I would assume (correct me if you think otherwise) that the vast majority of data classes would use val and not var anyway, and then it’s just extra noise.

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2

I feel this would make things more concise but less readable.

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IMHO, val and var are not extra noise. Having val and var in data classes means their behavior will match what’s expected of non data classes as well.

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3

Perhaps.

If they were both used a lot I’d agree.

Does anyone make var data classes? Maybe they do but none of the people I work with do.

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In rare cases I had to use data classes with mutable fields. Example of this are classes used for JAXB unmarshalling :slight_smile:

To assure that collections are always non-nullable I had to apply this trick:

@XmlRootElement
data class Root(
	@field:XmlElement(name = "number")
	var numbers: SortedSet<Long>
) {
	private fun afterUnmarshal(unmarshaller: Unmarshaller, parent: Any) {
		numbers = numbers ?: sortedSetOf()
	}
}

Ideally I would more prefer that there would be a syntax to make setters of primary constructor properties private.

5

Doesn’t look like anyone has mentioned that without val or var, a constructor parameter doesn’t get a backing field. I think it would be super confusing to have the lack of val or var mean different things when you do or don’t use the data keyword on a class.

Yes, the Android Room ORM required (or at least until the most recent release) that properties be var because they had to be late initialized in some cases.

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6

@joshfriend in a data class, you have to include val or var right, you can’t “leave them out”.
So I’m just saying instead of doing this

data class Person(val name: String, val age: Int)

also allow this as an equiv

data class Person(name: String, age: Int)

You can still do

data class Person(var name: String, var age: Int)

Given that (my guess only) that 98% of people use vals here for immutability, then defaulting to that just saves a bunch of noise (imo). Plus it’s 100% backwards compatible.

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