add code tab in types-variance.md (#2756)

Author: benluo

_overviews/scala3-book/types-variance.md

@@ -11,13 +11,41 @@ next-page: types-opaque-types
 Type parameter _variance_ controls the subtyping of parameterized types (like classes or traits).
 
 To explain variance, let us assume the following type definitions:
+
+{% tabs types-variance-1 %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 trait Item { def productNumber: String }
 trait Buyable extends Item { def price: Int }
 trait Book extends Buyable { def isbn: String }
+
 ```
+{% endtab %}
+{% endtabs %}
 
 Let us also assume the following parameterized types:
+
+{% tabs types-variance-2 class=tabs-scala-version %}
+{% tab 'Scala 2' for=types-variance-2 %}
+```scala
+// an example of an invariant type
+trait Pipeline[T] {
+  def process(t: T): T
+}
+
+// an example of a covariant type
+trait Producer[+T] {
+  def make: T
+}
+
+// an example of a contravariant type
+trait Consumer[-T] {
+  def take(t: T): Unit
+}
+```
+{% endtab %}
+
+{% tab 'Scala 3' for=types-variance-2 %}
 ```scala
 // an example of an invariant type
 trait Pipeline[T]:
@@ -31,6 +59,9 @@ trait Producer[+T]:
 trait Consumer[-T]:
   def take(t: T): Unit
 ```
+{% endtab %}
+{% endtabs %}
+
 In general there are three modes of variance:
 
 - **invariant**---the default, written like `Pipeline[T]`
@@ -45,6 +76,22 @@ This means that types like `Pipeline[Item]`, `Pipeline[Buyable]`, and `Pipeline[
 
 And rightfully so! Assume the following method that consumes two values of type `Pipeline[Buyable]`, and passes its argument `b` to one of them, based on the price:
 
+{% tabs types-variance-3 class=tabs-scala-version %}
+{% tab 'Scala 2' for=types-variance-3 %}
+```scala
+def oneOf(
+  p1: Pipeline[Buyable],
+  p2: Pipeline[Buyable],
+  b: Buyable
+): Buyable = {
+  val b1 = p1.process(b)
+  val b2 = p2.process(b)
+  if (b1.price < b2.price) b1 else b2
+ } 
+```
+{% endtab %}
+
+{% tab 'Scala 3' for=types-variance-3 %}
 ```scala
 def oneOf(
   p1: Pipeline[Buyable],
@@ -55,10 +102,19 @@ def oneOf(
   val b2 = p2.process(b)
   if b1.price < b2.price then b1 else b2
 ```
+{% endtab %}
+{% endtabs %}
+
 Now, recall that we have the following _subtyping relationship_ between our types:
+
+{% tabs types-variance-4 %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 Book <: Buyable <: Item
 ```
+{% endtab %}
+{% endtabs %}
+
 We cannot pass a `Pipeline[Book]` to the method `oneOf` because in its implementation, we call `p1` and `p2` with a value of type `Buyable`.
 A `Pipeline[Book]` expects a `Book`, which can potentially cause a runtime error.
 
@@ -68,7 +124,6 @@ We cannot pass a `Pipeline[Item]` because calling `process` on it only promises
 In fact, type `Pipeline` needs to be invariant since it uses its type parameter `T` _both_ as an argument _and_ as a return type.
 For the same reason, some types in the Scala collection library---like `Array` or `Set`---are also _invariant_.
 
-
 ### Covariant Types
 In contrast to `Pipeline`, which is invariant, the type `Producer` is marked as **covariant** by prefixing the type parameter with a `+`.
 This is valid, since the type parameter is only used in a _return position_.
@@ -78,33 +133,47 @@ And in fact, this is sound. The type of `Producer[Buyable].make` only promises t
 As a caller of `make`, we will be happy to also accept a `Book`, which is a subtype of `Buyable`---that is, it is _at least_ a `Buyable`.
 
 This is illustrated by the following example, where the function `makeTwo` expects a `Producer[Buyable]`:
+
+{% tabs types-variance-5 %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 def makeTwo(p: Producer[Buyable]): Int =
   p.make.price + p.make.price
 ```
+{% endtab %}
+{% endtabs %}
+
 It is perfectly fine to pass a producer for books:
-```
+
+{% tabs types-variance-6 %}
+{% tab 'Scala 2 and 3' %}
+```scala
 val bookProducer: Producer[Book] = ???
 makeTwo(bookProducer)
 ```
-The call to `price` within `makeTwo` is still valid also for books.
+{% endtab %}
+{% endtabs %}
 
+The call to `price` within `makeTwo` is still valid also for books.
 
 #### Covariant Types for Immutable Containers
 You will encounter covariant types a lot when dealing with immutable containers, like those that can be found in the standard library (such as `List`, `Seq`, `Vector`, etc.).
 
 For example, `List` and `Vector` are approximately defined as:
 
+{% tabs types-variance-7 %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 class List[+A] ...
 class Vector[+A] ...
 ```
+{% endtab %}
+{% endtabs %}
 
 This way, you can use a `List[Book]` where a `List[Buyable]` is expected.
 This also intuitively makes sense: If you are expecting a collection of things that can be bought, it should be fine to give you a collection of books.
 They have an additional ISBN method in our example, but you are free to ignore these additional capabilities.
 
-
 ### Contravariant Types
 In contrast to the type `Producer`, which is marked as covariant, the type `Consumer` is marked as **contravariant** by prefixing the type parameter with a `-`.
 This is valid, since the type parameter is only used in an _argument position_.
@@ -116,20 +185,34 @@ Remember, for type `Producer`, it was the other way around, and we had `Producer
 And in fact, this is sound. The method `Consumer[Item].take` accepts an `Item`.
 As a caller of `take`, we can also supply a `Buyable`, which will be happily accepted by the `Consumer[Item]` since `Buyable` is a subtype of `Item`---that is, it is _at least_ an `Item`.
 
-
 #### Contravariant Types for Consumers
 Contravariant types are much less common than covariant types.
 As in our example, you can think of them as “consumers.” The most important type that you might come across that is marked contravariant is the one of functions:
 
+{% tabs types-variance-8 class=tabs-scala-version %}
+{% tab 'Scala 2' for=types-variance-8 %}
+```scala
+trait Function[-A, +B] {
+  def apply(a: A): B
+}
+```
+{% endtab %}
+
+{% tab 'Scala 3' for=types-variance-8 %}
 ```scala
 trait Function[-A, +B]:
   def apply(a: A): B
 ```
+{% endtab %}
+{% endtabs %}
+
 Its argument type `A` is marked as contravariant `A`---it consumes values of type `A`.
 In contrast, its result type `B` is marked as covariant---it produces values of type `B`.
 
 Here are some examples that illustrate the subtyping relationships induced by variance annotations on functions:
 
+{% tabs types-variance-9 %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 val f: Function[Buyable, Buyable] = b => b
 
@@ -139,6 +222,8 @@ val g: Function[Buyable, Item] = f
 // OK to provide a Book where a Buyable is expected
 val h: Function[Book, Buyable] = f
 ```
+{% endtab %}
+{% endtabs %}
 
 ## Summary
 In this section, we have encountered three different kinds of variance:

_zh-cn/overviews/scala3-book/types-variance.md

@@ -18,14 +18,39 @@ permalink: "/zh-cn/scala3/book/:title.html"
 
 为了解释型变，让我们假设以下类型定义：
 
+{% tabs types-variance-1 %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 trait Item { def productNumber: String }
 trait Buyable extends Item { def price: Int }
 trait Book extends Buyable { def isbn: String }
 ```
+{% endtab %}
+{% endtabs %}
 
 我们还假设以下参数化类型：
 
+{% tabs types-variance-2 class=tabs-scala-version %}
+{% tab 'Scala 2' for=types-variance-2 %}
+```scala
+// an example of an invariant type
+trait Pipeline[T] {
+  def process(t: T): T
+}
+
+// an example of a covariant type
+trait Producer[+T] {
+  def make: T
+}
+
+// an example of a contravariant type
+trait Consumer[-T] {
+  def take(t: T): Unit
+}
+```
+{% endtab %}
+
+{% tab 'Scala 3' for=types-variance-2 %}
 ```scala
 // an example of an invariant type
 trait Pipeline[T]:
@@ -39,8 +64,10 @@ trait Producer[+T]:
 trait Consumer[-T]:
   def take(t: T): Unit
 ```
+{% endtab %}
+{% endtabs %}
 
-一般来说，方差有三种模式：
+一般来说，型变有三种模式：
 
 - **不变的**---默认值，写成 `Pipeline[T]`
 - **协变**---用`+`注释，例如 `Producer[+T]`
@@ -55,6 +82,25 @@ trait Consumer[-T]:
 
 理所当然地！假设以下方法使用两个类型为`Pipeline[Buyable]` 的值，并根据价格将其参数 `b` 传递给其中一个：
 
+{% tabs types-variance-3 class=tabs-scala-version %}
+{% tab 'Scala 2' for=types-variance-3 %}
+```scala
+def oneOf(
+  p1: Pipeline[Buyable],
+  p2: Pipeline[Buyable],
+  b: Buyable
+): Buyable = {
+  val b1 = p1.process(b)
+  val b2 = p2.process(b)
+  if (b1.price < b2.price)
+    b1 
+  else
+    b2
+ } 
+```
+{% endtab %}
+
+{% tab 'Scala 3' for=types-variance-3 %}
 ```scala
 def oneOf(
   p1: Pipeline[Buyable],
@@ -65,12 +111,18 @@ def oneOf(
   val b2 = p2.process(b)
   if b1.price < b2.price then b1 else b2
 ```
+{% endtab %}
+{% endtabs %}
 
 现在，回想一下，我们的类型之间存在以下_子类型关系_：
 
+{% tabs types-variance-4 %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 Book <: Buyable <: Item
 ```
+{% endtab %}
+{% endtabs %}
 
 我们不能将 `Pipeline[Book]` 传递给 `oneOf` 方法，因为在其实现中，我们调用的 `p1` 和 `p2` 是 `Buyable` 类型的值。
 `Pipeline[Book]` 需要的是 `Book`，这可能会导致运行时错误。
@@ -93,17 +145,25 @@ Book <: Buyable <: Item
 
 以下示例说明了这一点，其中函数 `makeTwo` 需要一个 `Producer[Buyable]`：
 
+{% tabs types-variance-5 %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 def makeTwo(p: Producer[Buyable]): Int =
   p.make.price + p.make.price
 ```
+{% endtab %}
+{% endtabs %}
 
 通过书籍制作人是完全可以的：
 
-```
+{% tabs types-variance-6 %}
+{% tab 'Scala 2 and 3' %}
+```scala
 val bookProducer: Producer[Book] = ???
 makeTwo(bookProducer)
 ```
+{% endtab %}
+{% endtabs %}
 
 在 `makeTwo` 中调用 `price` 对书籍仍然有效。
 
@@ -113,10 +173,14 @@ makeTwo(bookProducer)
 
 例如，`List` 和 `Vector` 大致定义为：
 
+{% tabs types-variance-7 %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 class List[+A] ...
 class Vector[+A] ...
 ```
+{% endtab %}
+{% endtabs %}
 
 这样，您可以在需要 `List[Buyable]` 的地方使用 `List[Book]`。
 这在直觉上也是有道理的：如果您期望收藏可以购买的东西，那么给您收藏书籍应该没问题。
@@ -139,16 +203,30 @@ class Vector[+A] ...
 逆变类型比协变类型少得多。
 在我们的示例中，您可以将它们视为“消费者”。你可能来的最重要的类型标记为逆变的 cross 是函数之一：
 
+{% tabs types-variance-8 class=tabs-scala-version %}
+{% tab 'Scala 2' for=types-variance-8 %}
+```scala
+trait Function[-A, +B] {
+  def apply(a: A): B
+}
+```
+{% endtab %}
+
+{% tab 'Scala 3' for=types-variance-8 %}
 ```scala
 trait Function[-A, +B]:
   def apply(a: A): B
 ```
+{% endtab %}
+{% endtabs %}
 
 它的参数类型 `A` 被标记为逆变的 `A` ——它消费 `A` 类型的值。
 相反，它的结果类型 `B` 被标记为协变——它产生 `B` 类型的值。
 
 以下是一些示例，这些示例说明了由函数上可变注释引起的子类型关系：
 
+{% tabs types-variance-9 %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 val f: Function[Buyable, Buyable] = b => b
 
@@ -158,6 +236,8 @@ val g: Function[Buyable, Item] = f
 // OK to provide a Book where a Buyable is expected
 val h: Function[Book, Buyable] = f
 ```
+{% endtab %}
+{% endtabs %}
 
 ## 概括