Tree Lens

It might be just me, but I find updating Trees unpleasant. The API for each type of Tree has a trait that defines how to interact with it, and a val "Extractor" that provides apply and unapply.

So when you have a deeply nested immutable structure, what do you do? Throw a Lens at it!

I first looked at using shapeless for creating a lens for each part of the different trees (cause the macro auto gen), but found that this is not as nice since Tree is not a case class, and the API was a bit harder to build these trees by hand. I then switched to scalaz's Lens since they are simple to build up and compose (and I auto depend on it for every project...).

val modFlags = Lens.lensu[Modifiers, FlagSet](
  set = (m, flags) => Modifiers(flags, m.privateWithin, m.annotations),
  get = (m) => m.flags
)

val valMods = Lens.lensu[ValDef, Modifiers](
  set = (v, m) => ValDef(m, v.name, v.tpt, v.rhs),
  get = (v) => v.mods
)

val valFlags: Lens[ValDef, FlagSet] = valMods andThen modFlags

Now that I have done that, I am able to update deep leafs very simply

clazzFlags.mod(_ | CASE, clazzBody.set(clazz, newBody ::: caseDefs))

I am adding more lenses as I need them, but for a set of pre-made lenses you can find them in my playground.