为什么 Scala 特征不允许构造函数参数？

Question

I'm new to Scala, coming from Java, and I was just reading about traits. One thing that gets mentioned often is that traits don't (can't? won't?) have constructor parameters. I was curious to know if there was a reason for this.

Coming from a long ago maths/computer-science background I was was wondering if this was an inevitable consequence because of some language design decision, or if it was a conscious decision to avoid some inheritance/mix-in problem or another?

Was hoping someone might know because it feels like there might be something interesting behind the fact.

Answer 1

The other answers describe the language; I suspect your question may really be "why is it designed in this way".

I believe it arises out of the awkwardnesses and verboseness that would arise when extending multiple traits, especially with overrides and with types, and various mix-in strategies.

The Cake Pattern often results in various traits providing missing bits to each other in a way that is totally invisible - by design - in the mixing class. And mixing can be bi-directional, using self-types . So the construction of a class from traits can be a very messy business for the compiler. Scala often trades simplicity of compiler design and implementation for simplicity of language use and code reduction, and this is certainly a good example.

So while there may be simple, hierarchical cases where having a constructor might be useful and sufficient, it would almost certainly have to be redundant of other mechanisms for more difficult, non-hierarchical scenarios.

Answer 2

Scala 3 will allow trait parameters. Here's a sample from the docs

trait Greeting(val name: String) {
  def msg = s"How are you, $name"
}

class C extends Greeting("Bob") {
  println(msg)
}

Answer 3

The answer is: that's what Scala is right now.

But that might not be the case in the future: trait parameters can replace early initializers. (see Martin Odersky's recent Scala Days presentation page 34) Scala: Where It Came From & Where It is Going

Answer 4

Traits don't have constructor parameters because traits cannot be constructed. Given any trait T it's not possible to instantiate any object of type exactly T . You can override trait defs with vals though, so

trait Foo {
  def bar: String
}
class Baz(override val bar: String) extends Foo

You can't construct them directly because new MyTrait {} is actually sugar for an anonymous class of new Object with MyTrait {}

Answer 5

Scala 3 allows traits with parameters , just like classes have parameters.

Answer 6

Trait is analog for Java Interface. The main difference is that trait can have default implementation for their methods.

So Java interfaces can't have constructor so do Scala traits