When I use the following code:
import JsonImpl.graphFormat
val js = Json.toJson(g)(graphFormat)
My code compiles and works fine but when I do this it doesn't work and says: "No Json serializer found for type SGraph. Try to implement an implicit Writes or Format for this type."
import JsonImpl.graphFormat
val js = Json.toJson(g)
JsonImpl is:
object JsonImpl{
implicit val graphFormat = Json.format[SGraph]
}
I don't want to use companion object for my SGraph class. What is the problem and why it cannot find the implicit value?
For the sake of completeness: Json.format is a macro, and when you're dealing with macros it's a good idea to make sure what you're getting back is appropriately statically typed:
object JsonImpl{
implicit val graphFormat: Format[SGraph] = Json.format[SGraph]
}
In fact this is a good idea whenever you're dealing with implicit values, and it'll save you a lot of confusion (sometimes because you've done something wrong, and sometimes because the compiler has).
Related
import scala.reflect.runtime.{universe => ru}
trait someTrait{
def getType[T: ru.TypeTag](obj: T) = ru.typeOf[T]
def reflect()={
println(getType(this)) // got someTrait type, not A type.
}
}
class A extends someTrait{
}
main(){
new A().reflect()
}
When I run main function, I got someTrait type printed out.
How can I get A type in reflect function?
Using TypeTags or ClassTags, you can't (without doing extra work in every subtype, as Ramesh's answer does). Because the compiler inserts them based on static types only.
When it sees getType(this), it first infers type parameter to getType[someTrait](this), and then turns into getType[someTrait](this)(typeTag[someTrait]). You can see A is never considered and it can't be.
As the scala document says, we cant use java reflectoin since it might cause problem.
No, Scala documentation certainly doesn't say you can't use Java reflection for this. You need to understand its limitations but exactly the same applies to Scala reflection.
I have a method accepting T and I'd like to do Json.parse(someString).as[T].
Now the T classes that I pass in have implicit formats defined such as implicit lazy val format: Format[Foo] = .... However, I'd like to be able to tell the compiler to find the implicit formats at runtime instead of complaining "No Json deserializer found for type T".
The correct way to do this is to add a context bound on T:
def yourMethod[T: Reads](...) = ...
It won't be looking for the implicits at runtime (which Scala can't do), but won't let you call the method if there is no implicit like format available in scope. And when there is, it'll just pass it through to as and any other methods which need it.
If your method needs to serialize as well as serialize, you'll need both bounds: T: Reads: Writes or just T: Format.
I'm using the following code to instantiate a scala object. This works, but there seems to be one problem: the println is printed out twice, each time with another hashcode.
import scala.reflect.runtime.universe._
import scala.reflect.runtime.{universe => ru}
object Test2 { println("init"+hashCode())}
val mirror = ru.runtimeMirror(getClass.getClassLoader)
val m = ru.typeOf[Test2.type].members.filter(_.isConstructor).head.asMethod
val m2 = mirror.reflectClass(typeOf[Test2.type].typeSymbol.asClass)
val cm = m2.reflectConstructor(m)
val e = cm.apply()
Results in:
init472467991
init2051378291
e: Any = Test2$#7a458c73
the hashCode of e is equal to the latter one (2051378291). I'm wondering why this is because as far as I know there should be only one?
EDIT: using scala version 2.12.4
JVM has no singletons*
You're invoking a private constructor of a class. Scala reflection allows it. And when you invoke a constructor, you get a new instance back.
It's actually pretty hard to make a singleton in plain Java because there are ways to construct an instance except using new Something. For instance, de-serialization might not call any constructors besides one of Object. And there's sun.misc.Unsafe#allocateInstance that can conjure new instances of any class sans java.lang.Class without calling any constructor code.
Scala object does some job behind the hood to ensure you don't accidentally create a second instance during any normal usage (e.g. it hides the constructor and handles de-serialization), but it cannot protect you from deliberately creating one. When you start using reflection, you do exactly that.
Even Java enums can be instantiated at runtime using reflection. You cannot call Class#newInstance on an enum directly (the implementation forbids it), but knowing a bit of internal details can get you there**
import java.nio.file.StandardOpenOption // first std enum I could remember for a quick dirty sample
val ctor = classOf[StandardOpenOption].getDeclaredConstructors.head
val aac = ctor.getClass.getDeclaredMethod("acquireConstructorAccessor")
aac.setAccessible(true) // unlimited power!
val ctorAccess = aac.invoke(ctor)
val newInstanceCall = ctorAccess.getClass.getDeclaredMethod("newInstance", classOf[Array[AnyRef]])
newInstanceCall.setAccessible(true)
// note that it does not throw ClassCastException, so it's a fine instance
val uhOh = newInstanceCall.invoke(ctorAccess, Array("UhOh", 42)).asInstanceOf[StandardOpenOption]
assert(uhOh.name == "UhOh")
assert(uhOh.ordinal == 42)
(interactive version # Scastie)
To get the "default" instance, you can access a public static field named MODULE$ using reflection. You can also run whole scala compiler at runtime
It's likely to be the best bet for you to not rely on reflection in whatever you're trying to achieve.
BTW, it is possible to get ScalaReflectionException trying to run your code in IntelliJ worksheet or Scastie in worksheet mode, because these things wrap your code in another object with main method
* Only tested on few versions of HotSpot JVM
** Please don't do this in any serious code! I only use this to prove a point. This is also pretty useless because it does not change values or valueOf. And yes, I only checked it on HotSpot that comes with JDK8.
How can you make code in a Scala library call type-specific code for objects supplied by a caller to that library, where the decision about which type-specific code to call is made at compile-time (statically), not at run-time?
To illustrate the concept, suppose I want to make a library function that prints objects one way if there's a CanMakeDetailedString defined for them, or just as .toString if not. See nicePrint in this example code:
import scala.language.implicitConversions
trait CanMakeDetailedString[A] extends (A => String)
def noDetailedString[A] = new CanMakeDetailedString[A] {
def apply(a: A) = a.toString
}
object Util {
def nicePrint[A](a: A)
(implicit toDetail: CanMakeDetailedString[A] = noDetailedString[A])
: Unit = println(toDetail(a))
def doStuff[A](a: A)
: Unit = { /* stuff goes here */ nicePrint(a) }
}
Now here is some test code:
object Main {
import Util._
case class Rototiller(name: String)
implicit val rototillerDetail = new CanMakeDetailedString[Rototiller] {
def apply(r: Rototiller) = s"The rototiller named ${r.name}."
}
val r = Rototiller("R51")
nicePrint(r)
doStuff(r)
}
Here's the output in Scala 2.11.2:
The rototiller named R51.
Rototiller(R51)
When I call nicePrint from the same scope where rototillerDetail is defined, the Scala compiler finds rototillerDetail and passes it implicitly to nicePrint. But when, from the same scope, I call a function in a different scope (doStuff) that calls nicePrint, the Scala compiler doesn't find rototillerDetail.
No doubt there are good reasons for that. I'm wondering, though, how can I tell the Scala compiler "If an object of the needed type exists, use it!"?
I can think of two workarounds, neither of which is satisfactory:
Supply an implicit toDetail argument to doStuff. This works, but it requires me to add an implicit toDetail argument to every function that might, somewhere lower in the call stack, have a use for a CanMakeDetailedString object. That is going to massively clutter my code.
Scrap the implicit approach altogether and do this in object-oriented style, making Rototiller inherit from CanMakeDetailedString by overriding a special new method like .toDetail.
Is there some technique, trick, or command-line switch that could enable the Scala compiler to statically resolve the right implicit object? (Rather than figuring it out dynamically, when the program is running, as in the object-oriented approach.) If not, this seems like a serious limitation on how much use library code can make of "typeclasses" or implicit arguments. In other words, what's a good way to do what I've done badly above?
Clarification: I'm not asking how this can be done with implicit val. I'm asking how you can get the Scala compiler to statically choose type-appropriate functions in library code, without explicitly listing, in every library function, an implicit argument for every function that might get called lower in the stack. It doesn't matter to me if it's done with implicits or anything else. I just want to know how to write generic code that chooses type-specific functions appropriately at compile-time.
implicits are resolved at compile time so it can't know what A is in doStuff without more information.
That information can be provided through an extra implicit parameter or a base type / interface as you suggested.
You could also use reflection on the A type, use the getType that returns the child type, cast the object to that type, and call a predefined function that has the name of the type that writes the string details for you. I don't really recommend it as any OOP or FP solution is better IMHO.
I have a function with the following signature:
myFunc[T <: AnyRef](arg: T)(implicit m: Manifest[T]) = ???
How can I invoke this function if I do not know the exact type of the argument at the compile time?
For example:
val obj: AnyRef = new Foo() // At compile time obj is defined as AnyRef,
val objClass = obj.getClass // At runtime I can figure out that it is actually Foo
// Now I would need to call `myFunc[Foo](obj.asInstanceOf[Foo])`,
// but how would I do it without putting [Foo] in the square braces?
I would want to write something logically similar to:
myFunc[objClass](obj.asInstanceOf[objClass])
Thank you!
UPDATE:
The question is invalid - As #DaoWen, #Jelmo and #itsbruce correctly pointed, the thing I was trying to do was a complete nonsense! I just overthought the problem severely.
THANK YOU guys! It's too bad I cannot accept all the answers as correct :)
So, the problem was caused by the following situation:
I am using Salat library to serialize the objects to/from BSON/JSON representation.
Salat has an Grater[T] class which is used for both serialization and deserialization.
The method call for deserialization from BSON looks this way:
val foo = grater[Foo].asObject(bson)
Here, the role of type parameter is clear. What I was trying to do then is to use the same Grater to serialize any entity from my domain model. So I wrote:
val json = grater[???].toCompactJSON(obj)
I immediately rushed for reflection and just didn't see an obvious solution lying on the surface. Which is:
grater[Entity].toCompactJSON(obj) // where Entity...
#Salat trait Entity // is a root of the domain model hierarchy
Sometimes things are much easier than we think they are! :)
It appears that while I was writing this answer the author of the question realized that he does not need to resolve Manifests at runtime. However, in my opinion it is perfectly legal problem which I resolved successfully when I was writing Yaml [de]serialization library, so I'm leaving the answer here.
It is possible to do what you want using ClassTags or even TypeTags. I don't know about Manifests because that API is deprecated and I haven't worked with it, but I believe that with manifests it will be easier since they weren't as sophisticated as new Scala reflection. FYI, Manifest's successor is TypeTag.
Suppose you have the following functions:
def useClasstag[T: ClassTag](obj: T) = ...
def useTypetag[T: TypeTag](obj: T) = ...
and you need to call then with obj: AnyRef as an argument while providing either ClassTag or TypeTag for obj.getClass class as the implicit parameter.
ClassTag is the easiest one. You can create ClassTag directly from Class[_] instance:
useClasstag(obj)(ClassTag(obj.getClass))
That's all.
TypeTags are harder. You need to use Scala reflection to obtain one from the object, and then you have to use some internals of Scala reflection.
import scala.reflect.runtime.universe._
import scala.reflect.api
import api.{Universe, TypeCreator}
// Obtain runtime mirror for the class' classloader
val rm = runtimeMirror(obj.getClass.getClassLoader)
// Obtain instance mirror for obj
val im = rm.reflect(obj)
// Get obj's symbol object
val sym = im.symbol
// Get symbol's type signature - that's what you really want!
val tpe = sym.typeSignature
// Now the black magic begins: we create TypeTag manually
// First, make so-called type creator for the type we have just obtained
val tc = new TypeCreator {
def apply[U <: Universe with Singleton](m: api.Mirror[U]) =
if (m eq rm) tpe.asInstanceOf[U # Type]
else throw new IllegalArgumentException(s"Type tag defined in $rm cannot be migrated to other mirrors.")
}
// Next, create a TypeTag using runtime mirror and type creator
val tt = TypeTag[AnyRef](rm, tc)
// Call our method
useTypetag(obj)(tt)
As you can see, this machinery is rather complex. It means that you should use it only if you really need it, and, as others have said, the cases when you really need it are very rare.
This isn't going to work. Think about it this way: You're asking the compiler to create a class Manifest (at compile time!) for a class that isn't known until run time.
However, I have the feeling you're approaching the problem the wrong way. Is AnyRef really the most you know about the type of Foo at compile time? If that's the case, how can you do anything useful with it? (You won't be able to call any methods on it except the few that are defined for AnyRef.)
It's not clear what you are trying to achieve and a little more context could be helpful. Anyway, here's my 2 cents.
Using Manifest will not help you here because the type parameter needs to be known at compile time. What I propose is something along these lines:
def myFunc[T](arg: AnyRef, klass: Class[T]) = {
val obj: T = klass.cast(arg)
//do something with obj... but what?
}
And you could call it like this:
myFunc(obj, Foo.class)
Note that I don't see how you can do something useful inside myFunc. At compile time, you cannot call any method on a object of type T beside the methods available for AnyRef. And if you want to use reflection to manipulate the argument of myFunc, then there is no need to cast it to a specific type.
This is the wrong way to work with a type-safe OO language. If you need to do this, your design is wrong.
myFunc[T <: AnyRef](arg: T)(implicit m: Manifest[T]) = ???
This is, of course, useless, as you have probably discovered. What kind of meaningful function can you call on an object which might be anything? You can't make any direct reference to its properties or methods.
I would want to write something logically similar to:
myFunc[objClass](obj.asInstanceOf[objClass])
Why? This kind of thing is generally only necessary for very specialised cases. Are you writing a framework that will use dependency injection, for example? If you're not doing some highly technical extension of Scala's capabilities, this should not be necessary.
I bet you know something more about the class, since you say you don't know the exact type. One big part of the way class-based OO works is that if you want to do something to a general type of objects (including all its subtypes), you put that behaviour into a method belonging to the class. Let subclasses override it if they need to.
Frankly, the way to do what you are attempting is to invoke the function in a context where you know enough about the type.