Scala:没有getter就不能写setter？

Question

这有效:

class ButtonCountObserver {
  private var cnt = 0  // private field
  def count = cnt      // reader method
  def count_=(newCount: Int) = cnt = newCount  // writer method
 // ...
}

val b = new ButtonCountObserver 
b.count = 0

但这不

class ButtonCountObserver {
  private var cnt = 0  // private field
  def count_=(newCount: Int) = cnt = newCount  // writer method
 // ...
}

val b = new ButtonCountObserver 
b.count = 0

我得到: error: value count is not a member of ButtonCountObserver
是否可以在没有 getter 的情况下创建一个 setter(使用语法糖)？

Answer 1

该规范要求 setter 和 getter 都被定义为能够使用语法糖来调用 setter:

The interpretation of an assignment to a simple variable x = e depends on the definition of x. If x denotes a mutable variable, then the assignment changes the current value of x to be the result of evaluating the expression e. The type of e is expected to conform to the type of x. If x is a parameterless function defined in some template, and the same template contains a setter function x_= as member, then the assignment x = e is interpreted as the invocation x_=(e ) of that setter function. Analogously, an assignment f .x = e to a parameterless function x is interpreted as the invocation f .x_=(e ). An assignment f (args) = e with a function application to the left of the ‘=’ operator is interpreted as f .update(args, e ), i.e. the invocation of an update function defined by f .

此外，为了使用 setter，getter 必须可见。我不确定这是否被指定

setter/getter 不可见 #1

// error: method x cannot be accessed in x.Test
object x {
  class Test { 
    private[this] var x0: Int = 0
    private[Test] def x = x0
    def x_=(a: Int) = x0 = a 
  }
  val t = new Test
  t.x = 1
}

setter/getter 不可见 #2

//<console>:11: error: type mismatch; found   : x.Test required: ?{val x: ?}
object x {
  class Test { 
    private[this] var x0: Int = 0
    private[this] def x = x0
    def x_=(a: Int) = x0 = a 
  }
  val t = new Test
  t.x = 1
}

setter/getter 可见

object x {
  class Test { 
    private[this] var x0: Int = 0
    private[x] def x = x0
    def x_=(a: Int) = x0 = a 
  }
  val t = new Test
  t.x = 1
}