In optics, is there a way to focus on a `Either` only if it is a `Left`? I have a state that I want ...

# arrow

[Luke]

In optics, is there a way to focus on a Either only if it is a Left? I have a state that I want to modify its Either variable, something like:

data class State(val either: Either<Error, Something>)

And I want something like this:

State.either./*focusIfLeft()*/.modify(state) { left: Left<Error> -> Something().right() }

Is there a way to do that already or a need to define my own extension?

[Luke]

I wrote this for now which seems to work, but I was wondering if something already existed.

fun <T, R, L> Lens<T, Either<L, R>>.onLeft(): POptional<T, T, L, Either<L, R>> = plus(
    POptional(
        getOrModify = { (it as? Either.Left<L>)?.value?.right() ?: it.left() },
        set = { _, newValue -> newValue }
    )
)

[Ties]

I think you need a prism for that