scala – 如何开发宏来短路null？

我写的一些关于宏的评论(来源包括在下面)：

>它返回一个Option,当某个时候存在空取消引用时它将是None.
>它使用if-else将每个成员访问点转换为空值保护访问,当前缀为null时返回None.
>它有点复杂,我想象它…
>有一些不适用的极端情况,我所知道的是使用getClass方法 – 这是由编译器特别针对其返回类型处理的.
>隐式转换可能存在不一致.想象一下,你有一个表达式a.b和b是通过隐式转换达到的,所以有效地说,这个表达式就像conv(a).b.现在,问题出现了：我们应该检查a是null还是conv(a)是null还是两者都是？目前,我的宏只检查a是否为null,因为它对我来说似乎更自然,但在某些情况下这可能不是理想的行为.此外,在我的宏中检测隐式转换是一个小问题,在this question中描述.

宏：

def withNullGuards[T](expr: T): Option[T] = macro withNullGuards_impl[T]

def withNullGuards_impl[T](c: Context)(expr: c.Expr[T]): c.Expr[Option[T]] = {
  import c.universe._

  def eqOp = newTermName("==").encodedName
  def nullTree = c.literalNull.tree
  def noneTree = reify(None).tree
  def someApplyTree = Select(reify(Some).tree,newTermName("apply"))

  def wrapInSome(tree: Tree) = Apply(someApplyTree,List(tree))

  def canBeNull(tree: Tree) = {
    val sym = tree.symbol
    val tpe = tree.tpe

    sym != null &&
      !sym.isModule && !sym.isModuleClass &&
      !sym.isPackage && !sym.isPackageClass &&
      !(tpe <:< typeOf[AnyVal])
  }

  def isInferredImplicitConversion(apply: Tree,fun: Tree,arg: Tree) =
    fun.symbol.isImplicit && (!apply.pos.isDefined || apply.pos == arg.pos)

  def nullGuarded(originalPrefix: Tree,prefixTree: Tree,whenNonNull: Tree => Tree): Tree =
    if (canBeNull(originalPrefix)) {
      val prefixVal = c.fresh()
      Block(
        ValDef(Modifiers(),prefixVal,TypeTree(null),prefixTree),If(
          Apply(Select(Ident(prefixVal),eqOp),List(nullTree)),noneTree,whenNonNull(Ident(prefixVal))
        )
      )
    } else whenNonNull(prefixTree)

  def addNullGuards(tree: Tree,whenNonNull: Tree => Tree): Tree = tree match {
    case Select(qualifier,name) =>
      addNullGuards(qualifier,guardedQualifier =>
        nullGuarded(qualifier,guardedQualifier,prefix => whenNonNull(Select(prefix,name))))
    case Apply(fun,List(arg)) if (isInferredImplicitConversion(tree,fun,arg)) =>
      addNullGuards(arg,guardedArg =>
        nullGuarded(arg,guardedArg,prefix => whenNonNull(Apply(fun,List(prefix)))))
    case Apply(Select(qualifier,name),args) =>
      addNullGuards(qualifier,prefix => whenNonNull(Apply(Select(prefix,args))))
    case Apply(fun,args) =>
      addNullGuards(fun,guardedFun => whenNonNull(Apply(guardedFun,args)))
    case _ => whenNonNull(tree)
  }

  c.Expr[Option[T]](addNullGuards(expr.tree,tree => wrapInSome(tree)))
}

编辑

这是一段额外的代码,使语法更好：

def any2question_impl[T,R >: T](c: Context {type PrefixType = any2question[T]})(default: c.Expr[R]): c.Expr[R] = {
  import c.universe._

  val Apply(_,List(prefix)) = c.prefix.tree
  val nullGuardedPrefix = withNullGuards_impl(c)(c.Expr[T](prefix))
  reify {
    nullGuardedPrefix.splice.getOrElse(default.splice)
  }
}

implicit class any2question[T](any: T) {
  def ?[R >: T](default: R): R = macro any2question_impl[T,R]
}

最后,您可以使用以下代码：

val str1: String = "hovercraftfullofeels"
val result1 = str1.substring(3).toUpperCase ? "THERE WAS NULL"
println(result1) // prints "ERCRAFTFULLOFEELS"

val str2: String = null
val result2 = str2.substring(3).toUpperCase ? "THERE WAS NULL"
println(result2) // prints "THERE WAS NULL"