在 Scala 3 中，元组类型如何工作？

Question

我被告知 Scala 3 元组的行为类似于 Scala 2 中的无形状 HList，所以我做了一个简单的实验：

    val tuple = (1, "String", 1.1)

    val _: *:[Int, *:[String, Double]] = tuple // not working

    val _: *:[Double, *:[String, Int]] = tuple // not working

其中任何一个都应该基于 HList 定义工作，这是怎么发生的？

完整错误：

[Error] /home/peng/git/dottyspike/src/main/scala/com/tribbloids/spike/dotty/NewTuple.scala:15:42: Found:    (tuple : (Int, String, Double))
Required: Int *: String *: Double

Explanation
===========

Tree: tuple
I tried to show that
  (tuple : (Int, String, Double))
conforms to
  Int *: String *: Double
but none of the attempts shown below succeeded:

  ==> (tuple : (Int, String, Double))  <:  Int *: String *: Double class dotty.tools.dotc.core.Types$CachedTermRef class dotty.tools.dotc.core.Types$CachedAppliedType
    ==> (Int, String, Double)  <:  Int *: String *: Double class dotty.tools.dotc.core.Types$CachedAppliedType class dotty.tools.dotc.core.Types$CachedAppliedType
      ==> (String, Double)  <:  String *: Double class dotty.tools.dotc.core.Types$CachedAppliedType class dotty.tools.dotc.core.Types$CachedAppliedType
        ==> Double *: EmptyTuple.type  <:  Double class dotty.tools.dotc.core.Types$CachedAppliedType class dotty.tools.dotc.core.Types$CachedTypeRef  = false

The tests were made under the empty constraint

[Error] /home/peng/git/dottyspike/src/main/scala/com/tribbloids/spike/dotty/NewTuple.scala:17:42: Found:    (tuple : (Int, String, Double))
Required: Double *: String *: Int

Explanation
===========

Tree: tuple
I tried to show that
  (tuple : (Int, String, Double))
conforms to
  Double *: String *: Int
but none of the attempts shown below succeeded:

  ==> (tuple : (Int, String, Double))  <:  Double *: String *: Int class dotty.tools.dotc.core.Types$CachedTermRef class dotty.tools.dotc.core.Types$CachedAppliedType
    ==> (Int, String, Double)  <:  Double *: String *: Int class dotty.tools.dotc.core.Types$CachedAppliedType class dotty.tools.dotc.core.Types$CachedAppliedType
      ==> Int  <:  Double class dotty.tools.dotc.core.Types$CachedTypeRef class dotty.tools.dotc.core.Types$CachedTypeRef  = false

The tests were made under the empty constraint

two errors found

Answer 1

找到正确的定义：

    val _: Int *: String *: Double *: EmptyTuple = tuple