Say I have 2 case classes:
case class Basic(id: String, name) extends SomeBasicTrait
case class Info (age: Int, country: String, many other fields..) extends SomeInfoTrait
and want to create a case class that has all fields from both of those case classes. This is a possible way:
case class Full(bs: Basic, meta: Info) extends SomeBasicTrait with SomeInfoTrait {
val id = bs.id
val name = bs.name
val age = meta.age
val country = meta.country
// etc
}
But it's a lot of boilerplate code. Is there any way to avoid this?
I couldn't find a way to achieve this in Shapeless but maybe there is..
[Update]
#jamborta 's comment helps and is basically this:
case class FullTwo(id: String, name: String, age:Int, country:String)
val b = Basic("myid", "byname")
val i = Info(12, "PT")
Generic[FullTwo].from(Generic[Basic].to(b) ++ Generic[Info].to(i))
The problem with this solution is that it still requires defining every field in the arguments of the FullTwo class, so that every time a change to Basic or Info is made, we also have to remember to change FullTwo as well.
Is there any way to create dynamically at compile time a case class equal to FullTwo?
Related
I want to use optics in Scala to not go down a nested structure but rather traverse the other way around like going to Parent from Child.
case class Parent(param1: Int)
class Child(param2: Int)
val parent = Parent(param1)
val child = Child(param2)
// I would like something of this sort.
val parentParam1 = Lens[Child, Parent](_.parent)
I have tried creating a trait WithParent[T] that the child class extends. For example -
trait WithParent[T] extends scala.AnyRef with scala.Product {
var parent: T = ???
}
case class Parent(param1: Int)
case class Child(param2: Int) extends WithParent[Parent]
object WithParentCheck extends App {
val a = Parent(1)
val b = Child(2)
val parentParam1 = Lens[Child, Parent](_.parent)
}
I have 2 questions -
Will this method work? If so can I define parent in the trait itself?
Is there a better approach to this question?
Edit:
Edit 2: Account has an unique customer identifier.
Real World Scenario
case class Customer(id: String, account: Account)
case class Account(id: String, balance: Double, customerId: String)
val account = Account('01', 100, '1')
val cust = Customer('1', account)
// Accessing customer from account
val customerToAccount = Lens[Customer, Account](_.account)
My question: Is the reverse possible? Accessing customer from the account object?
Logic for my approach:
trait MyParent[T] would contain a object/variable parent of type T which would be accessible from the case class extending it.
Thanks!
case class Person(name: String,
override val age: Int,
override val address: String
) extends Details(age, address)
class Details(val age: Int, val address: String)
val person = Person("Alex", 33, "Europe")
val details = person.asInstanceOf[Details] // ???
println(details) // I want only Details class fields
I have these 2 classes. In reality, both have a lot of fields. Somewhere, I need only field of superclass, taken from Person class.
There is a nice way to get only super class values and not mapping them field by field?
*I'm pretty sure I'll have some problems with json writes for class Details (which is not a case class and have not a singleton object, but this is another subject)
If I get your question correctly, then you might be asking me runtime polymorphism or dynamic method dispatch from java.
If so, you may have to create both the class and not case class
class Details( val age: Int, val address: String)
class Person(name: String,
override val age: Int,
override val address: String
) extends Details(age, address) {
}
Now create the object of person and reference to superclass (Details)
val detail:Details = new Person("Alex", 33, "Europe")
println(detail.address)
println(detail.age)
This way you will be able to get the only address and age
Another way is like , why can't we create the Details a separate entity like:
case class Details( age: Int, address: String)
case class Person(name: String,
details: Details
)
val detail = Person("Alex", Details(10,"Europe") )
Output:
println(detail.details)
Details(10,Europe)
I will post a solution that leverages scala macro system (old kind, not the newest introduced with Scala 3.0). It could be an overkill for you...
BTW, if you want to access to only parent values (for example for getting key, value pair), you can:
given a type tag, get all parents;
from them, extract all the accessors (vals);
for each val, get its value;
and finally returns a list with all accessors taken
So, I try to solve each point step by step.
First of all, we have to write the macro definition as:
object Macros {
def accessors[T](element : T): String = macro MacrosImpl.accessors[T]
}
object MacrosImpl {
def accessors[T: c.WeakTypeTag](c: whitebox.Context): c.Expr[String] = ...
}
for the first point, we can leverage the reflection macroprogramming API using c.universe:
import c.universe._
val weakType = weakTypeTag[T] //thanks to the WeakTypeTag typeclass
val parents = weakType.tpe.baseClasses
for the second point, we can iterate over the parent classes and then take only the public accessors:
val accessors = parents
.map(weakType.tpe.baseType(_))
.flatMap(_.members)
.filter(_.isPublic)
.filter(_.isMethod)
.map(_.asMethod)
.filter(_.isAccessor)
.toSet
So, for example, if the we write Macros.accessors[Details](person), accessors will yield age and address.
To take the value, we can leverage quasiqouting. So, first we take only the values name:
val names = accessors
.map(_.fullName)
.map(_.split("\\."))
.map(_.reverse.head)
Then we convert them into a TermName:
val terms = names.map(TermName(_))
And finally, we convert each term to a key value tuple containing the val name and its value:
val accessorValues = terms
.map(name => c.Expr[(String, Any)](q"(${name.toString}, ${element}.${name})"))
.toSeq
The last step consist in convert a Seq[Expr[(String, Any)] into a Expr[Seq[(String, Any)]. A way to do that, could be leveraging recursion, reify, and splicing expression:
def seqToExprs(seq: Seq[Expr[(String, Any)]]): c.Expr[Seq[(String, Any)]] =
seq.headOption match {
case Some(head) =>
c.universe.reify(
Seq((head.splice._1, head.splice._2)) ++
seqToExprs(seq.tail).splice
)
case _ => c.Expr[Seq[(String, Any)]](q"Seq.empty")
}
So now I decide to return a String representation (but you can manipulate it as you want):
val elements = seqToExprs(accessorValues)
c.Expr[String](q"${elements}.mkString")
You can use it as:
import Macros._
class A(val a : Int)
class B(val b : Int) extends A(b)
class C(val c: Int) extends B(c)
//println(typeToString[List[Set[List[Double]]]])
val c = new C(10)
println(accessors[C](c)) // prints (a, 10)(b, 10)(c, 10)
println(accessors[B](c)) // prints (a, 10)(b, 10)
println(accessors[A](c)) // prints (a, 10)
And, using your example:
// Your example:
case class Person(name: String,
override val age: Int,
override val address: String
) extends Details(age, address)
class Details(val age: Int, val address: String)
val person = Person("Alex", 33, "Europe")
println(accessors[Details](person)) // prints (address,Europe)(age,33)
println(accessors[Person](person)) // prints (address,Europe)(age,33)(name,Alex)
Here there is a repository with the macro implemented.
Scala 3.0 introduce a safer and cleaner macro system, if you use it and you want to go further you can read these articles:
macros tips and tricks
short tutorial
another tutorial
I'm attempting to write some code that tracks changes to a record and applies them at a later date. In a dynamic language I'd do this by simply keeping a log of List[(String, Any)] pairs, and then simply applying these as an update to the original record when I finally decide to commit the changes.
I need to be able to introspect over the updates, so a list of update functions isn't appropriate.
In Scala this is fairly trivial using reflection, however I'd like to implement a type-safe version.
My first attempt was to try with shapeless. This works well if we know specific types.
import shapeless._
import record._
import syntax.singleton._
case class Person(name:String, age:Int)
val bob = Person("Bob", 31)
val gen = LabelledGeneric[Person]
val updated = gen.from( gen.to(bob) + ('age ->> 32) )
// Result: Person("Bob", 32)
However I can't figure out how to make this work generically.
trait Record[T]
def update( ??? ):T
}
Given the way shapeless handles this, I'm not sure if this would even be possible?
If I accept a lot of boilerplate, as a poor mans version I could do something along the lines of the following.
object Contact {
sealed trait Field[T]
case object Name extends Field[String]
case object Age extends Field[Int]
}
// A typeclass would be cleaner, but too verbose for this simple example.
case class Contact(...) extends Record[Contact, Contact.Field] {
def update[T]( field:Contact.Field[T], value:T ) = field match {
case Contact.Name => contact.copy( name = value )
case Contact.Age => contact.copy( age = value )
}
}
However this isn't particularly elegant and requires a lot of boilerplate. I could probably write my own macro to handle this, however it seems like a fairly common thing - is there a way to handle this with Shapeless or a similar macro library already?
How about using the whole instance of the class as an update?
case class Contact(name: String, age: Int)
case class ContactUpdate(name: Option[String] = None, age: Option[Int] = None)
object Contact {
update(target: Contact, delta: ContactUpdate) = Contact(
delta.name.getOrElse(target.name)
target.age.getOrElse(delta.age)
)
}
// also, optionally this:
object ContactUpdate {
apply(name: String) = ContactUpdate(name = Option(name))
apply(age: Int) = ContactUpdate(age = Option(age))
}
I think, if you want the really type-safe solution, this is the cleanest and most readable, and also, possibly the least pain to implement, as you don't need to deal with Records, lenses and individual field descriptors, just ContactUpdate(name="foo") creates an update, and updates.map(Contact.update(target, _)) applies them all in sequence.
I have a Base class have some function and val, I want to inherent them in my inherent case class how to do it ?
This is my base class:
class Base(val name:String, val number:int) extends Sometrait {
def copy(name:String=this.name, number:int=this.number){
new Base(name, number)
}
}
I want to write the:
case class SomeCase(val name:String, val number:int, val id:int)extends Base(String, number){
...
}
But the compiler always told me:
value **** needs `override' modifier social.scala /scalatest/src/scalatest line 35 Scala
But I really want to is just do as inherent not override, how to do it.
(I need to put the child class as case class, as it is easy for me to use in slick. (here is my another question for how to use class as table content class in slick, someone give me really great answer, but I still mass.))
Because name and number fields in both Base and SomeCase are defined with val without any modifiers like private, they are both public members and participate in inheritance. Because these fields have same names in base and child classes, you have to add override modifier before val name and val number in the child class:
case class SomeCase(override val name: String,
override val number: Int,
val id: Int) extends Base(name, number) { ... }
I have an application based on Squeryl. I define my models as case classes, mostly since I find convenient to have copy methods.
I have two models that are strictly related. The fields are the same, many operations are in common, and they are to be stored in the same DB table. But there is some behaviour that only makes sense in one of the two cases, or that makes sense in both cases but is different.
Until now I only have used a single case class, with a flag that distinguishes the type of the model, and all methods that differ based on the type of the model start with an if. This is annoying and not quite type safe.
What I would like to do is factor the common behaviour and fields in an ancestor case class and have the two actual models inherit from it. But, as far as I understand, inheriting from case classes is frowned upon in Scala, and is even prohibited if the subclass is itself a case class (not my case).
What are the problems and pitfalls I should be aware in inheriting from a case class? Does it make sense in my case to do so?
My preferred way of avoiding case class inheritance without code duplication is somewhat obvious: create a common (abstract) base class:
abstract class Person {
def name: String
def age: Int
// address and other properties
// methods (ideally only accessors since it is a case class)
}
case class Employer(val name: String, val age: Int, val taxno: Int)
extends Person
case class Employee(val name: String, val age: Int, val salary: Int)
extends Person
If you want to be more fine-grained, group the properties into individual traits:
trait Identifiable { def name: String }
trait Locatable { def address: String }
// trait Ages { def age: Int }
case class Employer(val name: String, val address: String, val taxno: Int)
extends Identifiable
with Locatable
case class Employee(val name: String, val address: String, val salary: Int)
extends Identifiable
with Locatable
Since this is an interesting topic to many, let me shed some light here.
You could go with the following approach:
// You can mark it as 'sealed'. Explained later.
sealed trait Person {
def name: String
}
case class Employee(
override val name: String,
salary: Int
) extends Person
case class Tourist(
override val name: String,
bored: Boolean
) extends Person
Yes, you have to duplicate the fields. If you don't, it simply would not be possible to implement correct equality among other problems.
However, you don't need to duplicate methods/functions.
If the duplication of a few properties is that much of an importance to you, then use regular classes, but remember that they don't fit FP well.
Alternatively, you could use composition instead of inheritance:
case class Employee(
person: Person,
salary: Int
)
// In code:
val employee = ...
println(employee.person.name)
Composition is a valid and a sound strategy that you should consider as well.
And in case you wonder what a sealed trait means — it is something that can be extended only in the same file. That is, the two case classes above have to be in the same file. This allows for exhaustive compiler checks:
val x = Employee(name = "Jack", salary = 50000)
x match {
case Employee(name) => println(s"I'm $name!")
}
Gives an error:
warning: match is not exhaustive!
missing combination Tourist
Which is really useful. Now you won't forget to deal with the other types of Persons (people). This is essentially what the Option class in Scala does.
If that does not matter to you, then you could make it non-sealed and throw the case classes into their own files. And perhaps go with composition.
case classes are perfect for value objects, i.e. objects that don't change any properties and can be compared with equals.
But implementing equals in the presence of inheritance is rather complicated. Consider a two classes:
class Point(x : Int, y : Int)
and
class ColoredPoint( x : Int, y : Int, c : Color) extends Point
So according to the definition the ColorPoint(1,4,red) should be equal to the Point(1,4) they are the same Point after all. So ColorPoint(1,4,blue) should also be equal to Point(1,4), right? But of course ColorPoint(1,4,red) should not equal ColorPoint(1,4,blue), because they have different colors. There you go, one basic property of the equality relation is broken.
update
You can use inheritance from traits solving lots of problems as described in another answer. An even more flexible alternative is often to use type classes. See What are type classes in Scala useful for? or http://www.youtube.com/watch?v=sVMES4RZF-8
In these situations I tend to use composition instead of inheritance i.e.
sealed trait IVehicle // tagging trait
case class Vehicle(color: String) extends IVehicle
case class Car(vehicle: Vehicle, doors: Int) extends IVehicle
val vehicle: IVehicle = ...
vehicle match {
case Car(Vehicle(color), doors) => println(s"$color car with $doors doors")
case Vehicle(color) => println(s"$color vehicle")
}
Obviously you can use a more sophisticated hierarchy and matches but hopefully this gives you an idea. The key is to take advantage of the nested extractors that case classes provide