How can I have a type that represents a Double tha...

# announcements

[Viktor Qvarfordt]

How can I have a type that represents a Double that is within a given range? For example I’d like to have a type that holds a double that is guaranteed to be between 0 and 1. I had a look at inline class but it does not provide init so I have nowhere to place an assert. Any ideas?

[Dominaezzz]

You can provide a static constructor.

[Dominaezzz]

Make the actual constructor internal.

[Viktor Qvarfordt]

How can the static constructor access the argument?

[Dominaezzz]

My bad. By static constructor I meant a top-level function.

[Viktor Qvarfordt]

How would that look? Let’s say we start with this normal class, which I would like to be inline class (for performance and so that my type Score can be used directly without doing Score.score)

class Score(val score: Double) {
    init {
        if (score < 0 || score > 1) throw IllegalArgumentException("Score must be in range [0, 1].")
    }
}

[Dominaezzz]

inline class Score internal constructor(val score: Double)

fun createScore(score: Doublue): Score {
    if (score < 0 || score > 1) throw IllegalArgumentException("Score must be in range [0, 1].")
    return Score(score)
}

[Viktor Qvarfordt]

Oh, I see! However, this still allows a Score to be initialized incorrectly by not using the createScore wrapper

[SiebelsTim]

Not outside the same module though

[Viktor Qvarfordt]

Fair point, however, I now see that Primary constructor of inline class must be public.

[Dominaezzz]

You can use the @PublishedApi attribute, or whatever it's called.

[karelpeeters]

I don't think that's right @Dominaezzz, that for when you use a private property from within a public inline function.

[karelpeeters]

Also AFAIK there's no way to constrain the allowed values in an inline class (yet?).

[karelpeeters]

(because Java code would always be able to pass illegal values)

[Dominaezzz]

I've seen stuff like this used in stdlib.... but then it's stdlib 🤷🏼 .

[Dominaezzz]

Check out the Result inline class for instance.

[Dominaezzz]

Can Java create inline classes? Could've sworn it's hidden from java.

[karelpeeters]

That's what I said right, they use @PublishedApi to use an internal property in public inline functions.

[karelpeeters]

But yeah, Result is weird in a couple of ways.