scala - 隐藏和限定DSL中的隐式变量创建

在开发DSL时，最简单的方法是限制隐式变量的范围，同时隐藏定义了此类隐式变量的事实？

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
}

以下内容接近，但不具有所有所需的属性...

// 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
}

最佳答案

您可以创建一个宏来进行转换

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

进入

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

我认为没有比上一个没有宏的示例更好的方法了。

关于scala - 隐藏和限定DSL中的隐式变量创建，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/23190917/