["Request"] Optic composition with standard Kotlin KPropertys

[magneticflux-]

What version are you currently using?
0.13.2

What would you like to see?

Having extension functions similar to the existing ones like Getter<A,B> + Getter<B,C> -> Getter<A,C> would be super handy.

I found out that this code works (because Getter is a functional interface?):

fun example(getter: arrow.optics.Getter<T, V>) { }

fun main() {
    val prop: KProperty1<T, V> = TODO("External")
    example(prop)
}

So naturally now I want to be able to use typical optic compositions on the results.

This won't be able to replace the @optics annotation since it can't derive the equivalent Setter-type lenses, but it would make it easier to create optics on non-constructor parameters of data classes.

I am currently using this snippet:

operator fun <T, V, U> KProperty1<T, V>.plus(other: Getter<V, U>): Getter<T, U> {
    return Getter {
        other.get(this(it))
    }
}

[nomisRev]

Hey @magneticflux-,

That's a great idea, and a great addition to Optics! If you're up for adding your snippet in a small PR that'd be awesome.

(because Getter is a functional interface?):

Since Getter<S, A> is a functional interface it matches anything of the shape (source: S) -> A.
fun interface works like @FunctionalInterface in Java where you can pass anything of the same shape of the single abstract method.

[serras]

@magneticflux- since some time we have an arrow-optics-reflect which allows turning any KProperty into a lens. If you have time to have a look and give feedback, that would be awesome!

[magneticflux-]

Looks good, thank you!

Something like Scala's implicit conversions would be awesome, but unfortunately I don't think that's on the roadmap for Kotlin since it's a bit niche.