scala - 在 DSL 中隐藏和限定隐式变量的创建

Question

在开发 DSL 时，限制隐式变量的范围并同时隐藏定义了此类隐式变量的事实的最简洁方法是什么？

例如，这是所需的行为......

object External
{
    def funNeedingValue(implicit a : String)
    {
        println(a)
    }
}

object Main extends App
{
    useValue("Hi") {
        // Implicit string "Hi" is only defined in this block
        External.funNeedingValue // Prints "Hi"
    }

    External.funNeedingValue // Compilation error: No implicit String defined
}

以下接近，但没有所有所需的属性......

// The following works, but does not hide the fact that there is an implicit  
// variable defined.

object Main extends App
{
    {
        implicit val implicitValue = "Hi"
        External.funNeedingValue // Prints "Hi"
    }

    External.funNeedingValue // Compilation error: No implicit String defined
}

// The following hides that there is an implicit variable defined, but breaks
// the scoping requirement and destroys thread safety.

abstract class Parent
{
    implicit var implicitValue = ""

    def useValue(valueToMakeImplicit : String)(f : => Unit)
    {
        implicitValue = valueToMakeImplicit
        f()
    }
}

class Child extends Parent
{
    def go()
    {
        useValue("Hi") {
            External.funNeedingValue // Prints "Hi"
        }

        External.funNeedingValue // Scoping issue: also prints "Hi"
    }
}

object Main extends App
{
    new Child().go()
}

// The following works, but is harder to read and still doesn't really
// hide the implicit value

object Main extends App
{
    def useValue(valueToMakeImplicit : String)(f : String => Unit)
    {
        f(valueToMakeImplicit)
    }

    useValue("Hi") { 
        implicit value : String => {
            External.funNeedingValue // Prints "Hi"
        }
    }

    External.funNeedingValue // Compilation error: No implicit String defined
}

Answer 1

你可以制作一个转换的宏

useValue("Hi") {
    // Implicit string "Hi" is only defined in this block
    External.funNeedingValue // Prints "Hi"
}

进入

{
  implicit val iString: String = "Hi"
  External.funNeedingValue
}

我认为没有宏就不可能比上一个例子做得更好。