Related
Are there any methods in Scala's Vector that are not declared by its superclasses like AbstractSeq?
I am working on providing language localization (translation) for a learning environment/IDE built on top of Scala called Kojo (see kojo.in). I have translated most commonly used methods of Seq. Vector inherits them automatically, so I don't need to duplicated the translation code (keeping DRY). E.g.,
implicit class TurkishTranslationsForSeqMethods[T](s: Seq[T]) {
def başı: T = s.head
def kuyruğu: Seq[T] = s.tail
def boyu: Int = s.length
def boşMu: Boolean = s.isEmpty
// ...
}
implicit class TranslationsForVectorMethods[T](v: Vector[T]) {
??? // what to translate here?
}
Hence the question. Maybe, more importantly, is there a way to find out such novel additions for any class without having to do a manual diff?
The scaladoc provides a way to filter methods to not see the ones inherited from Seq for instance: https://www.scala-lang.org/api/current/scala/collection/immutable/Vector.html a'd click on "Filter all members".
Or, probably easier, IDEs usually provide a "Hierarchy" view of a class and its methods that would give you the information quickly.
Over the past week or so I've been working on a typed, indexed array trait for Scala. I'd like to supply the trait as a typeclass, and allow the library user to implement it however they like. Here's an example, using a list of lists to implement the 2d array typeclass:
// crate a 2d Array typeclass, with additional parameters
trait IsA2dArray[A, T, Idx0, Idx1] {
def get(arr: A, x: Int, y: Int): T // get a single element of the array; its type will be T
}
// give this typeclass method syntax
implicit class IsA2dArrayOps[A, T, Idx0, Idx1](value: A) {
def get(x: Int, y: Int)(implicit isA2dArrayInstance: IsA2dArray[A, T, Idx0, Idx1]): T =
isA2dArrayInstance.get(value, x, y)
}
// The user then creates a simple case class that can act as a 2d array
case class Arr2d[T, Idx0, Idx1] (
values: List[List[T]],
idx0: List[Idx0],
idx1: List[Idx1],
)
// A couple of dummy index element types:
case class Date(i: Int) // an example index element
case class Item(a: Char) // also an example
// The user implements the IsA2dArray typeclass
implicit def arr2dIsA2dArray[T, Idx0, Idx1] = new IsA2dArray[Arr2d[T, Idx0, Idx1], T, Idx0, Idx1] {
def get(arr: Arr2d[T, Idx0, Idx1], x: Int, y: Int): T = arr.values(x)(y)
}
// create an example instance of the type
val arr2d = Arr2d[Double, Date, Item] (
List(List(1.0, 2.0), List(3.0, 4.0)),
List(Date(0), Date(1)),
List(Item('a'), Item('b')),
)
// check that it works
arr2d.get(0, 1)
This all seems fine. Where I am having difficulties is that I would like to constrain the index types to a list of approved types (which the user can change). Since the program is not the original owner of all the approved types, I was thinking to have a typeclass to represent these approved types, and to have the approved types implement it:
trait IsValidIndex[A] // a typeclass, indicating whether this is a valid index type
implicit val dateIsValidIndex: IsValidIndex[Date] = new IsValidIndex[Date] {}
implicit val itemIsValidIndex: IsValidIndex[Item] = new IsValidIndex[Item] {}
then change the typeclass definition to impose a constraint that Idx0 and Idx1 have to implement the IsValidIndex typeclass (and here is where things start not to work):
trait IsA2dArray[A, T, Idx0: IsValidIndex, Idx1: IsValidIndex] {
def get(arr: A, x: Int, y: Int): T // get a single element of the array; its type will be T
}
This won't compile because it requires a trait to have an implicit parameter for the typeclass, which they are not allowed to have: (Constraining type parameters on case classes and traits).
This leaves me with two potential solutions, but both of them feel a bit sub-optimal:
Implement the original IsA2dArray typeclass as an abstract class instead, which then allows me to use the Idx0: IsValidIndex syntax directly above (kindly suggested in the link above). This was my original thinking, but
a) it is less user friendly, since it requires the user to wrap whatever type they are using in another class which then extends this abstract class. Whereas with a typeclass, the new functionality can be directly bolted on, and
b) this quickly got quite fiddly and hard to type - I found this blog post (https://tpolecat.github.io/2015/04/29/f-bounds.html) relevant to the problems - and it felt like taking the typeclass route would be easier over the longer term.
The contraint that Idx0 Idx0 and Idx1 must implement IsValidIndex can be placed in the implicit def to implement the typeclass:
implicit def arr2dIsA2dArray[T, Idx0: IsValidIndex, Idx1: IsValidIndex] = ...
But this is then in the user's hands rather than the library writer's, and there is no guarantee that they will enforce it.
If anyone could suggest either a work-around to square this circle, or an overall change of approach which achieves the same goal, I'd be most grateful. I understand that Scala 3 allows traits to have implicit parameters and therefore would allow me to use the Idx0: IsValidIndex constraint directly in the typeclass generic parameter list, which would be great. But switching over to 3 just for that feels like quite a big hammer to crack a relatively small nut.
I guess the solution is
Implement the original IsA2dArray typeclass as an abstract class instead, which then allows me to use the Idx0: IsValidIndex syntax directly above (kindly suggested in the link above).
This was my original thinking, but a) it is less user friendly, since it requires the user to wrap whatever type they are using in another class which then extends this abstract class.
No, abstract class will not be extended*, it will be still a type class, just abstract-class type class and not trait type class.
Can I just assume that trait and abstract class are interchangeable when defining typeclasses?
Mostly.
What is the advantage of using abstract classes instead of traits?
https://www.geeksforgeeks.org/difference-between-traits-and-abstract-classes-in-scala/
Unless you have hierarchy of type classes (like Functor, Applicative, Monad... in Cats). Trait or abstract class (a type class) can't extend several abstract classes (type classes) while it can extend several traits (type classes). But anyway inheritance of type classes is tricky
https://typelevel.org/blog/2016/09/30/subtype-typeclasses.html
* Well, when we write implicit def arr2dIsA2dArray[T, Idx0, Idx1] = new IsA2dArray[Arr2d[T, Idx0, Idx1], T, Idx0, Idx1] {... technically it's extending IsA2dArray but this is similar for IsA2dArray being a trait and abstract class.
Squeryl defines a trait KeyedEntity which overrides equals, checking for several conditions in an if and calling super.equals in the end. Since super is Object, it will always fail.
Consider:
trait T { override def equals(z: Any):Boolean = super.equals(z)} }
case class A(a: Int) extends T
val a = A(1); val b = A(1)
a==b // false
Thus, if you declare
case class Record(id: Long, name: String ...) extends KeyedEntity[Long] { ... }
-- and you create several Record instances but do not persist them, their comparison will break. I found this by implementing both Salat and Squeryl back ends for the same class, and then all Salat tests fail since isPersisted from KeyedEntity is false.
Is there a design whereby KeyedEntity will preserve case class equality if mixed into a case class? I tried self-typing and parameterizing BetterKeyedEntity[K,P] { self: P => ... } for the case class type as P but it causes infinite recursion in equals.
As things stand right now, super is Object so the final branch of the overridden equals in KeyedEntity will always return false.
The structural equality check usually generated for case classes seems not to be generated if there is an equals override. However some subtleties have to be noted.
Mixing the concept of id-based equality falling back to structural equality might not be a good idea, since I can imagine that it may lead to subtle bugs. For example:
x: A(1) and and y: A(1) are not yet persisted, so they are equal
then they get persisted, and since they are separate objects, the persistence framework may persist them as separate entities (I don't know Squeryl, maybe not an issue there, but this is a thin line to walk)
after persisting, they are suddenly not equal since the id differs.
Even worse, if x and y get persisted to the same id, the hashCode will differ before and after persisting (the source shows that if persisted it is the hashCode of the id). This breaks immutability, and will lead to very bad behavior (when put in maps for example). See this gist in which I demonstrate the assert failing.
So don't mix structural and id-based equality implicitly. Also see this explained in the context of Hibernate.
Typeclasses
It have to be noted that others pointed out (ref needed) that the concept of method-based equality is flawed, for such reasons (there is not only one way two thing can be equal). Therefore you can define a typeclass which describes Equality:
trait Eq[A] {
def equal(x: A, y: A): Boolean
}
and define (possibly multiple) instances of that typeclass for your classes:
// structural equality
implicit object MyClassEqual extends Eq[MyClass] { ... }
// id based equality
def idEq[K, A <: KeyedEntity[K]]: Eq[A] = new Eq[A] {
def equal(x: A, y: A) = x.id == y.id
}
then you can request that things are members of the Eq typeclass:
def useSomeObjects[A](a: A, b: A)(implicit aEq: Eq[A]) = {
... aEq.equal(a, b) ...
}
So you can decide which notion of equality to use by importing the appropriate typeclass in scope, or passing the typeclass instance directly as in useSomeObjects(x, y)(idEq[Int, SomeClass])
Note that you might also need a Hashable typeclass, similarly.
Autogenerating Eq instances
This situation is pretty similar to the Scala stdlib's scala.math.Ordering typeclass. Here is an example for auto-deriving structural Ordering instances for case classes using the excellent shapeless library.
The same would easy to be done for Eq and Hashable.
Scalaz
Note that scalaz has Equal typeclass, with nice pimp patterns with which you can write x === y instead of eqInstance.equal(x, y). I'm not aware it has Hashable typeclass, yet.
I understand covariance and contravariance in scala. Covariance has many applications in the real world, but I can not think of any for contravariance applications, except the same old examples for Functions.
Can someone shed some light on real world examples of contravariance use?
In my opinion, the two most simple examples after Function are ordering and equality. However, the first is not contra-variant in Scala's standard library, and the second doesn't even exist in it. So, I'm going to use Scalaz equivalents: Order and Equal.
Next, I need some class hierarchy, preferably one which is familiar and, of course, it both concepts above must make sense for it. If Scala had a Number superclass of all numeric types, that would have been perfect. Unfortunately, it has no such thing.
So I'm going to try to make the examples with collections. To make it simple, let's just consider Seq[Int] and List[Int]. It should be clear that List[Int] is a subtype of Seq[Int], ie, List[Int] <: Seq[Int].
So, what can we do with it? First, let's write something that compares two lists:
def smaller(a: List[Int], b: List[Int])(implicit ord: Order[List[Int]]) =
if (ord.order(a,b) == LT) a else b
Now I'm going to write an implicit Order for Seq[Int]:
implicit val seqOrder = new Order[Seq[Int]] {
def order(a: Seq[Int], b: Seq[Int]) =
if (a.size < b.size) LT
else if (b.size < a.size) GT
else EQ
}
With these definitions, I can now do something like this:
scala> smaller(List(1), List(1, 2, 3))
res0: List[Int] = List(1)
Note that I'm asking for an Order[List[Int]], but I'm passing a Order[Seq[Int]]. This means that Order[Seq[Int]] <: Order[List[Int]]. Given that Seq[Int] >: List[Int], this is only possible because of contra-variance.
The next question is: does it make any sense?
Let's consider smaller again. I want to compare two lists of integers. Naturally, anything that compares two lists is acceptable, but what's the logic of something that compares two Seq[Int] being acceptable?
Note in the definition of seqOrder how the things being compared becomes parameters to it. Obviously, a List[Int] can be a parameter to something expecting a Seq[Int]. From that follows that a something that compares Seq[Int] is acceptable in place of something that compares List[Int]: they both can be used with the same parameters.
What about the reverse? Let's say I had a method that only compared :: (list's cons), which, together with Nil, is a subtype of List. I obviously could not use this, because smaller might well receive a Nil to compare. It follows that an Order[::[Int]] cannot be used instead of Order[List[Int]].
Let's proceed to equality, and write a method for it:
def equalLists(a: List[Int], b: List[Int])(implicit eq: Equal[List[Int]]) = eq.equal(a, b)
Because Order extends Equal, I can use it with the same implicit above:
scala> equalLists(List(4, 5, 6), List(1, 2, 3)) // we are comparing lengths!
res3: Boolean = true
The logic here is the same one. Anything that can tell whether two Seq[Int] are the same can, obviously, also tell whether two List[Int] are the same. From that, it follows that Equal[Seq[Int]] <: Equal[List[Int]], which is true because Equal is contra-variant.
This example is from the last project I was working on. Say you have a type-class PrettyPrinter[A] that provides logic for pretty-printing objects of type A. Now if B >: A (i.e. if B is superclass of A) and you know how to pretty-print B (i.e. have an instance of PrettyPrinter[B] available) then you can use the same logic to pretty-print A. In other words, B >: A implies PrettyPrinter[B] <: PrettyPrinter[A]. So you can declare PrettyPrinter[A] contravariant on A.
scala> trait Animal
defined trait Animal
scala> case class Dog(name: String) extends Animal
defined class Dog
scala> trait PrettyPrinter[-A] {
| def pprint(a: A): String
| }
defined trait PrettyPrinter
scala> def pprint[A](a: A)(implicit p: PrettyPrinter[A]) = p.pprint(a)
pprint: [A](a: A)(implicit p: PrettyPrinter[A])String
scala> implicit object AnimalPrettyPrinter extends PrettyPrinter[Animal] {
| def pprint(a: Animal) = "[Animal : %s]" format (a)
| }
defined module AnimalPrettyPrinter
scala> pprint(Dog("Tom"))
res159: String = [Animal : Dog(Tom)]
Some other examples would be Ordering type-class from Scala standard library, Equal, Show (isomorphic to PrettyPrinter above), Resource type-classes from Scalaz etc.
Edit:
As Daniel pointed out, Scala's Ordering isn't contravariant. (I really don't know why.) You may instead consider scalaz.Order which is intended for the same purpose as scala.Ordering but is contravariant on its type parameter.
Addendum:
Supertype-subtype relationship is but one type of relationship that can exist between two types. There can be many such relationships possible. Let's consider two types A and B related with function f: B => A (i.e. an arbitrary relation). Data-type F[_] is said to be a contravariant functor if you can define an operation contramap for it that can lift a function of type B => A to F[A => B].
The following laws need to be satisfied:
x.contramap(identity) == x
x.contramap(f).contramap(g) == x.contramap(f compose g)
All of the data types discussed above (Show, Equal etc.) are contravariant functors. This property lets us do useful things such as the one illustrated below:
Suppose you have a class Candidate defined as:
case class Candidate(name: String, age: Int)
You need an Order[Candidate] which orders candidates by their age. Now you know that there is an Order[Int] instance available. You can obtain an Order[Candidate] instance from that with the contramap operation:
val byAgeOrder: Order[Candidate] =
implicitly[Order[Int]] contramap ((_: Candidate).age)
An example based on a real-world event-driven software system. Such a system is based on broad categories of events, like events related to the functioning of the system (system events), events generated by user actions (user events) and so on.
A possible event hierarchy:
trait Event
trait UserEvent extends Event
trait SystemEvent extends Event
trait ApplicationEvent extends SystemEvent
trait ErrorEvent extends ApplicationEvent
Now the programmers working on the event-driven system need to find a way to register/process the events generated in the system. They will create a trait, Sink, that is used to mark components in need to be notified when an event has been fired.
trait Sink[-In] {
def notify(o: In)
}
As a consequence of marking the type parameter with the - symbol, the Sink type became contravariant.
A possible way to notify interested parties that an event happened is to write a method and to pass it the corresponding event. This method will hypothetically do some processing and then it will take care of notifying the event sink:
def appEventFired(e: ApplicationEvent, s: Sink[ApplicationEvent]): Unit = {
// do some processing related to the event
// notify the event sink
s.notify(e)
}
def errorEventFired(e: ErrorEvent, s: Sink[ErrorEvent]): Unit = {
// do some processing related to the event
// notify the event sink
s.notify(e)
}
A couple of hypothetical Sink implementations.
trait SystemEventSink extends Sink[SystemEvent]
val ses = new SystemEventSink {
override def notify(o: SystemEvent): Unit = ???
}
trait GenericEventSink extends Sink[Event]
val ges = new GenericEventSink {
override def notify(o: Event): Unit = ???
}
The following method calls are accepted by the compiler:
appEventFired(new ApplicationEvent {}, ses)
errorEventFired(new ErrorEvent {}, ges)
appEventFired(new ApplicationEvent {}, ges)
Looking at the series of calls you notice that it is possible to call a method expecting a Sink[ApplicationEvent] with a Sink[SystemEvent] and even with a Sink[Event]. Also, you can call the method expecting a Sink[ErrorEvent] with a Sink[Event].
By replacing invariance with a contravariance constraint, a Sink[SystemEvent] becomes a subtype of Sink[ApplicationEvent]. Therefore, contravariance can also be thought of as a ‘widening’ relationship, since types are ‘widened’ from more specific to more generic.
Conclusion
This example has been described in a series of articles about variance found on my blog
In the end, I think it helps to also understand the theory behind it...
Short answer that might help people who were super confused like me and didn't want to read these long winded examples:
Imagine you have 2 classes Animal, and Cat, which extends Animal. Now, imagine that you have a type Printer[Cat], that contains the functionality for printing Cats. And you have a method like this:
def print(p: Printer[Cat], cat: Cat) = p.print(cat)
but the thing is, that since Cat is an Animal, Printer[Animal] should also be able to print Cats, right?
Well, if Printer[T] were defined like Printer[-T], i.e. contravariant, then we could pass Printer[Animal] to the print function above and use its functionality to print cats.
This is why contravariance exists. Another example, from C#, for example, is the class IComparer which is contravariant as well. Why? Because we should be able to use Animal comparers to compare Cats, too.
I'm trying to get a better grasp of structural type dispatch. For instance, assume I have an iterable object with a summary method that computes the mean. So o.summary() gives the mean value of the list. I might like to use structural type dispatch to enable summary(o).
Is there a set of best practices regarding o.summary() vs. summary(o)?
How does scala resolve summary(o) if I have a method summary(o: ObjectType) and summary(o: { def summary: Double})
How does structural type dispatch differ from multimethods or generic functions?
Michael Galpin gives the following discription about structural type dispatch:
Structural types are Scala’s version of “responds-to” style programming as seen in many dynamic languages. So like
def sayName ( x : { def name:String }){
println(x.name)
}
Then any object with a method called name that takes no parameters and returns a string, can be passed to sayName:
case class Person(name:String)
val dean = Person("Dean")
sayName(dean) // Dean
-- 1. In your example, I wouldn't use the summary(o) version, as this is not a very object oriented style of programming. When calling o.summary (you could drop the brackets as it has no side-effects), you are asking for the summary property of o. When calling summary(o), you are passing o to a method that calculates the summary of o. I believe that the first approach is nicer :).
I haven't used structural type dispatch much, but I assume that it is best suited (in a large system) for the case where you would have to write an interface just because one method wants a type that has some method defined. Sometimes creating that interface and forcing the clients to implement it can be awkward. Sometimes you want to use a client defined in another API which conforms to your interface but doesn't explicitly implement it. So, in my opinion, structural type dispatch serves as a nice way to make the adapter pattern implicitly (saves on boilerplate, yay!).
-- 2. Apparently if you call summary(o) and o is of ObjectType, summary(o: ObjectType) gets called (which does make sense). If you call summary(bar), in which bar is not of ObjectType, two things can happen. The call compiles if bar has the method summary() of the right signature and name or otherwise, the call doesn't compile.
Example:
scala> case class ObjectType(summary: Double)
defined class ObjectType
scala> val o = ObjectType(1.2)
o: ObjectType = ObjectType(1.2)
scala> object Test {
| def summary(o: ObjectType) { println("1") }
| def summary(o: { def summary: Double}) { println("2")}
| }
defined module Test
scala> Test.summary(o)
1
Unfortunately, something like the following does not compile due to type erasure:
scala> object Test{
| def foo(a: {def a: Int}) { println("A") }
| def foo(b: {def b: Int}) { println("B") }
| }
:6: error: double definition:
method foo:(AnyRef{def b(): Int})Unit and
method foo:(AnyRef{def a(): Int})Unit at line 5
have same type after erasure: (java.lang.Object)Unit
def foo(b: {def b: Int}) { println("B") }
-- 3. In a sense, structural type dispatch is more dynamic than generic methods, and also serves a different purpose. In a generic method you can say either: a. I want something of any type; b. I want something of a type that is a subtype of A; c. I'll take something that is a supertype of B; d. I'll take something that has an implicit conversion to type C. All of these are much stricter than just "I want a type that has the method foo with the right signature". Also, structural type dispatch does use reflection, as they are implemented by type erasure.
I don't know much about multimethods, but looking at the wikipedia article, it seems that multimethods in Scala can be achieved using pattern matching. Example:
def collide(a: Collider, b: Collider) = (a, b) match {
case (asteroid: Asteroid, spaceship: Spaceship) => // ...
case (asteroid1: Asteroid, asteroid2: Asteroid) => // ...
...
Again, you could use structural type dispatch - def collide(a: {def processCollision()}), but that depends on a design decision (and I would create an interface in this example).
-- Flaviu Cipcigan
Structural data types aren't really all that useful. That's not to say they are useless, but they definitely a niche thing.
For instance, you might want to write a generic test case for the "size" of something. You might do it like this:
def hasSize(o: { def size: Int }, s: Int): Boolean = {
o.size == s
}
This can then be used with any object that implements the "size" method, no matter its class hierarchy.
Now, they are NOT structural type dispatches. They are NOT related to dispatching, but to type definition.
And Scala is an object oriented language always. You must call methods on objects. Function calls are actually "apply" method calls. Things like "println" are just members of objects imported into scope.
I think you asked what Scala does with a call on a structural type. It uses reflection. For example, consider
def repeat(x: { def quack(): Unit }, n: Int) {
for (i <- 1 to n) x.quack()
}
The call x.quack() is compiled into a lookup of the quack method, and then a call, both using Java reflection. (You can verify that by looking at the byte codes with javap. Look for a class with a funny name like Example$$anonfun$repeat$1.)
If you think about it, it's not surprising. There is no way of making a regular method call because there is no common interface or superclass.
So, the other respondents were absolutely right that you don't want to do this unless you must. The cost is very high.