Is there a way to get the parent class from an instance of an inner class using macros rather than run-time reflection?
I have a set of classes like this:
trait IdProvider {
type IdObject = Id.type
case class Id(underlying: Int)
}
case class SomeEntity(id: SomeEntity.Id)
object SomeEntity extends IdProvider
And some code that works with arbitrary IdProvider#Ids:
val lookup = Map[IdProvider#IdObject, Set[Operation]]
def can(operation: Operation, id: IdProvider#Id): Boolean = {
val idObject = findIdTypeFromInstance(id) // This is what I don't have
lookup.get(idObject).exists(s => s(operation))
}
Taking a leaf out of this gist by Paul P. I now have this macro:
def findIdTypeFromInstance[T <: AnyRef : c.WeakTypeTag](
c: blackbox.Context)(thing: c.Expr[T]): c.Expr[T] = {
import c.universe._
val companion = thing.actualType.typeSymbol.companion match {
case NoSymbol =>
c.abort(c.enclosingPosition, s"Instance of ${thing.actualType} has no companion object")
case sym => sym
}
def make[U: c.WeakTypeTag] = c.Expr[U](internal.gen.mkAttributedRef(companion))
make(c.WeakTypeTag(companion.typeSignature))
}
This works for simpler cases (top level case classes, classes and objects, and even nested case classes). However, when dealing with the IdProvider setup above the macro tries to generate this tree:
Select(This(TypeName("IdProvider")), TermName("Id"))
This results in an extremely long stack trace in my test, which starts with:
scala.reflect.internal.Types$TypeError: value is not a member of my.spec.MacroSpec
I have not been able to find a path from the instance or the companion (IdProvider#Id) to the parent class (in this case SomeEntity). Is there a way to get to SomeEntity or do I have to use run-time reflection?
The Id companion is basically a lazy val. You need the enclosing instance to retrieve its value because it's not a statically defined stable path.
With -Yshow-syms you can see it get added in mixin phase:
object SomeEntity
constructor SomeEntity
* method Id$lzycompute (private)
method apply (case <synthetic>)
value id
method readResolve (private <synthetic>)
method unapply (case <synthetic>)
value x$0 (<synthetic>)
* object Id (<synthetic> <stable>)
value <local SomeEntity>
* variable Id$module (private <local> <synthetic>)
The $outer field of an Id is added in explicitouter.
Is it easier just to expose the companion reference explicitly?
case class Id(underlying: Int) {
def c = Id
}
This is just a quick look; maybe there's a clever way to do it.
Related
I need to access a companion class with a specified trait -- from a trait intended for case classes. I am almost certain that the Scala reflection library can accomplish this but I haven't quite been able to piece it together.
I created test code below that requires one section of ??? be filled in with some reflection magic. The code compiles and runs as is -- with a notification due to the missing functionality.
Some related answers that I have seen on StackOverflow were from 2.10. Scala 2.12 compatible please.
import scala.reflect.{ClassTag, classTag}
//for companion object
//accesses Fields of the associated case class to ensure the correctness
//note: abstract class -- not a trait due to issues using ClassTag on a trait
abstract class SupportsField1Companion[T: ClassTag] {
//gets the names of all Fields on the associated case class
val fieldNamesOfInstancedClass: Array[String] =
classTag[T].runtimeClass.getDeclaredFields.map(_.getName)
//prints the name and fields of the associated case class -- plus extra on success
def printFieldNames(extra: String = ""): Unit = {
val name = classTag[T].runtimeClass.getCanonicalName
val fields = fieldNamesOfInstancedClass.reduceLeft(_ + ", " + _)
println(s"Fields of $name: $fields" + extra)
}
}
//for case classes
//IMPORTANT -- please do not parameterize this if possible
trait SupportsField1 {
//some data for printing
val field1: String = this.getClass.getCanonicalName + ": field1"
//should get a reference to the associated companion object as instance of SupportsFieldsCompanion
def getSupportsFieldsCompanion: SupportsField1Companion[this.type] = //this.type may be wrong
??? //TODO reflection magic required -- need functionality to retrieve companion object cast as type
//calls a function on the associated Companion class
def callPrintFuncOnCompanion(): Unit =
getSupportsFieldsCompanion.printFieldNames(s" -- from ${this.getClass.getCanonicalName}")
}
//two case classes with the SupportsFieldsCompanion trait to ensure data is accessed correctly
object ExampleA extends SupportsField1Companion[ExampleA] {}
case class ExampleA() extends SupportsField1 {
val fieldA: String = "ExampleA: fieldA"
}
object ExampleB extends SupportsField1Companion[ExampleB] {}
case class ExampleB() extends SupportsField1 {
val fieldB: String = "ExampleB: fieldB"
}
object Run extends App {
//create instanced classes and print some test data
val exampleA = ExampleA()
println(exampleA.field1) //prints "ExampleA: field1" due to trait SupportsFields
println(exampleA.fieldA) //prints "ExampleA: fieldA" due to being of class ExampleA
val exampleB = ExampleB()
println(exampleB.field1) //prints "ExampleB: field1" due to trait SupportsFields
println(exampleB.fieldB) //prints "ExampleB: fieldB" due to being of class ExampleB
//via the SupportsFieldsCompanion trait on the companion objects,
//call a function on each companion object to show that each companion is associated with the correct case class
ExampleA.printFieldNames() //prints "Fields of ExampleA: fieldA, field1"
ExampleB.printFieldNames() //prints "Fields of ExampleB: fieldB, field1"
//test access of printFieldNames on companion object from instanced class
try {
exampleA.callPrintFuncOnCompanion() //on success, prints "Fields of ExampleA: fieldA, field1 -- from ExampleA"
exampleB.callPrintFuncOnCompanion() //on success, prints "Fields of ExampleB: fieldB, field1 -- from ExampleB"
} catch {
case _: NotImplementedError => println("!!! Calling function on companion(s) failed.")
}
}
There are lots of ways you can do this, but the following is probably one of the simplest that doesn't involve making assumptions about how Scala's companion object class name mangling works:
def getSupportsFieldsCompanion: SupportsField1Companion[this.type] =
scala.reflect.runtime.ReflectionUtils.staticSingletonInstance(
this.getClass.getClassLoader,
this.getClass.getCanonicalName
).asInstanceOf[SupportsField1Companion[this.type]]
This works as desired, but I'd probably type it as SupportsField1Companion[_], and ideally I'd probably avoid having public methods on SupportsField1 that refer to SupportsField1Companion—actually ideally I'd probably avoid this approach altogether, but if you're committed I think the ReflectionUtil solution above is probably reasonable.
Note: There's an EDIT below!
Note: There's another EDIT below!
I have written a Scala annotation macro that is being passed a class and creates (or rather populates) a case object. The name of the case object is the same as the name of the passed class. More importantly, for every field of the passed class, there will be a field in the case object of the same name. The fields of the case object, however, are all of type String, and their value is the name of the type of the respective field in the passed class. Example:
// Using the annotation macro to populate a case object called `String`
#RegisterClass(classOf[String]) case object String
// The class `String` defines a field called `value` of type `char[]`.
// The case object also has a field `value`, containing `"char[]"`.
println(String.value) // Prints `"char[]"` to the console
This, however, seems to only work with pre-defined classes such as String. If I define a case class A(...) and try to do #RegisterClass(classOf[A]) case object A, I get the following error:
[info] scala.tools.reflect.ToolBoxError: reflective compilation has failed:
[info]
[info] not found: type A
What have I done wrong? The code of my macro can be found below. Also, if someone notices un-idiomatic Scala or bad practices in general, I wouldn't mind a hint. Thank you very much in advance!
class RegisterClass[T](clazz: Class[T]) extends StaticAnnotation {
def macroTransform(annottees: Any*) =
macro RegisterClass.expandImpl[T]
}
object RegisterClass {
def expandImpl[T](c: blackbox.Context)(annottees: c.Expr[Any]*) = {
import c.universe._
val clazz: Class[T] = c.prefix.tree match {
case q"new RegisterClass($clazz)" => c.eval[Class[T]](c.Expr(clazz))
case _ => c.abort(c.enclosingPosition, "RegisterClass: Annotation expects a Class[T] instance as argument.")
}
annottees.map(_.tree) match {
case List(q"case object $caseObjectName") =>
if (caseObjectName.toString != clazz.getSimpleName)
c.abort(c.enclosingPosition, "RegisterClass: Annotated case object and class T of passed Class[T] instance" +
"must have the same name.")
val clazzFields = clazz.getDeclaredFields.map(field => field.getName -> field.getType.getSimpleName).toList
val caseObjectFields = clazzFields.map(field => {
val fieldName: TermName = field._1
val fieldType: String = field._2
q"val $fieldName = $fieldType"
})
c.Expr[Any](q"case object $caseObjectName { ..$caseObjectFields }")
case _ => c.abort(c.enclosingPosition, "RegisterClass: Annotation must be applied to a case object definition.")
}
}
}
EDIT: As Eugene Burmako pointed out, the error happens because class A hasn't been compiled yet, so a java.lang.Class for it doesn't exist. I have now started a bounty of 100 StackOverflow points for everyone who as an idea how one could get this to work!
EDIT 2: Some background on the use case: As part of my bachelor thesis I am working on a Scala DSL for expressing queries for event processing systems. Those queries are traditionally expressed as strings, which induces a lot of problems. A typical query would look like that: "select A.id, B.timestamp from pattern[A -> B]". Meaning: If an event of type A occurs and after that an event of type B occurs, too, give me the id of the A event and the timestamp of the B event. The types A and B usually are simple Java classes over which I have no control. id and timestamp are fields of those classes. I would like queries of my DSL to look like that: select (A.id, B.timestamp) { /* ... * / }. This means that for every class representing an event type, e.g., A, I need a companion object -- ideally of the same name. This companion object should have the same fields as the respective class, so that I can pass its fields to the select function, like so: select (A.id, B.timestamp) { /* ... * / }. This way, if I tried to pass A.idd to the select function, it would fail at compile-time if there was no such field in the original class -- because then there would not be one in the companion object either.
This isn't an answer to your macro problem, but it could be a solution to your general problem.
If you can allow a minor change to the syntax of your DSL this might be possible without using macro's (depending on other requirements not mentioned in this question).
scala> class Select[A,B]{
| def apply[R,S](fa: A => R, fb: B => S)(body: => Unit) = ???
| }
defined class Select
scala> def select[A,B] = new Select[A,B]
select: [A, B]=> Select[A,B]
scala> class MyA { def id = 42L }
defined class MyA
scala> class MyB { def timestamp = "foo" }
defined class MyB
scala> select[A,B](_.id, _.timestamp){ /* ... */ }
scala.NotImplementedError: an implementation is missing
I use the class Select here as a means to be able to specify the types of your event classes while letting the compiler infer the result types of the functions fa and fb. If your don't need those result types you could just write it as def select[A,B](fa: A => Any, fb: B => Any)(body: => Unit) = ???.
If necessary you can still implement the select or apply method as a macro. But using this syntax, you will no longer need to generate objects with macro annotations.
Assume the following pair of classes:
class A(arg:String)
class B(argList:Vector[String]) extends A(argList.first)
I want to be able to check for argList being empty before providing the base class constructor with its first element. Unfortunately, placing that check in the default constructor for B (e.g through require, as shown here) is way too late, since the base class' constructor will need to be called first.
This is probably a more general OOP question, but the solution is likely to be Scala-specific.
What do you expect to pass if argList is empty? In any case, you could just use the following:
class B(argList:Vector[String]) extends A(argList.headOption.getOrElse("your default string here")
One way to deal with this is via a companion object. You can mark the constructor for B as private to ensure no-one can by-pass the check, then add a suitable apply method to the companion object that pre-checks the input value(s):
class A(arg:String)
class B private(argList:Vector[String]) extends A(argList.head)
object B {
def apply(argList:Vector[String]): B = argList.headOption.map(_ => new B(argList)).getOrElse(throw new RuntimeException("Oops"))
}
Usage examples:
scala> B(Vector("foo", "bar"))
res2: B = B#328e9109
scala> B(Vector())
java.lang.RuntimeException: Oops
at B$$anonfun$apply$2.apply(<console>:24)
...
Note that for simplicity's sake, I simply throw an exception when handling bad data, but would probably try some other way of handling this situation (a default value per #Zoltan's answer is one such way).
That's why in most places constructors are replaced by factory objects. In Scala it's idiomatic to use companion object as such factories.
class A(arg: String)
abstract class B(arg: String) extends A(arg) {
def argList: IndexedSeq[String]
}
object B {
case object Empty extends B("") {
def argList = IndexedSeq.empty
}
case class NonEmpty private[B](argList: Vector[String]) extends B(argList.head)
def apply(argList: Vector[String]) =
if (argList.isEmpty) Empty else NonEmpty(argList)
def unapplySeq(b:B): Option[IndexedSeq[String]] = b match {
case Empty ⇒ Some(IndexedSeq.empty)
case NonEmpty(args) ⇒ Some(args)
}
}
you could verify that
B(Vector()) == B.Empty
B(Vector("x", "y")).isInstanceOf[B.NonEmpty]
For a project of mine I have implemented a Enum based upon
trait Enum[A] {
trait Value { self: A =>
_values :+= this
}
private var _values = List.empty[A]
def values = _values
}
sealed trait Currency extends Currency.Value
object Currency extends Enum[Currency] {
case object EUR extends Currency
case object GBP extends Currency
}
from Case objects vs Enumerations in Scala. I worked quite nice, till I run into the following problem. Case objects seem to be lazy and if I use Currency.value I might actually get an empty List. It would have been possible to make a call against all Enum Values on startup so that the value list would be populated, but that would be kind of defeating the point.
So I ventured into the dark and unknown places of scala reflection and came up with this solution, based upon the following SO answers. Can I get a compile-time list of all of the case objects which derive from a sealed parent in Scala?
and How can I get the actual object referred to by Scala 2.10 reflection?
import scala.reflect.runtime.universe._
abstract class Enum[A: TypeTag] {
trait Value
private def sealedDescendants: Option[Set[Symbol]] = {
val symbol = typeOf[A].typeSymbol
val internal = symbol.asInstanceOf[scala.reflect.internal.Symbols#Symbol]
if (internal.isSealed)
Some(internal.sealedDescendants.map(_.asInstanceOf[Symbol]) - symbol)
else None
}
def values = (sealedDescendants getOrElse Set.empty).map(
symbol => symbol.owner.typeSignature.member(symbol.name.toTermName)).map(
module => reflect.runtime.currentMirror.reflectModule(module.asModule).instance).map(
obj => obj.asInstanceOf[A]
)
}
The amazing part of this is that it actually works, but it is ugly as hell and I would be interested if it would be possible to make this simpler and more elegant and to get rid of the asInstanceOf calls.
Here is a simple macro based implementation:
import scala.language.experimental.macros
import scala.reflect.macros.blackbox
abstract class Enum[E] {
def values: Seq[E] = macro Enum.caseObjectsSeqImpl[E]
}
object Enum {
def caseObjectsSeqImpl[A: c.WeakTypeTag](c: blackbox.Context) = {
import c.universe._
val typeSymbol = weakTypeOf[A].typeSymbol.asClass
require(typeSymbol.isSealed)
val subclasses = typeSymbol.knownDirectSubclasses
.filter(_.asClass.isCaseClass)
.map(s => Ident(s.companion))
.toList
val seqTSymbol = weakTypeOf[Seq[A]].typeSymbol.companion
c.Expr(Apply(Ident(seqTSymbol), subclasses))
}
}
With this you could then write:
sealed trait Currency
object Currency extends Enum[Currency] {
case object USD extends Currency
case object EUR extends Currency
}
so then
Currency.values == Seq(Currency.USD, Currency.EUR)
Since it's a macro, the Seq(Currency.USD, Currency.EUR) is generated at compile time, rather than runtime. Note, though, that since it's a macro, the definition of the class Enum must be in a separate project from where it is used (i.e. the concrete subclasses of Enum like Currency). This is a relatively simple implementation; you could do more complicated things like traverse multilevel class hierarchies to find more case objects at the cost of greater complexity, but hopefully this will get you started.
A late answer, but anyways...
As wallnuss said, knownDirectSubclasses is unreliable as of writing and has been for quite some time.
I created a small lib called Enumeratum (https://github.com/lloydmeta/enumeratum) that allows you to use case objects as enums in a similar way, but doesn't use knownDirectSubclasses and instead looks at the body that encloses the method call to find subclasses. It has proved to be reliable thus far.
The article "“You don’t need a macro” Except when you do" by Max Afonov
maxaf describes a nice way to use macro for defining enums.
The end-result of that implementation is visible in github.com/maxaf/numerato
Simply create a plain class, annotate it with #enum, and use the familiar val ... = Value declaration to define a few enum values.
The #enum annotation invokes a macro, which will:
Replace your Status class with a sealed Status class suitable for acting as a base type for enum values. Specifically, it'll grow a (val index: Int, val name: String) constructor. These parameters will be supplied by the macro, so you don't have to worry about it.
Generate a Status companion object, which will contain most of the pieces that now make Status an enumeration. This includes a values: List[Status], plus lookup methods.
Give the above Status enum, here's what the generated code looks like:
scala> #enum(debug = true) class Status {
| val Enabled, Disabled = Value
| }
{
sealed abstract class Status(val index: Int, val name: String)(implicit sealant: Status.Sealant);
object Status {
#scala.annotation.implicitNotFound(msg = "Enum types annotated with ".+("#enum can not be extended directly. To add another value to the enum, ").+("please adjust your `def ... = Value` declaration.")) sealed abstract protected class Sealant;
implicit protected object Sealant extends Sealant;
case object Enabled extends Status(0, "Enabled") with scala.Product with scala.Serializable;
case object Disabled extends Status(1, "Disabled") with scala.Product with scala.Serializable;
val values: List[Status] = List(Enabled, Disabled);
val fromIndex: _root_.scala.Function1[Int, Status] = Map(Enabled.index.->(Enabled), Disabled.index.->(Disabled));
val fromName: _root_.scala.Function1[String, Status] = Map(Enabled.name.->(Enabled), Disabled.name.->(Disabled));
def switch[A](pf: PartialFunction[Status, A]): _root_.scala.Function1[Status, A] = macro numerato.SwitchMacros.switch_impl[Status, A]
};
()
}
defined class Status
defined object Status
I want to do something like this:
sealed abstract class Base(val myparam:String)
case class Foo(override val myparam:String) extends Base(myparam)
case class Bar(override val myparam:String) extends Base(myparam)
def getIt( a:Base ) = a.copy(myparam="changed")
I can't, because in the context of getIt, I haven't told the compiler that every Base has a 'copy' method, but copy isn't really a method either so I don't think there's a trait or abstract method I can put in Base to make this work properly. Or, is there?
If I try to define Base as abstract class Base{ def copy(myparam:String):Base }, then case class Foo(myparam:String) extends Base results in class Foo needs to be abstract, since method copy in class Base of type (myparam: String)Base is not defined
Is there some other way to tell the compiler that all Base classes will be case classes in their implementation? Some trait that means "has the properties of a case class"?
I could make Base be a case class, but then I get compiler warnings saying that inheritance from case classes is deprecated?
I know I can also:
def getIt(f:Base)={
(f.getClass.getConstructors.head).newInstance("yeah").asInstanceOf[Base]
}
but... that seems very ugly.
Thoughts? Is my whole approach just "wrong" ?
UPDATE I changed the base class to contain the attribute, and made the case classes use the "override" keyword. This better reflects the actual problem and makes the problem more realistic in consideration of Edmondo1984's response.
This is old answer, before the question was changed.
Strongly typed programming languages prevent what you are trying to do. Let's see why.
The idea of a method with the following signature:
def getIt( a:Base ) : Unit
Is that the body of the method will be able to access a properties visible through Base class or interface, i.e. the properties and methods defined only on the Base class/interface or its parents. During code execution, each specific instance passed to the getIt method might have a different subclass but the compile type of a will always be Base
One can reason in this way:
Ok I have a class Base, I inherit it in two case classes and I add a
property with the same name, and then I try to access the property on
the instance of Base.
A simple example shows why this is unsafe:
sealed abstract class Base
case class Foo(myparam:String) extends Base
case class Bar(myparam:String) extends Base
case class Evil(myEvilParam:String) extends Base
def getIt( a:Base ) = a.copy(myparam="changed")
In the following case, if the compiler didn't throw an error at compile time, it means the code would try to access a property that does not exist at runtime. This is not possible in strictly typed programming languages: you have traded restrictions on the code you can write for a much stronger verification of your code by the compiler, knowing that this reduces dramatically the number of bugs your code can contain
This is the new answer. It is a little long because few points are needed before getting to the conclusion
Unluckily, you can't rely on the mechanism of case classes copy to implement what you propose. The way the copy method works is simply a copy constructor which you can implement yourself in a non-case class. Let's create a case class and disassemble it in the REPL:
scala> case class MyClass(name:String, surname:String, myJob:String)
defined class MyClass
scala> :javap MyClass
Compiled from "<console>"
public class MyClass extends java.lang.Object implements scala.ScalaObject,scala.Product,scala.Serializable{
public scala.collection.Iterator productIterator();
public scala.collection.Iterator productElements();
public java.lang.String name();
public java.lang.String surname();
public java.lang.String myJob();
public MyClass copy(java.lang.String, java.lang.String, java.lang.String);
public java.lang.String copy$default$3();
public java.lang.String copy$default$2();
public java.lang.String copy$default$1();
public int hashCode();
public java.lang.String toString();
public boolean equals(java.lang.Object);
public java.lang.String productPrefix();
public int productArity();
public java.lang.Object productElement(int);
public boolean canEqual(java.lang.Object);
public MyClass(java.lang.String, java.lang.String, java.lang.String);
}
In Scala, the copy method takes three parameter and can eventually use the one from the current instance for the one you haven't specified ( the Scala language provides among its features default values for parameters in method calls)
Let's go down in our analysis and take again the code as updated:
sealed abstract class Base(val myparam:String)
case class Foo(override val myparam:String) extends Base(myparam)
case class Bar(override val myparam:String) extends Base(myparam)
def getIt( a:Base ) = a.copy(myparam="changed")
Now in order to make this compile, we would need to use in the signature of getIt(a:MyType) a MyType that respect the following contract:
Anything that has a parameter myparam and maybe other parameters which
have default value
All these methods would be suitable:
def copy(myParam:String) = null
def copy(myParam:String, myParam2:String="hello") = null
def copy(myParam:String,myParam2:Option[Option[Option[Double]]]=None) = null
There is no way to express this contract in Scala, however there are advanced techniques that can be helpful.
The first observation that we can do is that there is a strict relation between case classes and tuples in Scala. In fact case classes are somehow tuples with additional behaviour and named properties.
The second observation is that, since the number of properties of your classes hierarchy is not guaranteed to be the same, the copy method signature is not guaranteed to be the same.
In practice, supposing AnyTuple[Int] describes any Tuple of any size where the first value is of type Int, we are looking to do something like that:
def copyTupleChangingFirstElement(myParam:AnyTuple[Int], newValue:Int) = myParam.copy(_1=newValue)
This would not be to difficult if all the elements were Int. A tuple with all element of the same type is a List, and we know how to replace the first element of a List. We would need to convert any TupleX to List, replace the first element, and convert the List back to TupleX. Yes we will need to write all the converters for all the values that X might assume. Annoying but not difficult.
In our case though, not all the elements are Int. We want to treat Tuple where the elements are of different type as if they were all the same if the first element is an Int. This is called
"Abstracting over arity"
i.e. treating tuples of different size in a generic way, independently of their size. To do it, we need to convert them into a special list which supports heterogenous types, named HList
Conclusion
Case classes inheritance is deprecated for very good reason, as you can find out from multiple posts in the mailing list: http://www.scala-lang.org/node/3289
You have two strategies to deal with your problem:
If you have a limited number of fields you require to change, use an approach such as the one suggested by #Ron, which is having a copy method. If you want to do it without losing type information, I would go for generifying the base class
sealed abstract class Base[T](val param:String){
def copy(param:String):T
}
class Foo(param:String) extends Base[Foo](param){
def copy(param: String) = new Foo(param)
}
def getIt[T](a:Base[T]) : T = a.copy("hello")
scala> new Foo("Pippo")
res0: Foo = Foo#4ab8fba5
scala> getIt(res0)
res1: Foo = Foo#5b927504
scala> res1.param
res2: String = hello
If you really want to abstract over arity, a solution is to use a library developed by Miles Sabin called Shapeless. There is a question here which has been asked after a discussion : Are HLists nothing more than a convoluted way of writing tuples? but I tell you this is going to give you some headache
If the two case classes would diverge over time so that they have different fields, then the shared copy approach would cease to work.
It is better to define an abstract def withMyParam(newParam: X): Base. Even better, you can introduce an abstract type to retain the case class type upon return:
scala> trait T {
| type Sub <: T
| def myParam: String
| def withMyParam(newParam: String): Sub
| }
defined trait T
scala> case class Foo(myParam: String) extends T {
| type Sub = Foo
| override def withMyParam(newParam: String) = this.copy(myParam = newParam)
| }
defined class Foo
scala>
scala> case class Bar(myParam: String) extends T {
| type Sub = Bar
| override def withMyParam(newParam: String) = this.copy(myParam = newParam)
| }
defined class Bar
scala> Bar("hello").withMyParam("dolly")
res0: Bar = Bar(dolly)
TL;DR: I managed to declare the copy method on Base while still letting the compiler auto generate its implementations in the derived case classes. This involves a little trick (and actually I'd myself just redesign the type hierarchy) but at least it goes to show that you can indeed make it work without writing boiler plate code in any of the derived case classes.
First, and as already mentioned by ron and Edmondo1984, you'll get into troubles if your case classes have different fields.
I'll strictly stick to your example though, and assume that all your case classes have the same fields (looking at your github link, this seems to be the case of your actual code too).
Given that all your case classes have the same fields, the auto-generated copy methods will have the same signature which is a good start. It seems reasonable then to just add the common definition in Base, as you did:
abstract class Base{ def copy(myparam: String):Base }
The problem is now that scala won't generate the copy methods, because there is already one in the base class.
It turns out that there is another way to statically ensure that Base has the right copy method, and it is through structural typing and self-type annotation:
type Copyable = { def copy(myParam: String): Base }
sealed abstract class Base(val myParam: String) { this : Copyable => }
And unlike in our earlier attempt, this will not prevent scala to auto-generate the copy methods.
There is one last problem: the self-type annotation makes sure that sub-classes of Base have a copy method, but it does not make it publicly availabe on Base:
val foo: Base = Foo("hello")
foo.copy()
scala> error: value copy is not a member of Base
To work around this we can add an implicit conversion from Base to Copyable. A simple cast will do, as a Base is guaranteed to be a Copyable:
implicit def toCopyable( base: Base ): Base with Copyable = base.asInstanceOf[Base with Copyable]
Wrapping up, this gives us:
object Base {
type Copyable = { def copy(myParam: String): Base }
implicit def toCopyable( base: Base ): Base with Copyable = base.asInstanceOf[Base with Copyable]
}
sealed abstract class Base(val myParam: String) { this : Base. Copyable => }
case class Foo(override val myParam: String) extends Base( myParam )
case class Bar(override val myParam: String) extends Base( myParam )
def getIt( a:Base ) = a.copy(myParam="changed")
Bonus effect: if we try to define a case class with a different signature, we get a compile error:
case class Baz(override val myParam: String, truc: Int) extends Base( myParam )
scala> error: illegal inheritance; self-type Baz does not conform to Base's selftype Base with Base.Copyable
To finish, one warning: you should probably just revise your design to avoid having to resort to the above trick.
In your case, ron's suggestion to use a single case class with an additional etype field seems more than reasonable.
I think this is what extension methods are for. Take your pick of implementation strategies for the copy method itself.
I like here that the problem is solved in one place.
It's interesting to ask why there is no trait for caseness: it wouldn't say much about how to invoke copy, except that it can always be invoked without args, copy().
sealed trait Base { def p1: String }
case class Foo(val p1: String) extends Base
case class Bar(val p1: String, p2: String) extends Base
case class Rab(val p2: String, p1: String) extends Base
case class Baz(val p1: String)(val p3: String = p1.reverse) extends Base
object CopyCase extends App {
implicit class Copy(val b: Base) extends AnyVal {
def copy(p1: String): Base = b match {
case foo: Foo => foo.copy(p1 = p1)
case bar: Bar => bar.copy(p1 = p1)
case rab: Rab => rab.copy(p1 = p1)
case baz: Baz => baz.copy(p1 = p1)(p1.reverse)
}
//def copy(p1: String): Base = reflect invoke
//def copy(p1: String): Base = macro xcopy
}
val f = Foo("param1")
val g = f.copy(p1="param2") // normal
val h: Base = Bar("A", "B")
val j = h.copy("basic") // enhanced
println(List(f,g,h,j) mkString ", ")
val bs = List(Foo("param1"), Bar("A","B"), Rab("A","B"), Baz("param3")())
val vs = bs map (b => b copy (p1 = b.p1 * 2))
println(vs)
}
Just for fun, reflective copy:
// finger exercise in the api
def copy(p1: String): Base = {
import scala.reflect.runtime.{ currentMirror => cm }
import scala.reflect.runtime.universe._
val im = cm.reflect(b)
val ts = im.symbol.typeSignature
val copySym = ts.member(newTermName("copy")).asMethod
def element(p: Symbol): Any = (im reflectMethod ts.member(p.name).asMethod)()
val args = for (ps <- copySym.params; p <- ps) yield {
if (p.name.toString == "p1") p1 else element(p)
}
(im reflectMethod copySym)(args: _*).asInstanceOf[Base]
}
This works fine for me:
sealed abstract class Base { def copy(myparam: String): Base }
case class Foo(myparam:String) extends Base {
override def copy(x: String = myparam) = Foo(x)
}
def copyBase(x: Base) = x.copy("changed")
copyBase(Foo("abc")) //Foo(changed)
There is a very comprehensive explanation of how to do this using shapeless at http://www.cakesolutions.net/teamblogs/copying-sealed-trait-instances-a-journey-through-generic-programming-and-shapeless ; in case the link breaks, the approach uses the copySyntax utilities from shapeless, which should be sufficient to find more details.
Its an old problem, with an old solution,
https://code.google.com/p/scala-scales/wiki/VirtualConstructorPreSIP
made before the case class copy method existed.
So in reference to this problem each case class MUST be a leaf node anyway, so define the copy and a MyType / thisType plus the newThis function and you are set, each case class fixes the type. If you want to widen the tree/newThis function and use default parameters you'll have to change the name.
as an aside - I've been waiting for compiler plugin magic to improve before implementing this but type macros may be the magic juice. Search in the lists for Kevin's AutoProxy for a more detailed explanation of why my code never went anywhere