Implement lens generation macros

[liufengyun]

The following macros are used to cut down boilerplate when generating lens.

• GenLens
• GenPrism
• GenIso
  • 0 field
  • 1 field
  • n fields (blocked by missing support for whitebox macros)

Links

• http://julien-truffaut.github.io/Monocle/optics/prism.html#prism-generation
• http://julien-truffaut.github.io/Monocle/optics/lens.html#lens-generation
• https://github.com/julien-truffaut/Monocle/tree/master/macro/shared/src/main/scala/monocle/macros

Thanks to @milessabin for refering the macros to us.

[julien-truffaut]

Thanks for looking into this! Another macro I wanted to implement but never got the time is GenLens equivalent for common fields of Coproduct e.g.

sealed trait Foo
case class Foo1(name: String, i: Int) extends Foo
case class Foo2(name: String, b: Boolean) extends Foo

val name = GenLens[Foo](_.name)

but it would fail if they were a 3 implementation of Foo like:

case class Foo3(x: Int) extends Foo

[milessabin]

I'm pretty sure the answer is "yes", but can you confirm for me that this generalizes to nested selectors, ie. _.address.street?

[julien-truffaut]

good question, it would be awesome if it was.

[milessabin]

It's pattern matching on the Tasty trees, so I think it could be generalized and Just Work, but it'd be nice to be sure.

[liufengyun]

@julien-truffaut There is some difficulty to implement the Coproduct lens in Dotty macros:

Dotty macros should type check without expansion.

Even for the non-Coproduct GenLens macro, programmers have to write GenLens[Foo, String](_.name) in Dotty instead of GenLens[Foo](_.name) in Scala2.

Nested selectors _.address.street should be able to be supported.

[milessabin]

Could you get it to typecheck by having the target type inferred as Nothing in the unexpanded context?

[liufengyun]

@milessabin The problem is that given the following macro signature:

object GenLens {
    inline def apply[S, T](get: S => T): Lens[S, T] = ~impl('(get))
}

Dotty does not support only supplying S:

5 |    val len = GenLens[Address](_.streetNumber)
  |              ^^^^^^^^^^^^^^^^^
  |              Not enough type arguments for GenLens.apply[S, T]
  |              expected: [S, T]
  |              actual:   [Address]

[liufengyun]

@nicolasstucki Any thoughts about this?

[milessabin]

In vanilla Scala the usual trick is something like,

object GenLens {
  def apply[S] = new MkGenLens[S]
  class MkGenLens[S] {
    def apply[T](get: S => T): Lens[S, T] = ...
  }
}

Then S can be explicit and T inferred. Could you do something like that here?

[liufengyun]

@milessabin Thanks a lot for sharing the trick, it works like a charm 👍 73524a7

[nafg]

I thought Dotty was supposed to solve those issues

…

On Mon, Feb 18, 2019, 4:15 PM Fengyun Liu ***@***.***> wrote: @milessabin <https://github.com/milessabin> Thanks a lot for sharing the trick, it works like a charm 👍 73524a7 <73524a7> — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#5941 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAGAUJm3K3LFbyhjTf6kcNGbY5pwlETaks5vOxftgaJpZM4bAlx5> .