Scala 3中元组类型工作原理：为何无法匹配HList形式类型？

问题背景 #

有人告知Scala 3的元组表现类似Scala 2中shapeless的HList，因此进行了如下简单实验：

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

val _: *:[Int, *:[String, Double]] = tuple // 无法通过编译
val _: *:[Double, *:[String, Int]] = tuple // 无法通过编译

根据HList的定义，上述两种类型匹配中至少有一种应该可行，但实际均失败，完整报错信息如下：

[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

原因解析 #

1. 类型结构不完整 #

Scala 3的元组确实基于*:（Cons运算符）和EmptyTuple构建，(A, B, C)的等价底层类型是A *: B *: C *: EmptyTuple。但你写的*:[Int, *:[String, Double]]等价于Int *: String *: Double，缺少了末尾的*: EmptyTuple，和元组的完整类型不匹配，这是第一个匹配失败的核心原因。

2. 类型顺序严格匹配 #

第二个匹配尝试Double *: String *: Int是元组类型的完全反转，不管是Scala 3元组还是shapeless HList，都是严格有序的异构列表，元素类型的顺序是类型的一部分。(Int, String, Double)对应的HList类型只能是Int *: String *: Double *: EmptyTuple，反转后的类型和原类型完全不兼容，自然无法通过编译。

3. 正确的写法示例 #

如果要让类型匹配通过，需要写出完整的HList风格类型，或者直接使用元组的标准类型：

// 完整HList风格类型匹配
val _: Int *: String *: Double *: EmptyTuple = tuple // 编译通过
// 标准元组类型匹配
val _: (Int, String, Double) = tuple // 编译通过

内容的提问来源于stack exchange，提问作者tribbloid