2023年3月23日 10:26:22go评论55阅读模式

英文:

"Deferred inline method `foo` in trait `Foo` cannot be invoked": Pairs

问题

以下是您提供的内容的中文翻译：

我只是在尝试Scala 3.2.2中given和inline的行为，遇到了以下示例：

trait Max[X]:
  inline def max(a: X, b: X)
  
inline given maxForDoubles: Max[Double] with
  inline def max(a: Double, b: Double) = if a &lt; b then b else a

inline given maxForPairs[A, B](using mA: Max[A], mB: Max[B]): Max[(A, B)] with
  inline def max(x: (A, B), y: (A, B)) =
    (mA.max(x._1, y._1), mB.max(x._2, y._2))

@main def entryPoint(): Unit = {
  println(summon[Max[(Double, Double)]].max((10.0, 3.0), (20.0, -7.0)))
}

不仅仅是打印出对偶(20, 3)，它导致以下错误编译失败：

12 |  println(summon[Max[(Double, Double)]].max((10.0, 3.0), (20.0, -7.0)))
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |          无法调用Trait Max中的延迟内联方法max
   |----------------------------------------------------------------------------
   |内联堆栈跟踪
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   |此位置包含从buggy-example.scala:9内联的代码
 9 |    (mA.max(x._1, y._1), mB.max(x._2, y._2))
   |     ^^^^^^
    ----------------------------------------------------------------------------

看起来应该在静态情况下知道一切。为什么会失败呢？

<details>
<summary>英文:</summary>

I was just experimenting with the behavior of `given`s and `inline` in Scala 3.2.2, and ran into the following example:

trait Max[X]:
inline def max(a: X, b: X): X

inline given maxForDoubles: Max[Double] with
inline def max(a: Double, b: Double) = if a < b then b else a

inline given maxForPairs[A, B](using mA: Max[A], mB: Max[B]): Max[(A, B)] with
inline def max(x: (A, B), y: (A, B)) =
(mA.max(x._1, y._1), mB.max(x._2, y._2))

@main def entryPoint(): Unit = {
println(summon[Max[(Double, Double)]].max((10.0, 3.0), (20.0, -7.0)))
}


Instead of just printing the pair `(20, 3)`, it fails the compilation with the following error:

12 | println(summon[Max[(Double, Double)]].max((10.0, 3.0), (20.0, -7.0)))
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Deferred inline method max in trait Max cannot be invoked
Inline stack trace
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
This location contains code that was inlined from buggy-example.scala:9
9
^^^^^^
----------------------------------------------------------------------------

It seems that it should know everything statically. Any clues why it&#39;s failing?

</details>


# 答案1
**得分**: 4

I'll provide a translation of the code snippets:

```scala
如果你尝试使用[别名 givens](https://docs.scala-lang.org/scala3/reference/contextual/givens.html#alias-givens)而不是[given 实例](https://docs.scala-lang.org/scala3/reference/contextual/givens.html#given-instances)(`with`-语法)

```scala
inline given maxForDoubles: Max[Double] = new Max[Double]:
  inline override def max(a: Double, b: Double): Double = if a &lt; b then b else a

inline given maxForPairs[A, B](using mA: Max[A], mB: Max[B]): Max[(A, B)] = new Max[(A, B)]:
  inline override def max(x: (A, B), y: (A, B)): (A, B) = (mA.max(x._1, y._1), mB.max(x._2, y._2))

这将无法工作，因为

实现限制：不支持嵌套内联方法

但是使用 with-语法，嵌套的内联是允许的，因为

inline given maxForDoubles: Max[Double] with
  inline override def max(a: Double, b: Double): Double = if a &lt; b then b else a

inline given maxForPairs[A, B](using mA: Max[A], mB: Max[B]): Max[(A, B)] with
  inline override def max(x: (A, B), y: (A, B)): (A, B) = (mA.max(x._1, y._1), mB.max(x._2, y._2))

实际上相当于

class maxForDoubles extends Max[Double]:
  inline override def max(a: Double, b: Double): Double = if a &lt; b then b else a

inline given maxForDoubles: maxForDoubles = new maxForDoubles

class maxForPairs[A, B](using mA: Max[A], mB: Max[B]) extends Max[(A, B)]:
  inline override def max(x: (A, B), y: (A, B)): (A, B) = (mA.max(x._1, y._1), mB.max(x._2, y._2))

inline given maxForPairs[A, B](using mA: Max[A], mB: Max[B]): maxForPairs[A, B] = new maxForPairs[A, B]

实际上，使用 -Xprint:pickleQuotes 选项（或 -Xprint:typer）with-语法会产生

//    given class maxForDoubles() extends Object(), App.Max[Double] {
//      override inline def max(a: Double, b: Double): Double = 
//        (if a.&lt;(b) then b else a):Double
//    }
//    final inline given def maxForDoubles: App.maxForDoubles = 
//      new App.maxForDoubles():App.maxForDoubles
//    given class maxForPairs[A &gt;: Nothing &lt;: Any, B &gt;: Nothing &lt;: Any](using 
//      mA: App.Max[A]
//    , mB: App.Max[B]) extends Object(), App.Max[ 
//      Tuple2[maxForPairs.this.A, maxForPairs.this.B]
//    ] {
//      A
//      B
//      protected given val mA: App.Max[A]
//      protected given val mB: App.Max[B]
//      override inline def max(x: Tuple2[maxForPairs.this.A, maxForPairs.this.B] 
//        ,  
//      y: Tuple2[maxForPairs.this.A, maxForPairs.this.B]): (A, B) = 
//        Tuple2.apply[A, B](this.App$maxForPairs$$inline$mA.max(x._1, y._1),  
//          this.App$maxForPairs$$inline$mB.max(x._2, y._2) 
//        ):(A, B) 
//      def App$maxForPairs$$inline$mA: App.Max[A] = maxForPairs.this.mA 
//      def App$maxForPairs$$inline$mB: App.Max[B] = maxForPairs.this.mB 
//    } 
//    final inline given def maxForPairs[A &gt;: Nothing &lt;: Any, B &gt;: Nothing &lt;: Any] 
//      ( 
//    using mA: App.Max[A], mB: App.Max[B]): App.maxForPairs[A, B] =  
//      new App.maxForPairs[A, B](using mA, mB)():App.maxForPairs[A, B]

让我们暂时简化 with-语法，去掉 using 和 maxForDoubles

class maxForPairs[A, B]/*(using mA: Max[A], mB: Max[B])*/ extends Max[(A, B)]:
  inline override def max(x: (A, B), y: (A, B)): (A, B) = ??? // (mA.max(x._1, y._1), mB.max(x._2, y._2))

// inline given maxForPairs[A, B]/*(using mA: Max[A], mB: Max[B])*/: maxForPairs[A, B] = new maxForPairs[A, B]

然后在

val inst: maxForPairs[Double, Double] = new maxForPairs[Double, Double]
inst.max((10.0, 3.0), (20.0, -7.0)) // 编译通过

和

val inst: Max[(Double, Double)] = new maxForPairs[Double, Double]
inst.max((10.0, 3.0), (20.0, -7.0)) // 编译失败：在特质 Max 中无法调用推迟的内联方法 max

之间的差异是可以理解的，因为有一个规则

> 3) 内联方法也可以是抽象的。抽象内联方法只能由其他内联方法实现。它不能直接调用：
>
> abstract class A:
> inline def f: Int
>
> object B extends A:
> inline def f: Int = 22
>
> B.f // OK
> val a: A = B
> a.f // error: 无法在 A 中内联 f。

https://docs.scala-lang.org/scala3/reference/metaprogramming/inline.html#rules-for-overriding

而 max 恰恰是特质 Max 中的一个抽象内联方法。

`sum

英文:

If you tried to use alias givens rather than given instances (with-syntax)

inline given maxForDoubles: Max[Double] = new Max[Double]:
  inline override def max(a: Double, b: Double): Double = if a &lt; b then b else a

inline given maxForPairs[A, B](using mA: Max[A], mB: Max[B]): Max[(A, B)] = new Max[(A, B)]:
  inline override def max(x: (A, B), y: (A, B)): (A, B) = (mA.max(x._1, y._1), mB.max(x._2, y._2))

this wouldn't work because of

Implementation restriction: nested inline methods are not supported

But with with-syntax nested inline are allowed because

inline given maxForDoubles: Max[Double] with
  inline override def max(a: Double, b: Double): Double = if a &lt; b then b else a

inline given maxForPairs[A, B](using mA: Max[A], mB: Max[B]): Max[(A, B)] with
  inline override def max(x: (A, B), y: (A, B)): (A, B) = (mA.max(x._1, y._1), mB.max(x._2, y._2))

is actually

class maxForDoubles extends Max[Double]:
  inline override def max(a: Double, b: Double): Double = if a &lt; b then b else a

inline given maxForDoubles: maxForDoubles = new maxForDoubles

class maxForPairs[A, B](using mA: Max[A], mB: Max[B]) extends Max[(A, B)]:
  inline override def max(x: (A, B), y: (A, B)): (A, B) = (mA.max(x._1, y._1), mB.max(x._2, y._2))

inline given maxForPairs[A, B](using mA: Max[A], mB: Max[B]): maxForPairs[A, B] = new maxForPairs[A, B]

Indeed, with -Xprint:pickleQuotes option (or -Xprint:typer) the with-syntax produces

//    given class maxForDoubles() extends Object(), App.Max[Double] {
//      override inline def max(a: Double, b: Double): Double = 
//        (if a.&lt;(b) then b else a):Double
//    }
//    final inline given def maxForDoubles: App.maxForDoubles = 
//      new App.maxForDoubles():App.maxForDoubles
//    given class maxForPairs[A &gt;: Nothing &lt;: Any, B &gt;: Nothing &lt;: Any](using 
//      mA: App.Max[A]
//    , mB: App.Max[B]) extends Object(), App.Max[
//      Tuple2[maxForPairs.this.A, maxForPairs.this.B]
//    ] {
//      A
//      B
//      protected given val mA: App.Max[A]
//      protected given val mB: App.Max[B]
//      override inline def max(x: Tuple2[maxForPairs.this.A, maxForPairs.this.B]
//        , 
//      y: Tuple2[maxForPairs.this.A, maxForPairs.this.B]): (A, B) = 
//        Tuple2.apply[A, B](this.App$maxForPairs$$inline$mA.max(x._1, y._1), 
//          this.App$maxForPairs$$inline$mB.max(x._2, y._2)
//        ):(A, B)
//      def App$maxForPairs$$inline$mA: App.Max[A] = maxForPairs.this.mA
//      def App$maxForPairs$$inline$mB: App.Max[B] = maxForPairs.this.mB
//    }
//    final inline given def maxForPairs[A &gt;: Nothing &lt;: Any, B &gt;: Nothing &lt;: Any]
//      (
//    using mA: App.Max[A], mB: App.Max[B]): App.maxForPairs[A, B] = 
//      new App.maxForPairs[A, B](using mA, mB)():App.maxForPairs[A, B]

Let's temporarily simplify the with-syntax removing using and maxForDoubles

class maxForPairs[A, B]/*(using mA: Max[A], mB: Max[B])*/ extends Max[(A, B)]:
  inline override def max(x: (A, B), y: (A, B)): (A, B) = ??? // (mA.max(x._1, y._1), mB.max(x._2, y._2))

// inline given maxForPairs[A, B]/*(using mA: Max[A], mB: Max[B])*/: maxForPairs[A, B] = new maxForPairs[A, B]

Then the difference between

val inst: maxForPairs[Double, Double] = new maxForPairs[Double, Double]
inst.max((10.0, 3.0), (20.0, -7.0)) // compiles

and

val inst: Max[(Double, Double)] = new maxForPairs[Double, Double]
inst.max((10.0, 3.0), (20.0, -7.0)) // doesn&#39;t compile: Deferred inline method max in trait Max cannot be invoked

is understandable because there is a rule

> 3) Inline methods can also be abstract. An abstract inline method can be implemented only by other inline methods. It cannot be invoked directly:
>
> abstract class A:
> inline def f: Int
>
> object B extends A:
> inline def f: Int = 22
>
> B.f // OK
> val a: A = B
> a.f // error: cannot inline f in A.

https://docs.scala-lang.org/scala3/reference/metaprogramming/inline.html#rules-for-overriding

And max is exactly an abstract inline method in the trait Max.

summon[Max[(Double, Double)]] returns the precise type of implicit: maxForPairs[Double, Double] <: Max[(Double, Double)] (as shapeless.the in Scala 2), not just Max[(Double, Double)] (as implicitly in Scala 2).

But if we restore using and maxForDoubles then for some reason this confuses inlines: both options for val inst (: maxForPairs[Double, Double] and : Max[(Double, Double)]) produce Deferred inline method max in trait Max cannot be invoked.

This seems to be a bug or underspecified feature. Try to open a ticket at https://github.com/lampepfl/dotty/issues

By the way, sometimes with-syntax behaves weirdly in comparison with alias givens (even without inlines):
https://github.com/lampepfl/dotty/issues/8882

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