Scala - object extends abstract class and takes parameters - scala

Question

//File Animal.scala
abstract class Animal {
val name: String
def getSomething(tClass: TypeClass): String = {
tClass.tName.split('.').lift(0)
}
def apply(tClass: TypeClass): SomeOtherClassType = {
// something...
}
// File: DogSpike, this is used for some specific cases (overwrites
base class val)
object DogSpike extends Animal {
override val name: String = "Spike"
}
this call then works (calls apply)
myTransformation(() => DogSpike(this))
Now I would like to create a more generic object that one can pass arguments but I am unable to.
It would work to create a derived Object from Animal that takes one arguments and being able to use the apply call
object TheDog(name: String) extends Animal {
override val name: String = "Spike"
//...
}
not sure how to implicitly call Animal.apply for TheDog object where I could pass a parameter (name)
myTransformation(() => TheDog(this))
// also this seems to be incorrect "*Wrong top statement declaration*"
object TheDog(val n: String) extends Animal {
override val name: String = n
//...
}

Answer 1

As of *Wrong top statement declaration* (I can understand only this part of your question) - you can't have constructor in object as object is a singleton, so you should use a case class (ADT):
final case class TheDog(name: String) extends Animal
scala>TheDog("Spike")
res2_2: TheDog = TheDog("Spike")
val and companion object with apply is added automatically for case classes, so you don't need to define your own own apply in Animal. case class TheDog(val name: String) is same as case class TheDog(name: String).
I's also use traits instead of abstract class:
trait Animal {
val name: String
def getSomething: String = {
"Dog: " + name
}
}
I don't understand your TypeClass type, but if you really want type classes:
trait Animal {
def name: String
}
final case class TheDog(name: String) extends Animal
final case class TheCat(name: String) extends Animal
implicit class RichDog(dog: TheDog){
def getSomething: String = {
"Dog" + dog.name
}
}
implicit class RichCat(cat: TheCat){
def getSomething: String = {
"Cat: " + cat.name
}
}
scala> TheDog("Spike").getSomething
res4_5: String = "DogSpike"
scala> TheCat("Tom").getSomething
res4_6: String = "Cat: Tom"
About calling apply "implicitly", I don't know why would anyone need this, but:
trait AnimalFactory[A <: Animal] {
def apply(name: String)(implicit constructor: String => A) = constructor(name)
}
object TheeeDog extends AnimalFactory[TheDog]
implicit def createDog(name: String) = TheDog(name)
TheeeDog("Spike")
Of course you have to provide createDog and make it visible for a client, but it doesn't really make sense if you can just use ADTs and define additional required applys in companion object:
case class TheMouse(name: String)
object TheMouse{
def apply(isJerry: Boolean): TheMouse = if (isJerry) TheMouse("Jerry") else TheMouse("NotJerry")
}
TheMouse(true)
If you want to add some parameter to constructor, just add it:
class AnimalFactory(clazz: SomeClass){
def doSomething = clazz.name
def apply(name: String)
}
val dogFactory = new AnimalFactory(dogClassDescriptor)
val catFactory = new AnimalFactory(catClassDescriptor)
dogFactory("Spike")
catFactory("Tom")
You can even create a factory for factory (I wouldn't recommend - this solution already looks overcomplicated):
object AnimalFactory{ //please don't use classes for that - avoiding `new` is not their purpose
def apply(clazz: SomeClass) = new AnimalFactory(clazz)
}
val dogFactory = AnimalFactory(dogClassDescriptor)
//or even `val spike = AnimalFactory(dogClassDescriptor)("Spike")`
But still what's the point if you could just provide underlying clazz either as a member or just in a wrapper:
final case class ClazzWrapper[T <: Animal](clazz: SomeClass, animal: T)