scala – 在DSL中隐藏和确定隐式变量创建

举个例子,这是理想的行为……

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
}

以下是关闭,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
}