定义一个新类型，其域是另一类型的子集在Scala中

huangapple
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英文:

Define new type whose domain is subset of another type in Scala

问题

使用偶数的示例，我想定义一个名为“Even”的类型，其域是偶数整数使用函数
val isEven(i: Int): Boolean = i % 2 == 0

如何在Scala 3中使用不透明关键字实现这一点...

opaque type Even = ??? // use Int and filter using isEven 或者如果那行不通，

  1. type Even = ???

理念是使用函数作为构建新类型的基石，而不是通过使用newtype（如scala-even-type-number）或使用Refined Types来使事情复杂化。

英文:

Using the example of even numbers, I would like to define the Even type whose domain is even integers using the function
val isEven(i: Int): Boolean = i % 2 == 0

How would I do this in Scala 3 using the opaque keyword...

opaque type Even = ??? // use Int and filter using isEven or if that does not work,

  1. type Even = ???

The rationale is to use functions as a building block to create new types, rather that complicating matters by using newtype as scala-even-type-number or using Refined Types

答案1

得分: 4

  1. 透明类型 Evens:
  2.   不透明类型 Even = Int
  4.   对象 Even:
  5.     def apply(i: Int): Even = if i % 2 == 0 then i else sys.error(s"$i 是奇数")
  6.     def unsafe(i: Int): Even = i
  7.     def safe(i: Int): Option[Even] = Option.when(i % 2 == 0)(i)
  9.   扩展 (x: Even)：
  10.     def toInt: Int = x
  11.     def +(y: Even): Even = x + y
  12.     def *(y: Int):  Even = x * y
  13.     def half: Int = x / 2
  14. end Evens
  16. import Evens.*
  17. // val x: Even = 2 // 无法编译
  18. val x: Even = Even(2)
  19. // val x: Even = Even(3) // 运行时异常: 3 是奇数

https://docs.scala-lang.org/scala3/book/types-opaque-types.html

https://docs.scala-lang.org/scala3/reference/other-new-features/opaques.html

如果想让 Even(3) 在编译时而非运行时失败，请用以下一种实现替换 def apply(i: Int): Even = ...

  1. import scala.compiletime.{error, erasedValue, constValue, codeOf, summonFrom}
  2. import scala.compiletime.ops.int.%
  3. import scala.compiletime.ops.any.{==, ToString}
  5. inline def apply[I <: Int with Singleton](inline i: I): Even =
  6.   inline erasedValue[I % 2] match
  7.     case _: 0 => i
  8.     case _    => error(codeOf(i) + " 是奇数")
  1. inline def apply[I <: Int with Singleton](inline i: I): Even =
  2.   inline if constValue[I % 2] == 0 then i else error(codeOf(i) + " 是奇数")
  1. inline def apply[I <: Int with Singleton](inline i: I): Even = 
  2.   inline erasedValue[I % 2 == 0] match
  3.     case _: true => i
  4.     case _       => error(constValue[ToString[I]] + " 是奇数")
  1. inline def apply[I <: Int with Singleton](inline i: I): Even = 
  2.   summonFrom {
  3.     case _: (I % 2 =:= 0) => i
  4.     case _                => error(constValue[ToString[I]] + " 是奇数")
  5.   }
  1. inline def apply(inline i: Int): Even =
  2.   inline if i % 2 == 0 then i else error(codeOf(i) + " 是奇数")

https://docs.scala-lang.org/scala3/reference/metaprogramming/inline.html

https://docs.scala-lang.org/scala3/reference/metaprogramming/compiletime-ops.html

英文:

Something like the following:

  1. object Evens:
  2.   opaque type Even = Int
  4.   object Even:
  5.     def apply(i: Int): Even = if i % 2 == 0 then i else sys.error(s&quot;$i is odd&quot;)
  6.     def unsafe(i: Int): Even = i
  7.     def safe(i: Int): Option[Even] = Option.when(i % 2 == 0)(i)
  9.   extension (x: Even)
  10.     def toInt: Int = x
  11.     def +(y: Even): Even = x + y
  12.     def *(y: Int):  Even = x * y
  13.     def half: Int = x / 2
  14. end Evens
  16. import Evens.*
  17. // val x: Even = 2 // doesn&#39;t compile
  18. val x: Even = Even(2)
  19. // val x: Even = Even(3) // RuntimeException: 3 is odd

https://docs.scala-lang.org/scala3/book/types-opaque-types.html

https://docs.scala-lang.org/scala3/reference/other-new-features/opaques.html

If you'd like Even(3) to fail at compile time rather than runtime then replace def apply(i: Int): Even = ... with one of the following implementations

  1. import scala.compiletime.{error, erasedValue, constValue, codeOf, summonFrom}
  2. import scala.compiletime.ops.int.%
  3. import scala.compiletime.ops.any.{==, ToString}
  5. inline def apply[I &lt;: Int with Singleton](inline i: I): Even =
  6.   inline erasedValue[I % 2] match
  7.     case _: 0 =&gt; i
  8.     case _    =&gt; error(codeOf(i) + &quot; is odd&quot;)
  1. inline def apply[I &lt;: Int with Singleton](inline i: I): Even =
  2.   inline if constValue[I % 2] == 0 then i else error(codeOf(i) + &quot; is odd&quot;)
  1. inline def apply[I &lt;: Int with Singleton](inline i: I): Even = 
  2.   inline erasedValue[I % 2 == 0] match
  3.     case _: true =&gt; i
  4.     case _       =&gt; error(constValue[ToString[I]] + &quot; is odd&quot;)
  1. inline def apply[I &lt;: Int with Singleton](inline i: I): Even = 
  2.   summonFrom {
  3.     case _: (I % 2 =:= 0) =&gt; i
  4.     case _                =&gt; error(constValue[ToString[I]] + &quot; is odd&quot;)
  5.   }
  1. inline def apply(inline i: Int): Even =
  2.   inline if i % 2 == 0 then i else error(codeOf(i) + &quot; is odd&quot;)

https://docs.scala-lang.org/scala3/reference/metaprogramming/inline.html

https://docs.scala-lang.org/scala3/reference/metaprogramming/compiletime-ops.html

huangapple
  • 本文由 发表于 2023年4月4日 07:23:43
  • 转载请务必保留本文链接：https://go.coder-hub.com/75924406.html
  • domain-driven-design
  • scala
  • scala-3
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