Scala：编译错误：找不到类型

Question

I've started to learn some Spark\\Scala\\GraphX to use it with Pregel, I've found some simple code here: http://www.cakesolutions.net/teamblogs/graphx-pregel-api-an-example and wanted to try it. So I tried to compile this code as I thought it should be(It's my first time with Scala):

/* SimpleApp.scala */
import org.apache.spark.SparkContext
import org.apache.spark.SparkContext._
import org.apache.spark.SparkConf
import org.apache.spark._
import org.apache.spark.graphx._
import org.apache.spark.rdd.RDD

class Graph[VD, ED] (
        val vertices: VertexRDD[VD],
        val edges: EdgeRDD[ED]) {
}

object SimpleApp {
    val initialMsg = 9999

def vprog(vertexId: VertexId, value: (Int, Int), message: Int): (Int, Int) = {
    if (message == initialMsg)
        value
    else
        (message min value._1, value._1)
}

def sendMsg(triplet: EdgeTriplet[(Int, Int), Boolean]): Iterator[(VertexId, Int)] = {
    val sourceVertex = triplet.srcAttr

    if (sourceVertex._1 == sourceVertex._2)
        Iterator.empty
    else 
        Iterator((triplet.dstId, sourceVertex._1))
}

def mergeMsg(msg1: Int, msg2: Int): Int = msg1 min msg2

def pregel[A]
  (initialMsg: A,
   maxIter: Int = Int.MaxValue,
   activeDir: EdgeDirection = EdgeDirection.Out)
  (vprog: (VertexId, VD, A) => VD,
   sendMsg: EdgeTriplet[VD, ED] => Iterator[(VertexId, A)],
   mergeMsg: (A, A) => A)
: Graph[VD, ED]

def main(args: Array[String]) {
    val conf = new SparkConf().setAppName("Simple Application")
    val sc = new SparkContext(conf)

    val minGraph = graph.pregel(initialMsg, 
                            Int.MaxValue, 
                            EdgeDirection.Out)(
                            vprog,
                            sendMsg,
                            mergeMsg)

    minGraph.vertices.collect.foreach{
        case (vertexId, (value, original_value)) => println(value)
    }

    sc.stop()
    }
}

But I get this error:

$ C:\"Program Files (x86)"\sbt\bin\sbt package
Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=256m; support was removed in 8.0
[info] Set current project to Simple Project (in build file:/C:/spark/simple/)
[info] Compiling 1 Scala source to C:\spark\simple\target\scala-2.10\classes...
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:42: not found: type VD
[error]         : Graph[VD, ED]
[error]                 ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:42: not found: type ED
[error]         : Graph[VD, ED]
[error]                     ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:39: not found: type VD
[error]           (vprog: (VertexId, VD, A) => VD,
[error]                              ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:39: not found: type VD
[error]           (vprog: (VertexId, VD, A) => VD,
[error]                                        ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:40: not found: type VD
[error]            sendMsg: EdgeTriplet[VD, ED] => Iterator[(VertexId, A)],
[error]                                 ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:40: not found: type ED
[error]            sendMsg: EdgeTriplet[VD, ED] => Iterator[(VertexId, A)],
[error]                                     ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:35: only classes can have declared but undefined members
[error]         def pregel[A]
[error]             ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:48: not found: value graph
[error]                 val minGraph = graph.pregel(initialMsg,
[error]                                ^
[error] 8 errors found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 5 s, completed Jan 17, 2017 12:35:00 AM

I'm fairly new to Scala so I don't exactly understand VD\\ED role in these defines.

Answer 1

The problem is that you cannot use a type for your method definition that hasn't been declared as a type parameter somewhere in scope (eg that method, or containing class etc.).

Look at your method def pregel[A] . It returns a value of type Graph[VD, ED] . But how can compiler know what VD refers to? Without getting into what your code does, fixing this problem is easy if you just add VD as a method type parameter:

def pregel[A, VD]

Note that if pregel were a method within the class Graph , it would be OK because Graph defines that type: class Graph[VD, ED] . By the code you posted, it would seem that your methods are lingering in the middle of nowhere, which is not allowed - perhaps you may want to consider moving them inside the Graph class?