implement extractors

[dk14]

As we can't use:

scala> def extractor(p: Person) = p.address.street.name
extractor: (p: Person)String

scala> val p2 = person.modify(extractor).using(_.toUpperCase)
<console>:21: error: Path must have shape: _.field1.field2.each.field3.(...)

as it's hard to analyze such complex AST, it would be great to have something like:

person.modify(_.address.street.name).get

Which will return a List of values. This will allow to reuse a path for both read and write.
My practical need is to actually check a path at one instance of case class, but modify the same path in another instance of the same case class:

val accessor = (_: Person).modify(_.address.street.name)  //"access" alias for "modify" would be also great

if(accessor(person1).get == "Functional Road") accessor(person2).setTo("Imperative Road")

Currently, I've got to hack it with vars (or alternate the model), which is not very functional...

def getter[T, U](pm: T => PathModify[T, U])(in: T): List[U] = {
   var values: List[U] = Nil //list is just in case of "each"
   pm(in).using{x =>
     values ++= x
     x
   }
   values
}

[adamw]

so in fact you want to create lenses, right? :)

You could have: (_: Person).lens(_.x.y.z).get, .setTo, .modifyUsing, what do you think?
But maybe if you need lenses, https://github.com/julien-truffaut/Monocle already has what you need?

[jedesah]

👍 for (_: Person).lens(_.x.y.z).get, .setTo, .modifyUsing

I also have a use case where that would be super convenient. Of course I could use Monocle, but I prefer the terse syntax of quicklens.

[adamw]

There's one major problem here unfortunately: when you are using e.g. .each unwrapping. Then you wouldn't be able to get a single value (as there might be 0-many values).

A partial solution could be to constrain .lens no to allow .each unwrapping of lists and such (during compile-time), but then wouldn't the (now quite simple) API become complex?

[stanch]

FWIW, in Monocle the things that can extract/modify several values at once (but not get a single value) are called Traversals.

[jedesah]

I think it's possible to come up with both a simple and powerful API here, but that might require making the implementation more complex.

More concretely, I would suggest that get return a list if .each was used at some point in the chain. After all, the return type does not need to be static since the API makes use of macros.

[adamw]

@jedesah I would be against using whitebox macros, for two main reasons: they are not IDE-friendly (you don't know the return type until you actually run the compiler), and they won't be supported in the new scala-meta based macros (while you should be able to implement quicklens in its current form using scala meta)

[scalway]

I'm using such code right now to have .get method

class QuckLensExt[T,U](private val pm: PathModify[T, U]) extends AnyVal {
  def get = {
    var values: List[U] = Nil
    pm.using { x =>
      values ::= x
      x
    }
    values
  }
}

even if introducing get is not an option it would be nice to have cheaper way of implementing this hack. For example pm.usingPartial { case x if {values ::= x; false} => x } just to not copy object unnecessarily.