I have an interface defined using a structural type like this:
trait Foo {
def collection: {
def apply(a: Int) : String
def values() : collection.Iterable[String]
}
}
}
I wanted to have one of the implementers of this interface do so using a standard mutable HashMap:
class Bar {
val collection: HashMap[Int, String] = HashMap[Int, String]()
}
It compiles, but at runtime I get a NoSuchMethod exception when referring a Bar instance through a Foo typed variable. Dumping out the object's methods via reflection I see that the HashMap's apply method takes an Object due to type erasure, and there's some crazily renamed generated apply method that does take an int. Is there a way to make generics work with structural types? Note in this particular case I was able to solve my problem using an actual trait instead of a structural type and that is overall much cleaner.
Short answer is that the apply method parameter is causing you grief because it requires some implicit conversions of the parameter (Int => Integer). Implicits are resolved at compile time, the NoSuchMethodException is likely a result of these missing implicits.
Attempt to use the values method and it should work since there are no implicits being used.
I've attempted to find a way to make this example work but have had no success so far.
Related
I am trying to get the reflected runtime method in an instance but it is not shown in the decls result:
val foo: AnyRef = new Object {
def bar = 1
}
typeOf[foo.type].decls //Does not contain bar method
I tried to use Java reflection class and it works:
foo.getClass.getDeclaredMethods //It contains bar method
But I prefer to work with MethodSymbols and Scala Type than Java Class and Method reflection. How can I get the reflected MethodSymbol?
I want an method to look up an object passed as AnyRef for a method bar and call it. Something like below:
def getBarMethodFromObj(obj: AnyRef): MethodSymbol = {
//typeOf(obj).decl(TermName("bar")) this doesn't work
}
I cannot use trait because bar can have different argument and return types and numbers. As Scala does not support varadic generic arguments, I plan to use reflection to find the method and call, but this cannot be done in Scala as well. I am currently use Java solution:
val bar = foo.getClass.getDeclaredMethods.find(_.getName == "bar")
bar.invoke(foo, params: _*)
However Java reflection does not retain generic types as it creates problem for List and Map, etc. So I want to know if I can implement this in Scala, or is there any coming solution
I don't know what you're trying to do, but removing the AnyRef annotation makes your code work:
val foo = new { def bar = 1 }
typeOf[foo.type].decls // Contains bar method
If you need a type annotation (for example, in a method signature), you can use the same structural type that the compiler infers:
val foo: { def bar: Int } = new { def bar = 1 }
If you want to get the full list of methods from another method without knowing the exact type except via generics, you may be interested in TypeTag:
import scala.reflect.runtime.universe.{ TypeTag, typeTag }
val foo = new { def bar = 1 }
def getMethods[T: TypeTag](t: T) = typeTag[T].tpe.decls
getMethods(foo) // Contains bar
If you can't use TypeTag (maybe because you can't make API changes), then you're probably best off using the Java reflection API. The Scala reflection API is generally designed to use type information, so it may not work for you if you only know the type is AnyRef.
In response to your edit:
I cannot use trait because bar can have different argument and return types and numbers.
Sure you can:
trait Foo[A, B] {
def bar(a: A): B
}
If you need multiple arguments, just have bar take a tuple instead. If you need to do some list manipulation that tuples don't support, you might consider learning about HLists and shapeless.
However Java reflection does not retain generic types...
Well, no runtime-only reflection API will help you there. There is simply no way to find out what generic parameters an AnyRef has at runtime, as that information doesn't exist on the JVM. Use TypeTags, or use a trait in the manner described above.
I'm seeing something I do not understand. I have a hierarchy of (say) Vehicles, a corresponding hierarchy of VehicalReaders, and a VehicleReader object with apply methods:
abstract class VehicleReader[T <: Vehicle] {
...
object VehicleReader {
def apply[T <: Vehicle](vehicleId: Int): VehicleReader[T] = apply(vehicleType(vehicleId))
def apply[T <: Vehicle](vehicleType VehicleType): VehicleReader[T] = vehicleType match {
case VehicleType.Car => new CarReader().asInstanceOf[VehicleReader[T]]
...
Note that when you have more than one apply method, you must specify the return type. I have no issues when there is no need to specify the return type.
The cast (.asInstanceOf[VehicleReader[T]]) is the reason for the question - without it the result is compile errors like:
type mismatch;
found : CarReader
required: VehicleReader[T]
case VehicleType.Car => new CarReader()
^
Related questions:
Why cannot the compiler see a CarReader as a VehicleReader[T]?
What is the proper type parameter and return type to use in this situation?
I suspect the root cause here is that VehicleReader is invariant on its type parameter, but making it covariant does not change the result.
I feel like this should be rather simple (i.e., this is easy to accomplish in Java with wildcards).
The problem has a very simple cause and really doesn't have anything to do with variance. Consider even more simple example:
object Example {
def gimmeAListOf[T]: List[T] = List[Int](10)
}
This snippet captures the main idea of your code. But it is incorrect:
val list = Example.gimmeAListOf[String]
What will be the type of list? We asked gimmeAListOf method specifically for List[String], however, it always returns List[Int](10). Clearly, this is an error.
So, to put it in words, when the method has a signature like method[T]: Example[T] it really declares: "for any type T you give me I will return an instance of Example[T]". Such types are sometimes called 'universally quantified', or simply 'universal'.
However, this is not your case: your function returns specific instances of VehicleReader[T] depending on the value of its parameter, e.g. CarReader (which, I presume, extends VehicleReader[Car]). Suppose I wrote something like:
class House extends Vehicle
val reader = VehicleReader[House](VehicleType.Car)
val house: House = reader.read() // Assuming there is a method VehicleReader[T].read(): T
The compiler will happily compile this, but I will get ClassCastException when this code is executed.
There are two possible fixes for this situation available. First, you can use existential (or existentially quantified) type, which can be though as a more powerful version of Java wildcards:
def apply(vehicleType: VehicleType): VehicleReader[_] = ...
Signature for this function basically reads "you give me a VehicleType and I return to you an instance of VehicleReader for some type". You will have an object of type VehicleReader[_]; you cannot say anything about type of its parameter except that this type exists, that's why such types are called existential.
def apply(vehicleType: VehicleType): VehicleReader[T] forSome {type T} = ...
This is an equivalent definition and it is probably more clear from it why these types have such properties - T type is hidden inside parameter, so you don't know anything about it but that it does exist.
But due to this property of existentials you cannot really obtain any information about real type parameters. You cannot get, say, VehicleReader[Car] out of VehicleReader[_] except via direct cast with asInstanceOf, which is dangerous, unless you store a TypeTag/ClassTag for type parameter in VehicleReader and check it before the cast. This is sometimes (in fact, most of time) unwieldy.
That's where the second option comes to the rescue. There is a clear correspondence between VehicleType and VehicleReader[T] in your code, i.e. when you have specific instance of VehicleType you definitely know concrete T in VehicleReader[T] signature:
VehicleType.Car -> CarReader (<: VehicleReader[Car])
VehicleType.Truck -> TruckReader (<: VehicleReader[Truck])
and so on.
Because of this it makes sense to add type parameter to VehicleType. In this case your method will look like
def apply[T <: Vehicle](vehicleType: VehicleType[T]): VehicleReader[T] = ...
Now input type and output type are directly connected, and the user of this method will be forced to provide a correct instance of VehicleType[T] for that T he wants. This rules out the runtime error I have mentioned earlier.
You will still need asInstanceOf cast though. To avoid casting completely you will have to move VehicleReader instantiation code (e.g. yours new CarReader()) to VehicleType, because the only place where you know real value of VehicleType[T] type parameter is where instances of this type are constructed:
sealed trait VehicleType[T <: Vehicle] {
def newReader: VehicleReader[T]
}
object VehicleType {
case object Car extends VehicleType[Car] {
def newReader = new CarReader
}
// ... and so on
}
Then VehicleReader factory method will then look very clean and be completely typesafe:
object VehicleReader {
def apply[T <: Vehicle](vehicleType: VehicleType[T]) = vehicleType.newReader
}
It seems I don't understand something important, maybe about erasure (damn it).
I have a method, which I wanted to create array of size n filled with values from gen:
def testArray[T](n: Int, gen: =>T) {
val arr = Array.fill(n)(gen)
...
}
And use it, for example as:
testArray(10, util.Random.nextInt(10))
But I get error:
scala: could not find implicit value for evidence parameter of type scala.reflect.ClassManifest[T]
val arr = Array.fill(n)(gen)
^
Please, explain what I did wrong, why this error, and what kind of code it makes impossible?
That is because in testArray the concrete type of T is not known at compile time. Your signature has to look like def testArray[T : ClassManifest](n: Int, gen: =>T), this will add an implicit parameter of type ClassManifest[T] to your method, that is automatically passed to the call of testArray and then further passed to the Array.fill call. This is called a context bound.
The Array.fill method has the following signature:
def fill[T](n: Int)(elem: => T)(implicit arg0: ClassManifest[T]): Array[T]
In order to get an instance of ClassManifest[T] you need to know the concrete type. A ClassManifest can be obtained like this:
implicitly[ClassManifest[String]]
A ClassManifest is implicitly available for every concrete type.
For any implicit error, you can add the implicits you require to the method with the type parameter:
def wrap[T](n:Int)(elem: => T)(implicit c:ClassManifest[T], o:Ordering[T])
If you did not yourself introduce ClassManifest or Ordering, the writers of the library have (most likely) provided sensible defaults for you.
If you would call the wrap method:
wrap(2)(3)
It's expanded like this:
wrap[Int](2)(3)(implicitly[ClassManifest[Int]], implicitly[Ordering[Int]])
If you introduced a custom class Person here, you would get an error for not finding an implicit instance of Ordering[Person]. The writers of the library could not have known how to order Person. You could solve that like this:
class Person
implicit val o = new Ordering[Person] { // implement required methods }
wrap(2)(new Person)
The Scala compiler looks in different scopes for implicits, an Ordering would usually not be specified like this. I suggest you look up implicit resolution on the internet to learn more about it.
Can anyone explain the compile error below? Interestingly, if I change the return type of the get() method to String, the code compiles just fine. Note that the thenReturn method has two overloads: a unary method and a varargs method that takes at least one argument. It seems to me that if the invocation is ambiguous here, then it would always be ambiguous.
More importantly, is there any way to resolve the ambiguity?
import org.scalatest.mock.MockitoSugar
import org.mockito.Mockito._
trait Thing {
def get(): java.lang.Object
}
new MockitoSugar {
val t = mock[Thing]
when(t.get()).thenReturn("a")
}
error: ambiguous reference to overloaded definition,
both method thenReturn in trait OngoingStubbing of type
java.lang.Object,java.lang.Object*)org.mockito.stubbing.OngoingStubbing[java.lang.Object]
and method thenReturn in trait OngoingStubbing of type
(java.lang.Object)org.mockito.stubbing.OngoingStubbing[java.lang.Object]
match argument types (java.lang.String)
when(t.get()).thenReturn("a")
Well, it is ambiguous. I suppose Java semantics allow for it, and it might merit a ticket asking for Java semantics to be applied in Scala.
The source of the ambiguitity is this: a vararg parameter may receive any number of arguments, including 0. So, when you write thenReturn("a"), do you mean to call the thenReturn which receives a single argument, or do you mean to call the thenReturn that receives one object plus a vararg, passing 0 arguments to the vararg?
Now, what this kind of thing happens, Scala tries to find which method is "more specific". Anyone interested in the details should look up that in Scala's specification, but here is the explanation of what happens in this particular case:
object t {
def f(x: AnyRef) = 1 // A
def f(x: AnyRef, xs: AnyRef*) = 2 // B
}
if you call f("foo"), both A and B
are applicable. Which one is more
specific?
it is possible to call B with parameters of type (AnyRef), so A is
as specific as B.
it is possible to call A with parameters of type (AnyRef,
Seq[AnyRef]) thanks to tuple
conversion, Tuple2[AnyRef,
Seq[AnyRef]] conforms to AnyRef. So
B is as specific as A. Since both are
as specific as the other, the
reference to f is ambiguous.
As to the "tuple conversion" thing, it is one of the most obscure syntactic sugars of Scala. If you make a call f(a, b), where a and b have types A and B, and there is no f accepting (A, B) but there is an f which accepts (Tuple2(A, B)), then the parameters (a, b) will be converted into a tuple.
For example:
scala> def f(t: Tuple2[Int, Int]) = t._1 + t._2
f: (t: (Int, Int))Int
scala> f(1,2)
res0: Int = 3
Now, there is no tuple conversion going on when thenReturn("a") is called. That is not the problem. The problem is that, given that tuple conversion is possible, neither version of thenReturn is more specific, because any parameter passed to one could be passed to the other as well.
In the specific case of Mockito, it's possible to use the alternate API methods designed for use with void methods:
doReturn("a").when(t).get()
Clunky, but it'll have to do, as Martin et al don't seem likely to compromise Scala in order to support Java's varargs.
Well, I figured out how to resolve the ambiguity (seems kind of obvious in retrospect):
when(t.get()).thenReturn("a", Array[Object](): _*)
As Andreas noted, if the ambiguous method requires a null reference rather than an empty array, you can use something like
v.overloadedMethod(arg0, null.asInstanceOf[Array[Object]]: _*)
to resolve the ambiguity.
If you look at the standard library APIs you'll see this issue handled like this:
def meth(t1: Thing): OtherThing = { ... }
def meth(t1: Thing, t2: Thing, ts: Thing*): OtherThing = { ... }
By doing this, no call (with at least one Thing parameter) is ambiguous without extra fluff like Array[Thing](): _*.
I had a similar problem using Oval (oval.sf.net) trying to call it's validate()-method.
Oval defines 2 validate() methods:
public List<ConstraintViolation> validate(final Object validatedObject)
public List<ConstraintViolation> validate(final Object validatedObject, final String... profiles)
Trying this from Scala:
validator.validate(value)
produces the following compiler-error:
both method validate in class Validator of type (x$1: Any,x$2: <repeated...>[java.lang.String])java.util.List[net.sf.oval.ConstraintViolation]
and method validate in class Validator of type (x$1: Any)java.util.List[net.sf.oval.ConstraintViolation]
match argument types (T)
var violations = validator.validate(entity);
Oval needs the varargs-parameter to be null, not an empty-array, so I finally got it to work with this:
validator.validate(value, null.asInstanceOf[Array[String]]: _*)
I'm trying to get a better grasp of structural type dispatch. For instance, assume I have an iterable object with a summary method that computes the mean. So o.summary() gives the mean value of the list. I might like to use structural type dispatch to enable summary(o).
Is there a set of best practices regarding o.summary() vs. summary(o)?
How does scala resolve summary(o) if I have a method summary(o: ObjectType) and summary(o: { def summary: Double})
How does structural type dispatch differ from multimethods or generic functions?
Michael Galpin gives the following discription about structural type dispatch:
Structural types are Scala’s version of “responds-to” style programming as seen in many dynamic languages. So like
def sayName ( x : { def name:String }){
println(x.name)
}
Then any object with a method called name that takes no parameters and returns a string, can be passed to sayName:
case class Person(name:String)
val dean = Person("Dean")
sayName(dean) // Dean
-- 1. In your example, I wouldn't use the summary(o) version, as this is not a very object oriented style of programming. When calling o.summary (you could drop the brackets as it has no side-effects), you are asking for the summary property of o. When calling summary(o), you are passing o to a method that calculates the summary of o. I believe that the first approach is nicer :).
I haven't used structural type dispatch much, but I assume that it is best suited (in a large system) for the case where you would have to write an interface just because one method wants a type that has some method defined. Sometimes creating that interface and forcing the clients to implement it can be awkward. Sometimes you want to use a client defined in another API which conforms to your interface but doesn't explicitly implement it. So, in my opinion, structural type dispatch serves as a nice way to make the adapter pattern implicitly (saves on boilerplate, yay!).
-- 2. Apparently if you call summary(o) and o is of ObjectType, summary(o: ObjectType) gets called (which does make sense). If you call summary(bar), in which bar is not of ObjectType, two things can happen. The call compiles if bar has the method summary() of the right signature and name or otherwise, the call doesn't compile.
Example:
scala> case class ObjectType(summary: Double)
defined class ObjectType
scala> val o = ObjectType(1.2)
o: ObjectType = ObjectType(1.2)
scala> object Test {
| def summary(o: ObjectType) { println("1") }
| def summary(o: { def summary: Double}) { println("2")}
| }
defined module Test
scala> Test.summary(o)
1
Unfortunately, something like the following does not compile due to type erasure:
scala> object Test{
| def foo(a: {def a: Int}) { println("A") }
| def foo(b: {def b: Int}) { println("B") }
| }
:6: error: double definition:
method foo:(AnyRef{def b(): Int})Unit and
method foo:(AnyRef{def a(): Int})Unit at line 5
have same type after erasure: (java.lang.Object)Unit
def foo(b: {def b: Int}) { println("B") }
-- 3. In a sense, structural type dispatch is more dynamic than generic methods, and also serves a different purpose. In a generic method you can say either: a. I want something of any type; b. I want something of a type that is a subtype of A; c. I'll take something that is a supertype of B; d. I'll take something that has an implicit conversion to type C. All of these are much stricter than just "I want a type that has the method foo with the right signature". Also, structural type dispatch does use reflection, as they are implemented by type erasure.
I don't know much about multimethods, but looking at the wikipedia article, it seems that multimethods in Scala can be achieved using pattern matching. Example:
def collide(a: Collider, b: Collider) = (a, b) match {
case (asteroid: Asteroid, spaceship: Spaceship) => // ...
case (asteroid1: Asteroid, asteroid2: Asteroid) => // ...
...
Again, you could use structural type dispatch - def collide(a: {def processCollision()}), but that depends on a design decision (and I would create an interface in this example).
-- Flaviu Cipcigan
Structural data types aren't really all that useful. That's not to say they are useless, but they definitely a niche thing.
For instance, you might want to write a generic test case for the "size" of something. You might do it like this:
def hasSize(o: { def size: Int }, s: Int): Boolean = {
o.size == s
}
This can then be used with any object that implements the "size" method, no matter its class hierarchy.
Now, they are NOT structural type dispatches. They are NOT related to dispatching, but to type definition.
And Scala is an object oriented language always. You must call methods on objects. Function calls are actually "apply" method calls. Things like "println" are just members of objects imported into scope.
I think you asked what Scala does with a call on a structural type. It uses reflection. For example, consider
def repeat(x: { def quack(): Unit }, n: Int) {
for (i <- 1 to n) x.quack()
}
The call x.quack() is compiled into a lookup of the quack method, and then a call, both using Java reflection. (You can verify that by looking at the byte codes with javap. Look for a class with a funny name like Example$$anonfun$repeat$1.)
If you think about it, it's not surprising. There is no way of making a regular method call because there is no common interface or superclass.
So, the other respondents were absolutely right that you don't want to do this unless you must. The cost is very high.