Question

我是Parser Combinators的新手，在编译以下函数时遇到上述错误：

def attachRoad = "attach" ~ ("primary" | "secondary") ~ "road" ~ ident ~ "with" ~ "length" ~ floatingPointNumber ~ "at" ~ floatingPointNumber ^^
{
  case "attach" ~ "primary" ~ "road" ~ road ~ "with" ~ "length" ~ len ~ "at" ~ pos ~ "flow" ~ flow => attachRoadHelper(road, StreetType.PRIMARY, len, flow, pos)
  case "attach" ~ "secondary" ~ "road" ~ road ~ "with" ~ "length" ~ len ~ "at" ~ pos ~ "flow" ~ flow => attachRoadHelper(road, StreetType.SECONDARY, len, flow, pos)
}

所以这给了我

constructor cannot be initiated to expected type

在两种情况下都有解释。

再次查看了文档和StackOverflow上的一些线程，我未能找到解决方案或理解为什么会发生这种情况，因为我在“case”语句中没有看到任何构造函数。（那是Scala在幕后做的事情吗？）

我也尝试将“案例”结果更改为“无”无效。

非常感谢任何见解。

Answer 1

使用括号对所有重要捕获组进行分组并使用~>或<~运算符忽略所有常量标记可能是个好主意。在这种情况下，您只需要匹配重要结果：

def attachRoad =
  ("attach" ~> ("primary" | "secondary")) ~
    ("road" ~> ident) ~
    ("with" ~> "length" ~> floatingPointNumber) ~
    ("at" ~> floatingPointNumber) ^^ {
    case "primary" ~ road ~ len ~ pos =>
      attachRoadHelper(road, StreetType.PRIMARY, len, ???, pos)
    case "secondary" ~ road ~ len ~ pos =>
      attachRoadHelper(road, StreetType.SECONDARY, len, ???, pos)
  }

在你的情况下，很明显你的解析表达式返回四个结果的序列，而你期望五个。

scala解析器组合器 - 构造函数无法启动到预期类型

1 个答案: