scala generics covariance

We are assuming the type UnitTest is at the base of the hierarchy, and that at each step, we are adding more sophisticated and evolved features. O was able to pass an Orange to F[Fruit, Mammal] because Orange is a subtype of Fruit. The compiler would expect pets.petto be Cat, an object able to do, but is not Cat, it is an Animal. We can now conclude that functions are contravariant in their inputs, i.e. A function has two type parameters, one for the input, and one for the output. HashMap in scala.collection.mutable is invariant but immutable.HashMap is covariant, why? Starting from the notion of subtyping defined for simple types, we showed how the subtype concept translates to type constructors through the definition of variance. Therefore, the list of Assert can contain instances of either Assert[Employee] or Assert[Person]: We can execute an Assert[Empoyee] on an object of type Employee, testing the attribute name and the attribute salary. We have typedthe input parameter of the add () method as A (a covariant type becauseof Pets [+ A]). platform, Insight and perspective to help you to make when one thing changes, the other thing also changes "in the same direction"), they can change against each other (i.e. Now let's look at the output of F. What would happen if F were contravariant in B just like it is in A? 20 Git basic commands every QA Engineer should know, Reasons why Scala language climbs the ranks of popularity, Our experience migrating from Dagger to Koin, Passeig de Grcia 28, 4o, 08007 Barcelona, If you found this article aboutcovariance and contravariance in generics interesting, you might like. Note, however that it is not always obvious whether a type is an input or an output once you get to more complex types. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. with Knoldus Digital Platform, Accelerate pattern recognition and decision In the previous article welooked at Scala Type bounds, today we will continue with Scala generics and talk about Covariance and Contravariance in generics. Is there a way to generate energy using a planet's angular momentum, Sets with both additive and multiplicative gaps. Contravariant is the way to express that a Container can be either the basic type or only specialized for a given type? Even thougth I think there are a little mistake, in the contravariation point A is a supertype of B, right? Unfortunately, O calls the getMilk method on the result of F, but F just returned it a Chicken. Now, to come back to your question: you only have one stack. Other examples are comparison functions (you only put generics in, the output is fixed to be a boolean or an enum or an integer or whatever design your particular language chooses). Contravariant type parameter would be illegal in a position such as a method return type then Scala compiler would have reported an error. Difference between object and class in Scala. @FranArenas: No. Some of good examples are the following: What happened? A test suite is nothing more than a list of tests: We defined the type constructor TestsSuite as covariant, which means that the type TestsSuite[IntegrationTest] is a subtype of TestsSuite[UnitTest]. Well, in order to have related changes, we need two things which change, and they can either change together (i.e. Skipping a calculus topic (squeeze theorem). We can also run an Assert[Person] on an object of type Employee. disruptors, Functional and emotional journey online and

So it works. Spring-Webflux: How to test your controllers using WebTestClient? Assigning an object to a variable of one of its supertypes is always safe.

under production load, Data Science as a service for doing OTOH, what happens if we pass an F[Fruit, Cow]? Another example, in the case of the contravariance is the next one: What happened? Find centralized, trusted content and collaborate around the technologies you use most. If you have a few years of experience in the Scala ecosystem, and youre interested in sharing that experience with the community, have a look at our Contribution Guidelines. co- means "together" (think of cooperation, co-education, co-location), contra- means "against" (think of contradiction, counter-intelligence, counter-insurgency, contraceptive), and in- means "unrelated" or "non-" (think of involuntary, inaccessible, intolerant). needs to be added.

What is a real life example of generic
If I put Apple, Banana or Fruit in them, they all are accepted.

Note: subtyping of generic types is invariant. What if it were contravariant? We pass an F[Fruit, Animal] to O. The error makes it clear, although Car <: Vehicle, due to that Parking is invariant over A, Parking [Car] !<: Parking [Vehicle].

Lets define a hierarchy of classes, modeling an employees domain model: We can define a type constructor that represents a test assertion: An instance of type Assert is a function from a generic type T to Boolean. To illustrate it, we will use this set of classes. data-driven enterprise, Unlock the value of your data assets with Real-time information and operational agility Subtyping adds more constraints to the values of a type. Go to overview The covariance property allows us to declare a variable like: Every time we need to assign a variable of type TestsSuite[T], we can use an object of type TestsSuite[R], given that R is a subtype of T. In this case, covariance is type-safe because it reflects the standard behavior of subtyping. Please respond if you do not get it. To learn more, see our tips on writing great answers. This would be unsound because products, platforms, and templates that It is very hard to find a place in the syntax that actually conveys this relationship-relation. collaborative Data Management & AI/ML I have some operation O that wants to work with a function from Fruits to Mammals (yeah, I know, exciting original examples!) In that case, it would literally be the same the explanation for covariance, just written right-to-left. They provide type variables that we can bind to concrete types. Variance gives more flexible development. The contravariance allows us to type p1 as Parking [Car] and assign it a Parking [Vehicle] Summing up: For Parking[-A] If Vehicle >:(supertype of) Car Then Parking[Vehicle] <: Parking[Car]. Actually, that Stack code can't be made co- or contra-variant. Variance is the quality of being different. To [2022]. solutions that deliver competitive advantage. Here, Car is a subtype of Vehicle but Parking[Car] isnt subtype of Parking[Vehicle].

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market reduction by almost 40%, Prebuilt platforms to accelerate your development time A team of passionate engineers with product mindset who work Please see the code here. Oops. "co-") as its type parameter. insights to stay ahead or meet the customer It is safe (in the sense of the LSP) to make. The compiler has complained,it says that the add input parameter is in a contravariant position,it says that A is not valid. There are three types of variance: covariance, contravariance, and invariance. anywhere, Curated list of templates built by Knolders to reduce the Although pets are typed as Pets [Animal], it is actually a Pets [Cat], therefore, because pets are typed as Pets [Animal], pets.add () will accept Animal or any subtype of Animal.

In other words, given a class Thing [A], if A inherits from B (A <: B), then Thing [A] <: Thing [B]? We say that a type constructor F[_] is invariant if any subtype relationship between types A and B is not preserved in any order between types F[A] and F[B]. Its Liskov again. I would expect to be able to make (), sincepets its a Pets[Cat], show() will return a Cat. Which one is the right one is trickier, but thankfully, we have a powerful tool for analyzing when a subtype is a subtype of another type: Barbara Liskov's Substitution Principle tells us that a type S is a subtype of type T IFF any instance of T can be replaced with an instance of S without changing the observable desirable properties of the program. The LSP says that I should also be able to pass in a subtype of that function, and everything should still work. On the other hand, think of an output stream (such as a Logger), where you can only put stuff in but not get it out. As we have already seen, when we use a type variable as an input parameter to a function or a method, contravariance comes to the rescue, allowing us to define type-safe code. have a stack of characters of type Stack[Char] then it cannot be used Often, we need to execute tests in a suite, and not only in isolation.

In particular, the type Box[Apple] is now a sub-type of Box[Fruit], because Apple is a sub-type of Fruit, and we've instructed Box to vary its type relationships in the same manner (i.e. specifies that, in a relation of inheritance, a type defined as supertype must be in a context that in any case, it allows to substitute it by any of its derived classes. But why? And although Pets[-A] is contravariant over A, the value pet: A its not, once its type is defined (pets val: Pets [Cat] implies that will be Cat), this type is definitive. How did this note help previous owner of this old film camera? Before learning about variances, prerequisite is to first understand : Type System and Type Parameterization. US to Canada by car with an enhanced driver's license, no passport?

Well, O again tries to pass an Orange and it works: Orange is a Food, so F[Food, Mammal] knows how to deal with Oranges. In the generic types, the variance is the correlation between the inheritance relation of the abstract types and how it is transmitted to the inheritance in the generic classes.

Covariance is a concept that is very straightforward to understand. We bring 10+ years of global software delivery experience to And this is all for now in terms of covariance, contravariance and invariance! What are all the uses of an underscore in Scala? But do not forget that although p1 is typed as Parking[Vehicle], it is actually a Parking[Car], this can be confusing, but below I explain that they are thecovariant and contravariant positions and you will eventually get it.

In this article, we introduced the concept of type constructors or generic types. Tannakian-type reconstruction of etale fundamental group. Every programming language supports the concept of types. This is called subtyping and has nothing to do with variance at all. Let's say, F were covariant in A.

it says that type parameter should be covariant +A. Variance tells us if a type constructor (equivalent to a generic type in Java) is a subtype of another type constructor. For the same reason as before, the compiler says that the return type of a method is in a covariant position, A is contravariant.

The type constructor Assert is defined as a contravariant, which means that Assert[Person] is indeed a subtype of Assert[Employee]. Your email address will not be published. conclude, Stack[A] is only a subtype of Stack[B] if and only if B = A. I understand this completely that I cannot use Char where Int is required. Now let's make it covariant: class Box[+T].

Why do we need to have this type of relationship for type constructors? So, if you have a language which has both subtyping and parametric polymorphism, then it is important to know whether one type is a subtype of another type, and variance tells you whether or not a constructed type is a subtype of another constructed type of the same constructor based on the subtyping relationship between the type arguments. So, if I want to get a list of numbers, and someone hands me a list of integers, I am fine. As always, the code from this article can be found over on GitHub. along with your business to provide cutting edge of technology and processes By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy.

So, for example, immutable collections can generally be covariant in their element type, since they don't allow you to put something into the collection (you can only construct a new collection with a potentially different type) but only to get elements out. For this, it is safe to be contravariant. The variance models this correlation and allows us to create more reusable generic classes. The type constructor represents a function with one parameter of type T1. Let's take a simple generic type, a function. I.e. When we have a type constructor such as C[A], then our two things are: And what is our change with a sense of direction? clients think big. Asking for help, clarification, or responding to other answers. We concluded that if we use a generic type variable assignment, we need to use covariance, and if we use a generic type as an argument, we need to use contravariance. bash loop to replace middle of string after a certain character. The more popular are List[T], Option[T], and Try[T], to name a few. In this particular instance, we are talking about the context of subtyping and parametric polymorphism. and flexibility to respond to market has you covered. Or predicates, they only have generic inputs, the output is always fixed to be a boolean. In other words, given a class List [A],if A is a subclass of B then, List [A] can be a subclass of List [B].The variance models this correlation and allows us to create more reusable generic classes. Making statements based on opinion; back them up with references or personal experience.

We say that a type constructor F[_] is contravariant if B is a subtype of type A and F[A] is a subtype of type F[B]. There are four types of variance: covariance, contravariance, invariance and bivariance, three of which are expressed in Scala: A generic class invariantover its abstract type can only receive a parameter type of exactly that type. response

So here, we can substitute Car and Bike in place of Vehicle, since Car and Bike are subclass of Vehicle. Contravariant type parameter is usually used as a method argument type.

Which Terry Pratchett book starts with "Zoom in"? Yes, this is important. silos and enhance innovation, Solve real-world use cases with write once The covariance allows you to type p1 as Parking[Vehicle] and assign it a Parking[Car]. But this does not make sense, since in fact pets is a Pets [Cat] and add () can only accept Cats or Cat subtype.

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fintech, Patient empowerment, Lifesciences, and pharma, Content consumption for the tech-driven F[A, B] is a function that takes in an argument of type A and returns a result of type B. However, once the abstract type is defined, it can be used freely within the class, applying the Liskov substitution principle: A generic class covariantover its abstract type can receive a parameter type of that type or subtypes of that type. Everything still works! This has nothing to do with variance at all.

In this tutorial, we will be looking at Scala variances.

But as weve discovered before, its actually a Pets[Animal] and show() will return an Animal. super T>? Let me know if more Info. audience, Highly tailored products and real-time In this case, an assert can test only the property name that an Employee owns. >, Legal positions of covariant type parameter, Use cases for contravariant type parameter, Use restrictions of covariant type parameter. allow us to do rapid development. every partnership. In Scala, we declare a covariant type constructor using the notation F[+T], adding a plus sign on the left of the type variable. If we remove the contravariance property (-) from the previous Assert type, we obtain an invariant type constructor: Lets try to assign a variable of type Assert[Person] to an object of type Assert[Employee]: Due to the invariance on T, the compiler warns us correctly: Invariance is the only type of relationship available in many programming languages that allows the use of type constructors, similar to Java and C++. But why? You never ask whether one stack is a subtype of another stack. Type constructors are a mechanism that creates new types starting from old ones.

Variance makes Scala collections more Type-Safe. supertype.Contravariance allows assigning an instance to a variable whose type is one of the instances derived type; i.e.

Machine Learning and AI, Create adaptable platforms to unify business workshop-based skills enhancement programs, Over a decade of successful software deliveries, we have built Lets look at each of them in detail. A generic class covariant over its abstract type can receive a parameter type of that type or subtypes of that type. In the upcoming article, Scala Generics III: The Type constraints we will immerse ourselves in the world of type constraints and we will finish the introduction to Scala generics! Liskov substitution principle (the L. of S.O.L.I.D.) to deliver future-ready solutions. Variance in ScalaLearn more about variancesContravariance in details. millions of operations with millisecond

Variance annotation creates a type hierarchy between parameterized types. What is PECS (Producer Extends Consumer Super)?

Part of JournalDev IT Services Private Limited, Scala Variances: Covariant, Invariant and Contravariant, If S is subtype of T, then List[S] is also subtype of List[T], If S is subtype of T, then List[T] is also subtype of List[S]. Our You declare stack2 to be a Stack[Fruit], in other words, you declare that you are allowed to put anything into the Stack which is a Fruit. But, my Stack class accepts only A type (which is invariant). Ergo, functions can't be contravariant in their outputs. strategies, Upskill your engineering team with Thanks for contributing an answer to Stack Overflow!

Liskov substitution principle (the L. of SOLID principles) specifies that, in a relation of inheritance, a type defined as supertype allows to substitute it by any of its derived classes. Hence we arent able to assign the Parking[Car] in place of Parking[Vehicle]. We help our clients to One of the non-obvious things about Scala type variance is that the annotation, +A and -A, actually tells us more about the wrapper than it does about the type parameter. it would enable us to enter true integers into the character stack. What are good particle dynamics ODEs for an introductory scientific computing course? significantly, Catalyze your Digital Transformation journey Variance is related more with complex type rather then passing objects which is called subtyping. Laymen's description of "modals" to clients. An assert on a manager could test the attribute manages that an object of type Employee does not have: Therefore, the compiler warns us that we cannot use an object of type Assert[Manager]: In the Scala SDK, the most popular contravariant type constructor is Function1[-T1, +R].

Well also take a look at the three main types of variance invariance, covariance, and contravariance and how they differ. Save my name, email, and website in this browser for the next time I comment. The high level overview of all the articles on the site. You can now choose to sort by Trending, which boosts votes that have happened recently, helping to surface more up-to-date answers. All we need are two "things", some notion of "change", and this change needs to have some notion of "direction". Contravariance is most commonly associated with consumers (types that accept something). Variance is about the relationship between the relationship between (no, that's not a typo) Argument Types to the Type Constructor and the relationship between the Constructed Types.

Because T is invariant that means that some Box[Apple] is unrelated to a Box[Fruit]. If we try to remove the contravariance annotation from the definition of the Assert type constructor, the compiler warns us that we are missing something: What about the type Assert[Manager]? According to the LSP, if we are right and functions are contravariant in their output, nothing bad should happen. Lets say that we want to model the result of a test on an object of type generic type T. We can use a type constructor to model such a situation: Variance defines the subtyping relationship among type constructors, using the subtyping relationship among the types that bind their type variables. run anywhere smart contracts, Keep production humming with state of the art Therefore, variance doesn't come into play in your example. Now, that's of course very abstract. There are a great many tutorials explaining in detail how to break Java's or C's type-safety using their covariant mutable arrays, for example. Let's take a step back: what does variance actually mean? Scala variance positions - theory behind it?

"Selected/commanded," "indicated," what's the third word? times, Enable Enabling scale and performance for the

production, Monitoring and alerting for complex systems If you want to make a complex type that accepts some type as a child/parent of list that accepts certain other type, then idea of variance comes int effect.

Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Now lets define a function which takes a Vehicle as a parameter and returns the same vehicle value. What are the purpose of the extra diodes in this peak detector circuit (LM1815)? If we remove the covariant annotation from the type constructor TestsSuite[T], the compiler warns us that we cannot use an object of type UnitTest in the above example: There are many examples in the Scala SDK of type constructors declared as covariant concerning their type variable. as an integer stack of type Stack[Int]. For example, when your type parameter is used as the upper or lower bound of an abstract type member, or when you have a method which takes a function that returns a function whose argument type is your type parameter. Well, what are our two things? This relation is precisely the contrary of the covariance relation. If S is subtype of T, then List[S] and List[T] are unrelated. The covariant type parameter can be used as:immutable field type,method return type,method argument type( if the method argument type has a lower bound )Because of these restrictions, covariance is most commonly used in producers (types that return something) and immutable types. A generic class contravariant over its abstract type can receive a parameter type of that type or supertypes of that type. So, it looks like functions are covariant in their outputs. It is subtyping! It should verify that some property holds an object of type T, called target. Can anyone Identify the make, model and year of this car? How to check covariant and contravariant position of an element in the function? the right business decisions, Insights and Perspectives to keep you updated. But, a function from Apples doesn't know how to deal with Oranges, so it blows up. How do map designers subconsciously lead players? Connect and share knowledge within a single location that is structured and easy to search. And now we play through a couple of scenarios. In fact, this is just subtyping. Consider our Test type hierarchy from the previous section. But, on the next page (, it says that type parameter should be covariant +A, then how is the Fruit example working as even it is adding the subtypes with invariant. And again, there are three possibilities: There are two questions you might ask yourself now: This is useful for the same reason subtyping is useful. time to market. Announcing the Stacks Editor Beta release! That should be allowed! We have typed the input parameter as A (which is contravariant because Pets[-A]) and the compiler has told us that this parameter is in a covariant position, that wecan not type it as A. We looked at the three types of variance: covariance, contravariance, and invariance.

Enter your email address to subscribe our blog and receive e-mail notifications of new posts by email. And that is a general rule that applies to variance: In fact, that's why C's designers chose to re-use the already existing keywords in and out for variance annotations and Kotlin uses those same keywords. if I expect to be able to print strings, and someone hands me a printer that can print any object, then it can also print strings, and I am fine. At first sight, contravariance may seem counterintuitive. From deep technical topics to current business trends, our Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, Best explanation I've found. So, we have "change" and that change can be "together", "against" or "unrelated". In Scala, we declare a contravariant type constructor using the notation F[-T], adding a minus sign on the left of the type variable. when one thing changes, the other thing changes "in the opposite direction"), or they can be unrelated (i.e. Now, we can invoke the vehicle Identity function as. Thank you for your great explanation. make it either a subtype or a supertype), then how does C[A] relate to C[B]". To understand contravariance, stop thinking of types in terms of is a more specialized type and switch the focus to the idea of acceptance. The compiler prevents us from falling into the absurdity of calling pets.add (Dog ()), since it is a set of Cat. you can pass a function that takes a more general type as its input as a replacement for a function that takes a more restricted type and everything will work out fine.
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