Scala案例类副本并非总是与`_`存在类型一起使用

问题描述

我正在尝试copy()一个具有param类型的Scala case类.在呼叫站点，值的类型为Foo[_].

I'm trying to copy() a Scala case class which has a type param. At the call site, the type of the value is Foo[_].

这将按预期进行编译:

case class Foo[A](id: String, name: String, v1: Bar[A])
case class Bar[A](v: A)

val foo: Foo[_] = Foo[Int]("foo1", "Foo 1", Bar[Int](1))

foo.copy(id = "foo1.1")

但是如果我添加另一个类型为Bar[A]的成员，它将不再编译:

But if I add another member of type Bar[A], it doesn't compile anymore:

case class Foo[A](id: String, name: String, v1: Bar[A], v2: Bar[A])
case class Bar[A](v: A)

val foo: Foo[_] = Foo[Int]("foo1", "Foo 1", Bar[Int](1), Bar[Int](2))

foo.copy(id = "foo1.1") // compile error, see below

type mismatch;
 found   : Playground.Bar[_$1]
 required: Playground.Bar[Any]
Note: _$1 <: Any, but class Bar is invariant in type A.
You may wish to define A as +A instead. (SLS 4.5)
Error occurred in an application involving default arguments

Scastie

到目前为止，我发现了两种解决方法:

So far I found two workarounds:

• 在A中使Bar协变，然后问题隐藏起来，因为现在是Bar[_$1] <: Bar[Any]
• 在Foo上定义一个copyId(newId: String) = copy(id = newId)方法并调用它，那么我们就不会在类型为Foo[_]的值上调用copy.

• Make Bar covariant in A, then the problem hides itself because now Bar[_$1] <: Bar[Any]
• Define a copyId(newId: String) = copy(id = newId) method on Foo and call that instead, then we aren't calling copy on a value of type Foo[_].

但是，对于我的用例而言，这两个都不是真正可行的，Bar应该是不变的，并且我在Foo[_]实例上有太多不同的copy调用，无法为它们全部创建copyThisAndThat方法.

However, neither of those are really feasible for my use case, Bar should be invariant, and I have too many different copy calls on Foo[_] instances to make copyThisAndThat methods for them all.

我猜我真正的问题是，为什么Scala会这样?似乎是一个错误tbh.

I guess my real question is, why is Scala behaving this way? Seems like a bug tbh.

推荐答案

编译器处理命名参数和默认参数后，调用变为

After the compiler handles named and default parameters, the calls become

foo.copy("foo1.1", foo.name, foo.v1)

和

foo.copy("foo1.1", foo.name, foo.v1, foo.v2)

分别.或者，如果您将参数替换为类型，

respectively. Or, if you replace the parameters with types,

foo.copy[?](String, String, Bar[_])

和

foo.copy[?](String, String, Bar[_], Bar[_])

?是必须推断的copy的类型参数.在第一种情况下，编译器基本上会说"?是Bar[_]的类型参数，即使我不知道这是什么".

? is the type parameter of copy which has to inferred. In the first case the compiler basically says "? is the type parameter of Bar[_], even if I don't know what that is".

在第二种情况下，两个Bar[_]的类型参数必须确实相同，但是在编译器推断?时，该信息已丢失；它们只是Bar[_]，而不是Bar[foo's unknown type parameter]之类的东西.所以"?是第一​​个Bar[_]的类型参数，即使我不知道那是什么".无法工作，因为据编译器所知，第二个Bar[_]可能不同.

In the second case the type parameters of two Bar[_] must really be the same, but that information is lost by the time the compiler is inferring ?; they are just Bar[_], and not something like Bar[foo's unknown type parameter]. So e.g. "? is the type parameter of first Bar[_], even if I don't know what that is" won't work because so far as the compiler knows, the second Bar[_] could be different.

在遵循语言规范的意义上，这不是错误；并更改规范以允许这样做将花费大量精力，并使它和编译器更加复杂.在这种相对罕见的情况下，这可能不是一个很好的权衡.

It isn't a bug in the sense that it follows the language specification; and changing the specification to allow this would take significant effort and make both it and the compiler more complicated. It may not be a good trade-off for such a relatively rare case.

另一种解决方法是使用类型变量模式为_临时命名:

Another workaround is to use type variable pattern to temporarily give a name to _:

foo match { case foo: Foo[a] => foo.copy(id = "foo1.1") }

编译器现在看到foo.v1和foo.v2都是Bar[a]，因此copy的结果是Foo[a].离开case分支后，它将变为Foo[_].

The compiler now sees that foo.v1 and foo.v2 are both Bar[a] and so the result of copy is Foo[a]. After leaving the case branch it becomes Foo[_].

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