与具有泛型类型的值进行模式匹配

发布于 2024-11-29 12:15:42 字数 842 浏览 1 评论 0原文

我正在编写基于树的表达式计算器，并且在类型擦除方面遇到了一些麻烦。

树看起来像

sealed abstract class Node[+T]
case class Var[+T](name:String) extends Node[T]
/* SNIP */

评估器是

def eval[T](node:Node[T], context:Map[String, Any]):Option[T] = node match {
  case Var(name) => context.get(name) match {
    case Some(value:T) => Some(value)
    case _ => None
  }
  /* SNIP */
}

代码编译的，但对 Var 节点的类型检查不起作用。所以这个测试失败了：

class ContextEvaluatorTest extends FunSuite with ShouldMatchers {
  test("evaluation with type mismatch") {
    ContextEvaluator.eval(Var[Int]("a"), Map("a" -> "not int")) should equal (None)
  }
}

错误消息是

org.scalatest.TestFailedException: Some(not int) did not equal None

情况看起来像是清单的用例，但我无法正确添加它们。

原文

I'm writing tree-based expression evaluator, ans I've run into some troubles with type-erasure.

Tree looks like

sealed abstract class Node[+T]
case class Var[+T](name:String) extends Node[T]
/* SNIP */

The evaluator is

def eval[T](node:Node[T], context:Map[String, Any]):Option[T] = node match {
  case Var(name) => context.get(name) match {
    case Some(value:T) => Some(value)
    case _ => None
  }
  /* SNIP */
}

The code compiles, but type checks on Var nodes don't work. So this test fails:

class ContextEvaluatorTest extends FunSuite with ShouldMatchers {
  test("evaluation with type mismatch") {
    ContextEvaluator.eval(Var[Int]("a"), Map("a" -> "not int")) should equal (None)
  }
}

Error message is

org.scalatest.TestFailedException: Some(not int) did not equal None

Situation looks like a use-case for manifests, but I couldn't add them properly.

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苍景流年 2024-12-06 12:15:42

这似乎有效：

def eval[T:ClassManifest](node:Node[T], context:Map[String, Any]):Option[T] = node match {
  case Var(name) => context.get(name) match {
    case Some(value:T) if classManifest[T].erasure.isInstance(value) => Some(value)
    case _ => None
  }
  case _ => None
}

但请注意 T 必须是简单类型，据我所知 ClassManifest 无法区分 List[Int] 和列表[字符串]。也许 Manifest 可以做到这一点，但是调查比在底层类上调用 isInstance 更复杂。

This seems to work:

def eval[T:ClassManifest](node:Node[T], context:Map[String, Any]):Option[T] = node match {
  case Var(name) => context.get(name) match {
    case Some(value:T) if classManifest[T].erasure.isInstance(value) => Some(value)
    case _ => None
  }
  case _ => None
}

Note however that T must be a simple type, AFAIK ClassManifest can't distinguish between things like List[Int] and List[String]. Probably Manifestcan do that, but then the investigation is more complicated than calling isInstance on the underlying class.

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