Hi! I have a certain dificulty with optics which i...

# arrow

[dimsuz]

Hi! I have a certain dificulty with optics which is illustrated by example:

interface HasAge { val age: Int }

@optics
data class Form(
  override val age: Int
): HasAge {
  companion object
}
// is exported out of the current "module"
val lens: Lens<HasAge, Int> = Form.age // compilation error

// in another module
fun transformAge(form: HasAge, lens: Lens<HasAge, Int>)

Can i somehow generate lens so that it can be assignable in the val lens line?

Very likely that all this trouble points to the wrong design on my part, but the thing is that full type information is lost between the modules, I cannot export full data class Form to be accessible to another module, all I've got is Any which I can cast to HasAge and then I thought I'd apply the lens (which also can be exported).

[Alejandro Serrano Mena]

let’s see what happens if we try to write the Lens directly

val lens: Lens<HasAge, Int> = Lens(
  get = { it.age },
  set = { a, i -> /* what should I do here */ }
}

[Alejandro Serrano Mena]

so you cannot write it only using HasAge methods

[Alejandro Serrano Mena]

now, why doesn’t the assignment to Form.age work? Well, if you write Lens<HasAge, Int> that means you should be able to use any HasLens, but your Form.age only works for Form!

[Alejandro Serrano Mena]

my recommendation would be to write something like this:

interface HasAge<A> {
  val ageLens: Lens<A, Int>
}

@optics data class Form(val age: Int) {
  companion object: HasAge<Form> {
    override val ageLens = Form.age
  }
}

[dimsuz]

First, thanks for the clarity, I often ask the "stupid" questions, because when in the middle of something you often need a clear view on the problem and that's usually a side-view 🙂

Second, the last sample might be just what I need! I was searching for some way to have a lens and be able to apply it to some generic data of which shape I have a limited knowledge, not concrete and full structure.