Scala, gremlin-scala, HLists, Poly2, RightFold and a missing implicit Prepend

问题

So, I am trying to encapsulate a series of operations from gremlin-scala into an HList so I can do a RightFold over them (this would allow me to construct a gremlin query as data: specifically an HList of Operations).

Here is what I mean: usually you can make a call to gremlin-scala like so:

import gremlin.scala._
import org.apache.tinkerpop.gremlin.tinkergraph.structure.TinkerFactory

def graph = TinkerFactory.createModern.asScala
graph.V.hasLabel("person").out("created").as("creations").toList.map(_.valueMap)

---> List[Map[String,Any]] = List(Map(name -> lop, lang -> java), Map(name -> ripple, lang -> java), Map(name -> lop, lang -> java), Map(name -> lop, lang -> java))

This is all well and good, but I want to be able to construct a query as data. I am modeling this as an HList of Operations like so:

sealed trait Operation

case class VertexOperation[Labels <: HList](vertex: String) extends Operation {
  def operate(graph: Graph): GremlinScala[Vertex, Labels] = {
    graph.V.hasLabel(vertex).asInstanceOf[GremlinScala[Vertex, Labels]]
  }
}

case class OutOperation[Labels <: HList](out: String) extends Operation {
  def operate(vertex: GremlinScala[Vertex, Labels]): GremlinScala[Vertex, Labels] = {
    vertex.out(out)
  }
}

Then I would be able to create a query by putting these in an HList like so:

import shapeless._

val query = OutOperation("created") :: VertexOperation("person") :: HNil

Now that I have these in an HList, I can do a RightFold to apply them one by one to the graph:

trait ApplyOperationDefault extends Poly2 {
  implicit def default[T, L <: HList] = at[T, L] ((_, acc) => acc)
}

object ApplyOperation extends ApplyOperationDefault {
  implicit def vertex[T, L <: HList, S <: HList] = at[VertexOperation[S], Graph] ((t, acc) => t.operate(acc))
  implicit def out[T, L <: HList, S <: HList] = at[OutOperation[S], GremlinScala[Vertex, S]] ((t, acc) => t.operate(acc))
}

object Operation {
  def process[Input, Output, A <: HList](operations: A, input: Input) (implicit folder: RightFolder.Aux[A, Input, ApplyOperation.type, Output]): Output = {
    operations.foldRight(input) (ApplyOperation)
  }
}

And call it like so:

val result = Operation.process(query, graph).toList

This all works! And shows great promise.

Here is where I get to my issue: when I try to do this with the as operation, I can get the Operation to compile:

case class AsOperation[A, In <: HList](step: String) extends Operation {
  def operate(g: GremlinScala[A, In]) (implicit p: Prepend[In, ::[A, HNil]]): GremlinScala[A, p.Out] = {
    g.as(step)
  }
}

(I added that (implicit p: Prepend[In, ::[A, HNil]]) in there because the compiler was complaining otherwise)... but when I try to create the implicit handler for this case along with the others, it fails:

implicit def as[T, L <: HList, A, In <: HList] = at[AsOperation[A, In], GremlinScala[A, In]] ((t, acc) => t.operate(acc))

---> could not find implicit value for parameter p: shapeless.ops.hlist.Prepend[In,shapeless.::[A,shapeless.HNil]]

So, several questions here:

What is this implicit Prepend all about, and why do I need it?
Why is it able to find the implicit Prepend when calling as normally, but fails when trying to RightFold over it?
How do I create an implicit instance of Prepend?
Once created, how would I pass it in to the invocation of operate?
What is the right way to do this??

I probably have more questions, but these are the main ones. I have been reading up on type-level programming and shapeless in general, and I really kind of love it, but this kind of stuff is maddening. I know there is some subtle type thing I am missing here, but it is hard to know where to start deciphering what is missing.

Thanks for any help! I truly want to love scala and shapeless, hoping to get over this obstacle soon.

EDIT: I made a minimal repo reproducing the problem here: https://github.com/bmeg/leprechaun

Hopefully that helps!

回答1:

Your misunderstanding is the use of Prepend. The compiler will generate it for you automatically, you don't ever need to create it manually.

As mentioned in Shapeless and gremlin scala: How do I return the result of a call to `as`? Prepend is used to keep the types of the labelled steps. The gremlin-scala readme.md goes into more depth.

The compiler actually tells you exactly what it needs: could not find implicit value for parameter p: shapeless.ops.hlist.Prepend[In,shapeless.::[A,shapeless.HNil]]

So that's what I did: add an implicit Prepend to the scope :) I just sent you a PR, it compiles fine now.

PS: You might want to update your versions of gremlin-scala.

来源：https://stackoverflow.com/questions/40754229/scala-gremlin-scala-hlists-poly2-rightfold-and-a-missing-implicit-prepend

标签

scala

types

gremlin

shapeless

hlist