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So, I was trying to make a finagle server, talk to sentry (not important), and stumbled upon a case, where I needed to inherit from two classes (not traits) at the same time, let's call them class SentryHandler extends Handler and class TwitterHandler extends Handler, and assume, that I need to create MyHandler, that inherits from both of them.

After a moment of stupidity, when I thought it was impossible without using a dreaded "delegation pattern", I found a solution:

trait SentryTrait extends SentryHandler class MyHandler extends TwitterHandler with SentryTrait

Now, this got me thinking: what is the purpose of having the notion of "trait" to being with? If the idea was to enforce that you can inherit from multiple traits but only a single class, it seems awfully easy to get around. It kinda sounds like class is supposed to be the "main" line of inheritance (that you "extend a class with traits", but that isn't true either: you can extend a trait with (or without) a bunch of other traits, and no class at all.

You cannot instantiate a trait, but the same holds for an abstract class ...

The only real difference I can think of is that a trait cannot have constructor parameters. But what is the significance of that? I mean, why not? What would the problem with something like this?

class Foo(bar: String, baz: String) extends Bar(bar) with Baz(baz)
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  • Main idea is to enhance classical interfaces with implementation you can extend class with. Don't see much to add .. Commented Mar 18, 2016 at 11:10
  • @PavelOliynyk well, that's the point: there is already a term for an "interface enhanced with implementation" - it's called a "class" :). So, the question is why create a new term rather than using an existing one. Commented Mar 18, 2016 at 11:12
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    if your hierarchy looks like this: trait Handler; class SentryHandler extends Handler; class TwitterHandler extends Handler; trait SentryTrait extends SentryHandler; class MyHandler extends TwitterHandler with SentryTrait this doesn't compile on the REPL. Commented Mar 18, 2016 at 11:13
  • Game of words :) I would say that something everyone have to accept. Commented Mar 18, 2016 at 11:13
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    @YuvalItzchakov in theory, yes, but in practice ... :-/ For example, Either is a class, but Future is a trait (while "has a Future" doesn't even make any sense). Option is a class, but Map is a trait ("has-a Map"???). List is a class, but Seq is a trait ... etc. Clearly, the decision of whether the type you are designing should be a class or a trait is based on a plethora of considerations other than "is-a" vs. "has-a" dilemma, which is rather philosophical (in a bad sense, as in having no practical significance). Commented Mar 18, 2016 at 13:24

4 Answers 4

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Your solution (if I understood correctly) - doesn't work. You cannot multiinherit classes in scala:

scala> class Handler defined class Handler scala> class SentryHandler extends Handler defined class SentryHandler scala> class TwitterHandler extends Handler defined class TwitterHandler scala> trait SentryTrait extends SentryHandler defined trait SentryTrait scala> class MyHandler extends TwitterHandler with SentryTrait <console>:11: error: illegal inheritance; superclass TwitterHandler is not a subclass of the superclass SentryHandler of the mixin trait SentryTrait class MyHandler extends TwitterHandler with SentryTrait

As for the question - why traits, as I see it, this is because traits are stackable in order to solve the famous diamond problem

 trait Base { def x: Unit = () } trait A extends Base { override def x: Unit = { println("A"); super.x}} trait B extends Base { override def x: Unit = { println("B"); super.x}} class T1 extends A with B {} class T2 extends B with A {} (new T1).x // Outputs B then A (new T2).x // Outputs A then B

Even though trait A super is Base (for T1) it calls B implementation rather then Base. This is due to trait linearization

So for classes if you extend something - you can be sure that this base will be called next. But this is not true for traits. And that's probably why you do not have trait constructor parameters

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18 Comments

Ouch, that's a bummer. I was tricked by my IDE into believing that inheritance would work :( Too bad. Well, now I know what is differentiates traits from classes, but still don't understand why it needs to be this way. The linearization is a good theory, but I don't see any reason why the same process could not be applied to classes.
@Dima There are some languages like Perl or Python where I believe they did the stuff you are talking about. But I think this makes working with classes a little bit more difficult. You would want to avoid linearization - because it is complex. I am not that familiar with Perl or Python, but I think Python classes always have default constructor. And you cannot call other constructor when inheriting. And this is because of these problems. So the natural way to get rid of these problems - introduce traits.
@Dima There is also other thing about all of this. I don't think this is the reason why traits were introduced, but it definetely affected the decision. Scala is based on Java which does not have multi inheritence. So you just cannot make classes multiinherit in Scala
I am sure it's not the name (trait vs. class) that let's you multiinherit :) Scala has some clever tricks to allow classes inherit from multiple traits, and still work with java. I don't see how replacing the word "trait" with "class" in the last sentence would make it any more or less difficult.
@Dima Scala compiles traits into classes. You can't do that with classes and multiinheritence and expect Java to see those classes afterwards
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The question should rather be: why do we need classes in Scala? Martin Odersky has said that Scala could get by with just traits. We would need to add constructors to traits, so that instances of traits can be constructed. That's okay, Odersky has said that he has worked out a linearization algorithm for trait constructors.

The real purpose is platform interoperability.

Several of the platforms Scala intends to integrate with (currently Java, formerly .NET, maybe in the future Cocoa/Core Foundation/Swift/Objective-C) have a distinct notion of classes, and it is not always easy to have a 1:1 mapping between Scala traits and platform classes. This is different, for example, from interfaces: there is a trivial mapping between platform interfaces and Scala traits – a trait with only abstract members is isomorphic to an interface.

Classes, packages, and null are some examples of Scala features whose main purpose is platform integration.

The Scala designers try very hard to keep the language small, simple, and orthogonal. But Scala is also explicitly intended to integrate well with existing platforms. In fact, even though Scala is a fine language in itself, it was specifically designed as a replacement for the major platform languages (Java on the Java platform, C# on the .NET platform). And in order to do that, some compromises have to be made:

  • Scala has classes, even though they are redundant with traits (assuming we add constructors to traits), because it's easy to map Scala classes to platform classes and almost impossible to map traits to platform classes. Just look at the hoops Scala has to jump through to compile traits to efficient JVM bytecode. (For every trait there is an interface which contains the API and a static class which contains the methods. For every class the trait is mixed into, a forwarder class is generated that forwards the method calls to trait methods to the static class belonging to that trait.)
  • Scala has packages, even though they are redundant with objects. Scala packages can be trivially mapped to Java packages and .NET namespaces. Objects can't.
  • Package Objects are a way to overcome some of the limitations of packages, if we didn't have packages, we wouldn't need package objects.
  • Type Erasure. It is perfectly possible to keep generic types around when compiling to the JVM, e.g. you could store them in annotations. But third-party Java libraries will have their types erased anyway, and other languages won't understand the annotations and treat Scala types as erased, too, so you have to deal with Type Erasure anyway, and if you have to do it anyway, then why do both?
  • null, of course. It is just not possible to automatically map between null and Option in any sane way, when interoperating with real-world Java code. You have to have null in Scala, even though we rather wished it weren't there.
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The problem with having constructors and state in a trait (which then makes it a class) is with multiple inheritance. While this is technically possible in a hypothetical language, it is terrible for language definition and for understanding the program code. The diamond problem, mentioned in other responses to this question), causes the highest level base class constructor to be called twice (the constructor of A in the example below).

Consider this code in a Scala-like language that allows multiple inheritance:

Class A(val x: Int) class B extends A(1) class C extends A(2) class D extends B, C

If state is included, then you have to have two copies of the value x in class A. So you have two copies of class A (or one copy and the diamond problem - so called due to the diamond shape of the UML inheritance diagram).

Diamond Multiple Inheritance

The early versions of the C++ compiler (called C-Front) had lots of bugs with this and the compiler or the compiled code often crashed handling them. Issues include if you have a reference to B or C, how do you (the compiler, actually) determine the start of the object? The compiler needs to know that in order to cast the object from the Base type (in the image below, or A in the image above) to the Descendant type (D in the image above).

Multiple Inheritance Memory Layout

But, does this apply to traits? The way I understand it, Traits are an easy way to implement composition using the Delegation Pattern (I assume you all know the GoF patterns). When we implement Delegation in any other language (Java, C++, C#), we keep a reference to the other object and delegate a message to it by calling the method in its class. If traits are implemented in Scala internally by simply keeping a reference and calling its method, then traits do exactly the same thing as Delegation. So, why can't it have a constructor? I think it should be able to have one without violating its intent.

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You can have state and diamond inheritance with traits just as well as with classes, there is nothing preventing you from that. It's true, that you can't have (non-default) constructors in a trait, but I fail to see how it makes anything better. And no, traits aren't implemented by keeping a reference. They are actual real classes, generated on the fly by the compiler. It is not the delegation pattern at all, it's the actual inheritance.
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The only real difference I can think of is that a trait cannot have constructor parameters. But what is the significance of that? I mean, why not?

Consider

trait A(val x: Int) trait B extends A(1) trait C extends A(2) class D extends B with C

What should (new D {}).x be? Note: there are plans to add trait parameters in Scala 3, but still with restrictions, so that the above is not allowed.

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You can do trait A {def x: Int}, and then have two traits override it and inherit from both. Vals could be handled in the same way.

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