对于Scala,是否存在类型安全的等于===
实现,其开销超过==
?也就是说,与Scalaz和ScalaUtils中的===
不同,这是一个使用直接宏来执行检查的实现?
我想在很多地方使用===
,但这些都是热点,所以我不希望这会产生额外的运行时成本(比如构建类型等)。
答案 0 :(得分:0)
基于机械师的答案可能是最好的。这是一个更具有hackish的变体,用于检测推断AnyRef
或Product with Serializable
或两个不相关的案例类(import scala.collection.breakOut
import scala.language.experimental.macros
import scala.reflect.macros.blackbox
object Implicits {
implicit class TripleEquals[A](a: A) {
def === [B >: A](b: B): Boolean = macro Macros.equalsImpl[A, B]
}
}
object Macros {
private val positiveList = Set("scala.Boolean", "scala.Int", "scala.Long",
"scala.Float", "scala.Double", "scala.Option)
private val negativeList = Set("java.lang.Object", "java.io.Serializable",
"<refinement>")
def equalsImpl[A: c.WeakTypeTag, B: c.WeakTypeTag](c: blackbox.Context)
(b: c.Expr[A]): c.Tree = {
import c.universe._
val bTpe = weakTypeOf[B]
val base = bTpe.baseClasses
val names: Set[String] = base.collect {
case sym if sym.isClass => sym.fullName
} (breakOut)
// if a primitive is inferred, we're good. otherwise:
if (names.intersect(positiveList).isEmpty) {
// exclude all such as scala.Product, scala.Equals
val withoutTopLevel = names.filterNot { n =>
val i = n.lastIndexOf('.')
i == 5 && n.startsWith("scala")
}
// exclude refinements and known Java types
val excl = withoutTopLevel.diff(negativeList)
if (excl.isEmpty) {
c.abort(c.enclosingPosition, s"Inferred type is too generic: `$bTpe`")
}
}
// now simply rewrite as `a == b`
val q"$_($a)" = c.prefix.tree
q"$a == $b"
}
}
)的典型组合等案例:
Some(1) === Some("hello")
这不适用于更高级别的类型,但是,元组故意失败,而不幸的是{{1}}编译。
修改:内置的a small library,可以对其进行改进,以支持更高级别的类型。