方法“|”中解析器的预期类型

Question

我针对 scala 2.8.0 编译了以下代码：

import scala.util.parsing.combinator.{syntactical,PackratParsers}
import syntactical.StandardTokenParsers

object MyParser extends StandardTokenParsers with PackratParsers{
  lexical.reserved ++= Set("int","char","boolean")

  lazy val primitiveType:PackratParser[PrimitiveType[_]] = primitiveChar | primitiveInt | primitiveBool

  lazy val primitiveInt:PackratParser[PrimitiveType[Int]] = "int" ^^ { _ => PrimitiveType[Int]() }

  lazy val primitiveChar:PackratParser[PrimitiveType[Char]] = "char" ^^ { _ => PrimitiveType[Char]() }

  lazy val primitiveBool:PackratParser[PrimitiveType[Boolean]] = "boolean" ^^ { _ => PrimitiveType[Boolean]() }
}

object MyParser2 extends StandardTokenParsers with PackratParsers{
  lexical.reserved ++= Set("int","char","boolean")

  lazy val primitiveType:PackratParser[PrimitiveType[_]] =  primitiveChar | primitiveIntOrBool

  lazy val primitiveIntOrBool:PackratParser[PrimitiveType[_]] = "int" ^^ { _ => PrimitiveType[Int]() } | "boolean" ^^ {_ => PrimitiveType[Boolean]()}

  lazy val primitiveChar:PackratParser[PrimitiveType[Char]] = "char" ^^ { _ => PrimitiveType[Char]()} 
}

case class PrimitiveType[T]()

编译 MyParser1 给出：

error: inferred type arguments  [this.PrimitiveType[_ >: _1 with Boolean <: AnyVal]] do not conform to method |'s type parameter bounds [U >: this.PrimitiveType[_ >: Char with Int <: AnyVal]]

我相信它失败是因为 |方法类型签名，定义为：

def | [U >: T](q: => Parser[U]): Parser[U]

为什么 U 必须是 T 的超类型？ “primitiveType”的返回值应该是什么？

Answer 1

您需要将最后一行更改为

case class PrimitiveType[+T]()

Thisallows for PrimitiveType[Int] <: PrimitiveType[AnyVal] 当您想通过 | 合并 PrimitiveType[Boolean] 和 PrimitiveType[Int] 解析器的结果时需要它。

顺便说一句，我还建议写成

PrimitiveType[AnyVal]

not ，

PrimitiveType[_]

因为这对你的情况来说更准确。

Answer 2

您可能最好将通用 case class PrimitiveType[T] 更改为类层次结构。泛型类型参数在运行时不可用，因此您将无法对解析结果做太多事情...

这将为您提供以下内容（未测试的代码现已测试）：

object MyParser extends StandardTokenParsers with PackratParsers{
  lexical.reserved ++= Set("int","char","boolean")

  lazy val primitiveType:Parser[PrimitiveType] = primitiveChar | primitiveInt | primitiveBool

  lazy val primitiveInt:PackratParser[PrimitiveType] = "int" ^^^ PrimitiveInt

  lazy val primitiveChar:PackratParser[PrimitiveType] = "char" ^^^ PrimitiveChar

  lazy val primitiveBool:PackratParser[PrimitiveType] = "boolean" ^^^ PrimitiveBoolean
}

sealed trait PrimitiveType
case object PrimitiveInt extends PrimitiveType
case object PrimitiveChar extends PrimitiveType
case object PrimitiveBoolean extends PrimitiveType