Question

I have a script. It runs without warnings.

$ cat ~/tmp/so1.scala 
import org.yaml.snakeyaml.Yaml

class JavaMapIteratorWrapper[K,V] (map: java.util.Map[K,V]) {
  def foreach (f: Tuple2 [K, V] => Unit): Unit = {
    val iter = map.entrySet.iterator
    while (iter.hasNext) {
      val entry = iter.next
      f (entry.getKey, entry.getValue)
    }
  }
}

implicit def foreachJavaMap[K,V] (map: java.util.Map[K,V]): JavaMapIteratorWrapper[K,V] = new JavaMapIteratorWrapper[K,V](map)

val yaml = new Yaml;
(yaml load (io.Source.fromFile(argv(0)).mkString)) match {
  case map: java.util.Map [_, _] => {
    for (entry <- map) {
      entry match {
        case ("id", id: String) => System.out.println ("ID is " + id)
        case (n: String, v: String) => System.out.println (n + " = " + v)
      }
    }
  }
}

$ scala -unchecked -classpath jar/snakeyaml-1.7.jar ~/tmp/so1.scala eg/default.yaml
(program output as expected)

I’d like to extract the loop into its own function. So I try that.

$ cat ~/tmp/so2.scala 
import org.yaml.snakeyaml.Yaml

class JavaMapIteratorWrapper[K,V] (map: java.util.Map[K,V]) {
  def foreach (f: Tuple2 [K, V] => Unit): Unit = {
    val iter = map.entrySet.iterator
    while (iter.hasNext) {
      val entry = iter.next
      f (entry.getKey, entry.getValue)
    }
  }
}

implicit def foreachJavaMap[K,V] (map: java.util.Map[K,V]): JavaMapIteratorWrapper[K,V] = new JavaMapIteratorWrapper[K,V](map)

val processMap = (map: java.util.Map [_, _]) => {
  for (entry <- map) {      // line 16
    entry match {
      case ("id", id: String) => System.out.println ("ID is " + id)
      case (n: String, v: String) => System.out.println (n + " = " + v)
    }
  }
}

val yaml = new Yaml;
(yaml load (io.Source.fromFile(argv(0)).mkString)) match {
  case map: java.util.Map [_, _] => processMap (map)
}

$ scala -unchecked -classpath jar/snakeyaml-1.7.jar ~/tmp/so2.scala eg/default.yaml
(fragment of so2.scala):16: error: type mismatch;
 found   : map.type (with underlying type java.util.Map[_, _])
 required: java.util.Map[_$1,_$2] where type _$2, type _$1
  for (entry <- map) {
                 ^
one error found
!!!
discarding <script preamble>

The loop being in its own function means it requires a more specific type. Okay.

I’ll try with java.util.Map [AnyRef, AnyRef] instead of java.util.Map [_, _].

$ cat ~/tmp/so3.scala 
import org.yaml.snakeyaml.Yaml

class JavaMapIteratorWrapper[K,V] (map: java.util.Map[K,V]) {
  def foreach (f: Tuple2 [K, V] => Unit): Unit = {
    val iter = map.entrySet.iterator
    while (iter.hasNext) {
      val entry = iter.next
      f (entry.getKey, entry.getValue)
    }
  }
}

implicit def foreachJavaMap[K,V] (map: java.util.Map[K,V]): JavaMapIteratorWrapper[K,V] = new JavaMapIteratorWrapper[K,V](map)

val processMap = (map: java.util.Map [AnyRef, AnyRef]) => {
  for (entry <- map) {
    entry match {
      case ("id", id: String) => System.out.println ("ID is " + id)
      case (n: String, v: String) => System.out.println (n + " = " + v)
    }
  }
}

val yaml = new Yaml;
(yaml load (io.Source.fromFile(argv(0)).mkString)) match {
  case map: java.util.Map [AnyRef, AnyRef] => processMap (map)      // line 26
}

$ scala -unchecked -classpath jar/snakeyaml-1.7.jar ~/tmp/so3.scala eg/default.yaml
(fragment of so3.scala):26: warning: non variable type-argument AnyRef in type pattern is unchecked since it is eliminated by erasure
  case map: java.util.Map [AnyRef, AnyRef] => processMap (map)
                       ^
one warning found
!!!
discarding <script preamble>
(program output as expected)

So now it runs, but it gives me a warning. How do I eliminate that warning?

Notes:

1. org.yaml.snakeyaml.Yaml is written in Java, so I can’t use type manifests. (Can I?)
2. My real program uses several Java libraries, so I want to be warned when I make possibly false assumptions about what types
   I’m being given. But how do I tell the compiler “yes, I’ve checked this, it’s correct, don’t warn me about it again”?
3. I’m using scala 2.7.7 (because that’s the version that’s packaged with Ubuntu).

Answer 1

You could try removing your custom wrapper to start with. The (2.8.1) Scala standard library already includes a wrapper to use Java collection types more idiomatically, in scala.collection.JavaConverters. (note: the scala. prefix is not needed when importing this)

I’d also make processMap a method instead of a function, and add type params:

import collection.JavaConverters._

def processMap[K,V](map: Map[K, V]): Unit = {
  for (entry <- map) {
    entry match {
      case ("id", id: String) => System.out.println ("ID is " + id)
      case (n: String, v: String) => System.out.println (n + " = " + v)
    }
  }
}

val yaml = new Yaml
(yaml load (io.Source.fromFile(argv(0)).mkString)) match {
  case map: java.util.Map[_, _] => processMap(map.asScala)
}

Note the asScala method on the second to last line…

When dealing with Java/Scala interop, it’s generally a best practice to convert from Java to Scala collections at the earliest opportunity, and to convert back as late as possible.