“弱一致性"是什么概念?在斯卡拉?

问题描述

我最近刚遇到弱一致性"这个词(在 Stack Overflow 用户 retronym 对 的回答中如何设置隐式转换以允许数字类型之间的算术运算?).

I just recently encountered the term "Weak Conformance" (in Stack Overflow user retronym's answer to How to set up implicit conversion to allow arithmetic between numeric types?).

这是什么?

推荐答案

• 字节
• 字节 <: 字符
• 短 <: Int
• Int
• 长
• Float
  • Byte <: Short
  • Byte <: Character
  • Short <: Int
  • Int <: Long
  • Long <: Float
  • Float <: Double

  弱最小上限是弱的最小上限一致性.

  这个用在什么地方?一方面，它决定了 if 表达式的类型:

  Where is this used? For one thing, it determines the type of if expressions:

  条件表达式的类型是 e2 和 e3 类型的弱最小上限(第 3.5.3 节)

  在 Scala 2.7.x 中，这将是 AnyVal 类型，Int 和 Double 的最小上限.在 2.8.x 中，它的类型为 Double.

  In Scala 2.7.x, this would by of type AnyVal, the least uppper bound of Int and Double. In 2.8.x, it types as Double.

  scala> if (true) 1 else 1d
  res0: Double = 1.0
  

  同样:

  scala> try { 1 } catch { case _ => 1.0 }
  res2: Double = 1.0
  
  scala> (new {}: Any) match { case 1 => 1; case _ => 1.0 }
  res6: Double = 1.0
  
  scala> def pf[R](pf: PartialFunction[Any, R]): PartialFunction[Any, R] = pf
  pf: [R](pf: PartialFunction[Any,R])PartialFunction[Any,R]
  
  scala> pf { case 1 => 1; case _ => 1d }
  res4: PartialFunction[Any,Double] = <function1>
  

  它的另一个用处是类型推断:

  Another place it is used is in type inference:

  scala> def foo[A](a1: A, a2: A): A = a1
  foo: [A](a1: A,a2: A)A
  
  scala> foo(1, 1d)
  res8: Double = 1.0
  
  scala> def foos[A](as: A*): A = as.head
  foos: [A](as: A*)A
  
  scala> foos(1, 1d)
  res9: Double = 1.0
  

  也用于简单的数字加宽:

  And also for simple numeric widening:

  数字加宽.如果 e 有一个原语弱符合的数字类型(第 3.5.3 节)到预期的类型，它是使用一个扩展到预期类型的6.26 隐式转换 97 种数字转换方法 toShort、toChar、toInt, toLong, toFloat, toDouble在 §12.2.1 中定义.预期类型是原始数字类型 Byte、Short 或Char，表达式 e 是一个在范围内拟合的整数文字那种类型，它被转换为该类型中的相同文字.

  scala> 1: Double
  res10: Double = 1.0
  

  更新

  正如 Daniel 所指出的，规范关于哪些类型具有弱一致性是错误的.让我们问问编译器本身:

  As pointed out by Daniel, the spec is wrong about which types have weak conformance. Let's ask the compiler itself:

  scala> :power
  ** Power User mode enabled - BEEP BOOP      **
  ** scala.tools.nsc._ has been imported      **
  ** New vals! Try repl, global, power        **
  ** New cmds! :help to discover them         **
  ** New defs! Type power.<tab> to reveal     **
  
  scala> settings.maxPrintString = 10000
  
  
  scala> import global.definitions._
  import global.definitions._
  
  scala> (for{c1 <- ScalaValueClasses;
        c2 <- ScalaValueClasses
        isNSC = isNumericSubClass(c1, c2)
        if isNSC
    } yield ("isNumericSubClass (%s, %s) = %b" format (c1, c2, isNSC))).mkString("\n")
  
  
  res5: String =
  isNumericSubClass (class Byte, class Byte) = true
  isNumericSubClass (class Byte, class Short) = true
  isNumericSubClass (class Byte, class Int) = true
  isNumericSubClass (class Byte, class Long) = true
  isNumericSubClass (class Byte, class Float) = true
  isNumericSubClass (class Byte, class Double) = true
  isNumericSubClass (class Short, class Short) = true
  isNumericSubClass (class Short, class Int) = true
  isNumericSubClass (class Short, class Long) = true
  isNumericSubClass (class Short, class Float) = true
  isNumericSubClass (class Short, class Double) = true
  isNumericSubClass (class Int, class Int) = true
  isNumericSubClass (class Int, class Long) = true
  isNumericSubClass (class Int, class Float) = true
  isNumericSubClass (class Int, class Double) = true
  isNumericSubClass (class Long, class Long) = true
  isNumericSubClass (class Long, class Float) = true
  isNumericSubClass (class Long, class Double) = true
  isNumericSubClass (class Char, class Int) = true
  isNumericSubClass (class Char, class Long) = true
  isNumericSubClass (class Char, class Char) = true
  isNumericSubClass (class Char, class Float) = true
  isNumericSubClass (class Char, class Double) = true
  isNumericSubClass (class Float, class Float) = true
  isNumericSubClass (class Float, class Double) = true
  isNumericSubClass (class Double, class Double) = true
  

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