Why wouldn't the scala compiler dig this:
class Clazz
class Foo[C <: Clazz] {
val foo = new C
}
class type required but C found
[error] val a = new C
[error] ^
Related question - How to get rid of : class type required but T found
This is a classic generic problem that also happens in Java - you cannot create an instance of a generic type variable. What you can do in Scala to fix this, however, is to introduce a type evidence to your type parameter that captures the runtime type:
class Foo[C <: Clazz](implicit ct: ClassTag[C]) {
val foo = ct.runtimeClass.newInstance
}
Note that this only works if the class has a constructor without any arguments. Since the parameter is implicit, you don't need to pass it when calling the Foo constructor:
Foo[Clazz]()
I came up with this scheme, couldn't simplify it through a companion object thought.
class Clazz
class ClazzFactory {
def apply = new Clazz
}
class Foo(factory: ClazzFactory) {
val foo: Clazz = factory.apply
}
It's very annoying that ClazzFactory can't be an object rather than a class though. A simplified version:
class Clazz {
def apply() = new Clazz
}
class Foo(factory: Clazz) {
val foo: Clazz = factory.apply
}
This requires the caller to use the new keyword in order to provide the factory argument, which is already a minor enough annoyance relative to the initial problem. But, scala could have made this scenario all more elegant; I had to fallback here to passing a parameter of the type I wish to instantiate, plus the new keyword. Maybe there's a better way.
(motivation was to instantiate that type many times within the real Foo, that's why this is at all a solution; otherwise my pattern above is just redundantly meaningless).
Is there a way in Scala to modify a parameter passed to a single-argument case class constructor / apply() method before it becomes a val? E.g.
case class AbsVal private(aVal: Double)
object AbsVal {
def apply(aVal: Double): AbsVal = AbsVal(Math.abs(aVal)) // doesn't compile
}
This fails of course with ambiguous reference to overloaded definition. I thought maybe I could trick it with named parameters (and different parameter names for the constructor vs apply()), but that doesn't work either.
Of course instead of apply() I could just have the private constructor and a factory method, but it's annoying to have to litter the code with AbsVal.make(x) instead of just AbsVal(x).
abstract case class AbsVal private(value: Double)
object AbsVal {
def apply(value: Double): AbsVal = new AbsVal(Math.abs(value)) {}
}
Abstract case classes don't have an apply method automatically generated in their companion object. This lets us create our own. We have to create another class to inherit from the case class (anonymous, here), but the toString method generated for the case class will still display it as an AbsVal, so this should be invisible as well. A copy method is not automatically generated for abstract case classes, but for a class with a single parameter like this it wouldn't be useful anyway (and it can always be defined manually, see LimbSoup's answer).
Case class inheritance is usually a bad idea, but because of the private constructor the only subclass that will exist will be the anonymous one we define, so in this instance it is safe.
it's annoying to have to litter the code with AbsVal.make(x) instead of just AbsVal(x)
This is an extremely subjective point. Throughout different languages it is a common wisdom to prefer descriptive names to overloaded definitions, since they tend to introduce ambiguity. Yours is a perfect example of such a case.
Hence, AbsVal.make(x) is the correct way to go in your situation. Although I'd rather name it something like AbsVal.abs(x).
There doesn't seem to be a way to override apply on a case class. There are some syntactic errors in your code, but even changing it to override def apply(value: Double): AbsVal = new AbsVal(Math.abs(value)) will fail. I think this behavior is intentional, because when you define AbsVal(-1), you expect the value not to change (for any case class).
Nonetheless, the same feel can be achieved through a class with a private constructor. Obviously a bit more work to get the same functionality as a case class, but your AbsVal.make is gone..
class AbsVal private(val value: Double) {
def copy(value: Double = this.value): AbsVal = AbsVal(value)
def equals(that: AbsVal): Boolean = this.value.equals(that.value)
override def toString: String = "AbsVal(" + this.value.toString + ")"
}
object AbsVal {
def apply(value: Double):AbsVal = new AbsVal(Math.abs(value))
def unapply(a: AbsVal): Option[Double] = Some(a.value)
}
I'm writing a Play! 2.1 application using ReactiveMongo. each persistable case class has an object that holds 2 implicit objects, implementing BSONReader[...] and BSONWriter[...], and each case class has methods to return these:
trait Persistable {
implicit def getReader: BSONReader[Persistable]
implicit def getWriter: BSONWriter[Persistable]
val collectionName: String
}
case class MyObj () extends Persistable {
override val collectionName: String = MyObj.collectionName
override def getReader: BSONReader[MyObj] = MyObj.MyObjBSONReader
override def getWriter: BSONWriter[MyObj] = MyObj.MyObjBSONWriter
}
object MyObj{
val collectionName: String = "MyObj"
implicit object MyObjBSONReader extends BSONReader[MyObj] {
def fromBSON(document: BSONDocument): MyObj = {
val doc = document.toTraversable
new MyObj(
)
}
}
implicit object MyObjBSONWriter extends BSONWriter[MyObj] {
def toBSON(myObj: MyObj) = {
BSONDocument(
)
}
}
for some reason, getReader seems to work fine, but getWriter errors:
overriding method getWriter in trait Persistable of type =
reactivemongo.bson.handlers.BSONWriter[models.persistable.Persistable];
method getWriter has incompatible type
what am i doing wrong? both seem to have similar signatures.
another hint is that if i remove the return type from getWriter, i get complie time error in eclipse:
type mismatch; found : models.persistable.MyObj.MyObjBSONWriter.type required:
reactivemongo.bson.handlers.BSONWriter[models.persistable.Persistable]
UPDATE:
I did as #AndrzejDoyle said below, but then the implementation of Persister, which was the heart of this exercise, complains:
def insert(persistable: Persistable) = {
val collection = db(persistable.collectionName)
import play.api.libs.concurrent.Execution.Implicits._
implicit val reader = persistable.getReader
implicit val writer = persistable.getWriter
collection.insert(persistable)
}
error:
trait Persistable takes type
parameters
It is due to covariance and contravariance.
The mongodb reader is defined as BSONReader[+DocumentType]. The + in the generic parameter, means that this class is covariant in that parameter. Or more fully,
If B is a subclass of A, then BSONReader[B] is a subclass of BSONReader[A].
Therefore you can use a BSONReader[MyObj] everywhere that a BSONReader[Persistable] is required.
On the other hand, the writer is contravariant: BSONWriter[-DocumentType]. This means that
If B is a subclass of A, then BSONWriter[B] is a superclass of BSONWriter[A].
Therefore your BSONWriter[MyObj] is not a subclass of BSONWriter[Persistable], and so cannot be used in its place.
This might seem confusing initially (i.e. "why does contravariance make sense when it's 'backwards'?"). However if you think about what the classes are doing, it becomes clearer. The reader probably produces some instance of its generic parameter. A caller then might expect it to produce a Persistable - if you have a version that specifically produces MyObjs instead then this is fine.
The writer on the other hand, is probably given an object of its generic parameter. A caller with a BSONWriter[Persistable] will call the write() method, passing in an instance of Persistable to be written. Your implementation can only write instances of MyObj, and so it doesn't actually match the interface. On the other hand, a BSONWriter[Object] would be a subclass of any BSONWriter, since it can (from a type perspective) accept any type as an argument.
The fundamental problem seems to be that your Persistable trait is looser than you intended. You probably want each implementation to return a reader and writer parameterized on itself, rather than on Persistable in general. You can achieve this via self-bounded generics:
trait Persistable[T <: Persistable[T]] {
implicit def getReader: BSONReader[T]
implicit def getWriter: BSONWriter[T]
val collectionName: String
}
and then declare the class as MyObj[MyObj]. Now the reader and writer are expected to be parameterised on MyObj, and your existing implementations will compile.
I'm trying to figure out how to .clone my own objects, in Scala.
This is for a simulation so mutable state is a must, and from that arises the whole need for cloning. I'll clone a whole state structure before moving the simulation time ahead.
This is my current try:
abstract trait Cloneable[A] {
// Seems we cannot declare the prototype of a copy constructor
//protected def this(o: A) // to be defined by the class itself
def myClone= new A(this)
}
class S(var x: String) extends Cloneable[S] {
def this(o:S)= this(o.x) // for 'Cloneable'
def toString= x
}
object TestX {
val s1= new S("say, aaa")
println( s1.myClone )
}
a. Why does the above not compile. Gives:
error: class type required but A found
def myClone= new A(this)
^
b. Is there a way to declare the copy constructor (def this(o:A)) in the trait, so that classes using the trait would be shown to need to provide one.
c. Is there any benefit from saying abstract trait?
Finally, is there a way better, standard solution for all this?
I've looked into Java cloning. Does not seem to be for this. Also Scala copy is not - it's only for case classes and they shouldn't have mutable state.
Thanks for help and any opinions.
Traits can't define constructors (and I don't think abstract has any effect on a trait).
Is there any reason it needs to use a copy constructor rather than just implementing a clone method? It might be possible to get out of having to declare the [A] type on the class, but I've at least declared a self type so the compiler will make sure that the type matches the class.
trait DeepCloneable[A] { self: A =>
def deepClone: A
}
class Egg(size: Int) extends DeepCloneable[Egg] {
def deepClone = new Egg(size)
}
object Main extends App {
val e = new Egg(3)
println(e)
println(e.deepClone)
}
http://ideone.com/CS9HTW
It would suggest a typeclass based approach. With this it is possible to also let existing classes be cloneable:
class Foo(var x: Int)
trait Copyable[A] {
def copy(a: A): A
}
implicit object FooCloneable extends Copyable[Foo] {
def copy(foo: Foo) = new Foo(foo.x)
}
implicit def any2Copyable[A: Copyable](a: A) = new {
def copy = implicitly[Copyable[A]].copy(a)
}
scala> val x = new Foo(2)
x: Foo = Foo#8d86328
scala> val y = x.copy
y: Foo = Foo#245e7588
scala> x eq y
res2: Boolean = false
a. When you define a type parameter like the A it gets erased after the compilation phase.
This means that the compiler uses type parameters to check that you use the correct types, but the resulting bytecode retains no information of A.
This also implies that you cannot use A as a real class in code but only as a "type reference", because at runtime this information is lost.
b & c. traits cannot define constructor parameters or auxiliary constructors by definition, they're also abstract by definition.
What you can do is define a trait body that gets called upon instantiation of the concrete implementation
One alternative solution is to define a Cloneable typeclass. For more on this you can find lots of blogs on the subject, but I have no suggestion for a specific one.
scalaz has a huge part built using this pattern, maybe you can find inspiration there: you can look at Order, Equal or Show to get the gist of it.
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