I'm trying to port my application to Scala 2.10.0-M2. I'm seeing some nice improvements with better warnings from compiler. But I also got bunch of errors, all related to me mapping from Enumeration.values.
I'll give you a simple example. I'd like to have an enumeration and then pre-create bunch of objects and build a map that uses enumeration values as keys and then some matching objects as values. For example:
object Phrase extends Enumeration {
type Phrase = Value
val PHRASE1 = Value("My phrase 1")
val PHRASE2 = Value("My phrase 2")
}
class Entity(text:String)
object Test {
val myMapWithPhrases = Phrase.values.map(p => (p -> new Entity(p.toString))).toMap
}
Now this used to work just fine on Scala 2.8 and 2.9. But 2.10.0-M2 gives me following warning:
[ERROR] common/Test.scala:21: error: diverging implicit expansion for type scala.collection.generic.CanBuildFrom[common.Phrase.ValueSet,(common.Phrase.Value, common.Entity),That]
[INFO] starting with method newCanBuildFrom in object SortedSet
[INFO] val myMapWithPhrases = Phrase.values.map(p => (p -> new Entity(p.toString))).toMap
^
What's causing this and how do you fix it?
It's basically a type mismatch error. You can work around it by first converting is to a list:
scala> Phrase.values.toList.map(p => (p, new Entity(p.toString))).toMap
res15: scala.collection.immutable.Map[Phrase.Value,Entity] = Map(My phrase 1 -> Entity#d0e999, My phrase 2 -> Entity#1987acd)
For more information, see the answers to What's a “diverging implicit expansion” scalac message mean? and What is a diverging implicit expansion error?
As you can see from your error, the ValueSet that holds the enums became a SortedSet at some point. It wants to produce a SortedSet on map, but can't sort on your Entity.
Something like this works with case class Entity:
implicit object orderingOfEntity extends Ordering[Entity] {
def compare(e1: Entity, e2: Entity) = e1.text compare e2.text
}
Related
Suppose I have:
class X
{
val listPrimitive: List[Int] = null
val listX: List[X] = null
}
and I print out the return types of each method in Scala as follows:
classOf[ComplexType].getMethods().foreach { m => println(s"${m.getName}: ${m.getGenericReturnType()}") }
listPrimitive: scala.collection.immutable.List<Object>
listX: scala.collection.immutable.List<X>
So... I can determine that the listX's element type is X, but is there any way to determine via reflection that listPrimitive's element type is actually java.lang.Integer? ...
val list:List[Int] = List[Int](123);
val listErased:List[_] = list;
println(s"${listErased(0).getClass()}") // java.lang.Integer
NB. This seems not to be an issue due to JVM type erasure since I can find the types parameter of List. It looks like the scala compiler throws away this type information IFF the parameter type is java.lang.[numbers] .
UPDATE:
I suspect this type information is available, due to the following experiment. Suppose I define:
class TestX{
def f(x:X):Unit = {
val floats:List[Float] = x.listPrimitive() // type mismatch error
}
}
and X.class is imported via a jar. The full type information must be available in X.class in order that this case correctly fails to compile.
UPDATE2:
Imagine you're writing a scala extension to a Java serialization library. You need to implement a:
def getSerializer(clz:Class[_]):Serializer
function that needs to do different things depending on whether:
clz==List[Int] (or equivalently: List[java.lang.Integer])
clz==List[Float] (or equivalently: List[java.lang.Float])
clz==List[MyClass]
My problem is that I will only ever see:
clz==List[Object]
clz==List[Object]
clz==List[MyClass]
because clz is provided to this function as clz.getMethods()(i).getGenericReturnType().
Starting with clz:Class[_] how can I recover the element type information that was lost?
Its not clear to me that TypeToken will help me because its usages:
typeTag[T]
requires that I provide T (ie. at compile time).
So, one path to a solution... Given some clz:Class[_], can I determine the TypeTokens of its method's return types? Clearly this is possible as this information must be contained (somewhere) in a .class file for a scala compiler to correctly generate type mismatch errors (see above).
At the java bytecode level Ints have to be represented as something else (apparently Object) because a List can only contain objects, not primitives. So that's what java-level reflection can tell you. But the scala type information is, as you infer, present (at the bytecode level it's in an annotation, IIRC), so you should be able to inspect it with scala reflection:
import scala.reflect.runtime.universe._
val list:List[Int] = List[Int](123)
def printTypeOf[A: TypeTag](a: A) = println(typeOf[A])
printTypeOf(list)
Response to update2: you should use scala reflection to obtain a mirror, not the Class[_] object. You can go via the class name if need be:
import scala.reflect.runtime.universe._
val rm = runtimeMirror(getClass.getClassLoader)
val someClass: Class[_] = ...
val scalaMirrorOfClass = rm.staticClass(someClass.getName)
// or possibly rm.reflectClass(someClass) ?
val someObject: Any = ...
val scalaMirrorOfObject = rm.reflectClass(someObject)
I guess if you really only have the class, you could create a classloader that only loads that class? I can't imagine a use case where you wouldn't have the class, or even a value, though.
I am trying to parse the input data to case classes inside a TreeSet but it doesn't seem to work.
case class Block(letter: Char)
// This does not compile
val brokenBlocks: collection.immutable.TreeSet[Block] = "A B C".split(' ').map(letter => Block(letter(0)))(collection.breakOut)
// Although this compiles
val workingBlocks: collection.immutable.TreeSet[Int] = "A B C".split(' ').map(letter => letter(0).asDigit)(collection.breakOut)
Compilation error:
type mismatch; found : scala.collection.generic.CanBuildFrom[Any,Char,String]
required: scala.collection.generic.CanBuildFrom[Array[String],Block,scala.collection.immutable.TreeSet[Block]]
I am trying this in scala worksheet.
No relation with case classes at all. If you had tried to build a List or a Vector it would have worked fine.
A TreeSet requires that there is an Ordering on its element's type. There is one on Int, but none on Block.
If you add one, say in the companion object
object Block {
implicit val ordering: Ordering[Block] = Ordering.by(_.letter)
}
then it works.
Converting from java.util.ArrayList to scala.collection.immutable.List, the 2.10 compiler and run-time may seem to be in disagreement, about the type of val emits:
import org.ahocorasick.trie._
import scala.collection.JavaConverters._ // convert Java colllections to Scala ones
object wierd {
val trie = new Trie
def trieInit(patterns: List[String]) {
trie.onlyWholeWords();
for (pattern <- patterns)
trie.addKeyword(pattern)
}
def patternTest(text : String) : List[String] =
{
val emitsJ = trie.parseText(text)
val emits = emitsJ.asScala map (i => i.getKeyword)
println(s"converted from ${emitsJ.getClass} to ${emits.getClass}")
//return(emits)
return (List.empty[String])
}
trieInit(List("hello"))
patternTest("hello")
}
Yields:
converted from class java.util.ArrayList to class scala.collection.immutable.$colon$colon
Now changing to return the real value by changing only the return line -
import org.ahocorasick.trie._
import scala.collection.JavaConverters._ // convert Java colllections to Scala ones
object wierd {
val trie = new Trie
def trieInit(patterns: List[String]) {
trie.onlyWholeWords();
for (pattern <- patterns)
trie.addKeyword(pattern)
}
def patternTest(text : String) : List[String] =
{
val emitsJ = trie.parseText(text)
val emits = emitsJ.asScala map (i => i.getKeyword)
println(s"converted from ${emitsJ.getClass} to ${emits.getClass}")
return(emits)
//return (List.empty[String])
}
trieInit(List("hello"))
patternTest("hello")
}
Yields a compilation error:
[error] reproduce.scala:23: type mismatch;
[error] found : Iterable[String]
[error] required: List[String]
[error] return(emits)
[error] ^
[error] one error found
[error] (compile:compile) Compilation failed
What would be the simple explanation for this?
How would I better approach the conversion?
JavaConverters convert to the Scala collection closest to the Java collection that it has pimped. You still have to call toList to further transform it to the collection you want:
val emits = emitsJ.asScala.toList map (i => i.getKeyword)
See related: What is the difference between JavaConverters and JavaConversions in Scala?
So what's happening here is that the underlying object returned to you from asScala is a List but it's been downcast to an Iterable since List <: Iterable this is all fine until you want to use the Iterable as a list. The easiest option is to call toList on it.
For a little more detail you can browse the source:
case class JCollectionWrapper[A](underlying: ju.Collection[A]) extends AbstractIterable[A] with Iterable[A] {
def iterator = underlying.iterator
override def size = underlying.size
override def isEmpty = underlying.isEmpty
def newBuilder[B] = new mutable.ArrayBuffer[B]
}
So your call to asScala gives you back this wrapper. Next, your call to map is using this CanBuildFrom which is then used in this map operation, which finally gives you the result which happens to be a List because the builder is a ListBuffer and the result is a List, which again happens to be an Iterable because the can build from pattern downcasts to Iterable. Hope that explains everything :)
the return type of trie.parseText is declared as java.util.Collection[Emit]. This isn't very specific, there are lots of possible subtypes of Collection, they aren't specifying what specific type they are going to return, it could be a TreeSet, it could be a Vector, it could be an ArrayList. But as far as the compiler is concerned, it could be anything which is a sub-type of Collection. You have inspected it at run-time and seen that for some particular input, it happened to return an ArrayList, but there is no way the compiler could know this.
When you call .asScala on this, you are using this implicit definition from JavaConverters
implicit def iterableAsScalaIterableConverter[A](i: java.lang.Iterable[A]): convert.Decorators.AsScala[Iterable[A]]
Which converts a java.lang.Itaerable into a scala.collection.Iterable.
There is no converter for Collection so you get a converter for the next most specific thing, Iterable. you call map on this Iterable and get an Iterable back.
Now, just like you have inspected the runtime value of the Collection returned from parseText and seen that it was a ArrayList you have inspected the value returned from this map operation and have seen that it is a scala.collection.immutable.List, but again, the compiler can't know this, all that it can know is that you gotten something which is a subclass of Iterable back.
Simply calling .toList on the resulting Iterable should be all you need to do.
I have the following definition of an enum:
object GraphType extends Enumeration {
type Type = Value
val MESSAGE, REQUEST, ERRORS = Value
}
Now I am trying to map each of the type to the corresponding, new TimeSeries as follows:
val dataSets = ( GraphType.values map (graphType => graphType -> new TimeSeries(graphType)) ).toMap
the type system lists datasets as Map[GraphType.Value, TimeSeries] which is precisely what I want. However, compilation fails with error message:
error: diverging implicit expansion for type scala.collection.generic.CanBuildFrom[ird.replay.gui.GraphType.ValueSet,(ird.replay.gui.GraphType.Value, org.jfree.data.time.TimeSeries),That]
starting with method newCanBuildFrom in object SortedSet
val dataSets = GraphType.values map (graphType => graphType -> new TimeSeries(graphType)) toMap
COuld anyone provide some explanation for this, rather cryptic, error message? Thanks
Try converting the Set of values for the enum to a List first like so:
val dataSets = (GraphType.values.toList.map(gt => (gt, new TimeSeries(gt)))).toMap
Something about that being a Set did not agree with how you were attempting to convert it to a Map, but it seems to work just fine with a List
I can't seem to create a SortedSet that also mixes in SynchronizedSet. The crux of the problem is SortedSet requires an implicit Ordering object.
val orderByIdThenName = Ordering[(Int, String)].on[Foo](foo => foo.id -> foo.name)
new mutable.TreeSet[Foo]()(orderByIdThenName) // <- Works fine and is Ordered
new mutable.HashSet[Foo] with mutable.SynchronizedSet[Foo] // <- Mixin works
new mutable.TreeSet[Foo]()(orderByCount) with mutable.SynchronizedSet[Foo] // Fail!
The last line gives an error "Object creation impossible, since member Ordering[A] in scala.collection.SortedSetLike is undefined.
Any suggestions?
This looks to be a bug in IntelliJ. I was able to reproduce the issue and see the error in the editor, but there are no errors or warnings on compiling.
Because there's no definition of orderByCount given, I'm assuming it's something like:
val orderByCount = Ordering[Int].on[Foo](_.count)
new mutable.TreeSet[Foo]()(orderByCount) with mutable.SynchronizedSet[Foo]
UPDATE:
Came up with a way to eliminate the error in IntelliJ by overriding ordering:
new mutable.TreeSet[Foo]()(orderByCount) with mutable.SynchronizedSet[Foo] {
implicit override val ordering: Ordering[Foo] = orderByCount
}