Is there any dicussion about operator functions fo...

# language-proposals

[snrostov]

Is there any dicussion about operator functions for getting and setting properties?

Example:

class Builder<T> {
    operator fun <V> getProperty(key: KProperty1<T, V>): V
    operator fun <@kotlin.internal.OnlyInputTypes V> setProperty(key: KProperty1<T, V>, value: V)    
}

class MyUser(val name: String, val age: Int)

fun buildUser() {
    val userBuilder = Builder<MyUser>()

    userBuilder.name = "test"
    userBuilder.age = 132    

    // should be compiled to:
    // userBuilder.setProperty(MyUser::name, "test")
    // userBuilder.setProperty(MyUser::age, 132)   

    // @OnlyInputTypes is required to disallow userBuilder.name = 123, since R in KProperty1 is contravariant
}

Another use case is LINQ:

class Expression<T> {
    operator fun <V> getProperty(key: KProperty1<T, V>): Expression<V>
}

fun Expression<T>.equals(other: Expression<T>): Expression<Boolean> = TODO()
fun Expression<T>.greater(other: Expression<T>): Expression<Boolean> = TODO()
fun Expression<Boolean>.and(other: Expression<Boolean>): Expression<Boolean> = TODO()

inline fun <reified T> query(predicateProvider: Expression<T>.() -> Expression<Boolean>) = TODO()

fun makeQuery() = query<MyUser> { (name == "test") and (age greater name.size) }

// lambda should be compiled to:
// (getProperty(MyUser::name) == "test") and (getProperty(MyUser::age) greater getProperty(MyUser::age).getProperty(String::size))

Second use case may be also covered by expression trees, but first likely not.

[Ruckus]

Personally, I don't see how that's much better than using the get and set operators, in which case it would just be:

userBuilder["name"] = "test"
userBuilder["age"] = 132

[snrostov]

in your example it is not that type safe. in the provided example you can set only properties of MyUser.

[snrostov]

also ide completion will not work

[Ruckus]

I don't see how your proposal is any different, so I must be missing something.

[Ruckus]

Ah, wait, I misread. I see the difference.

[snrostov]

you cannot call Builder<MyUser>().setProperty(AnotherObj::anotherProp), only MyUser properties will be accepted

[Ruckus]

I see that now. I can't say I'm a fan of the idea, but I'm sure there are many that are (I've never particularly liked LINQ)