Question

我正在Scala（kiama）中创建一个迷你java编译器。我的一个名为“tipe”的代码块给了我一个错误，我在Scala的介绍性知识不能破解。

这是我的代码（有点不完整，但我不相信给我错误）

lazy val tipe : PackratParser[Type] =
 "bool"|
 "int" |
 "obj" |
 tipe ~("->" ~> tipe) |
 ("(" ~> tipe <~")")

当我尝试编译程序时出现以下错误：

发现类型不匹配：

找到：SyntaxAnalysis.this.Parser [Object]

必需：SyntaxAnalysis.this.PackratParser [funjs.FunJSTree.type]

tipe~（“ - ＆gt;”〜＆gt; tipe）|

用箭头指向|

任何帮助都会受到赞赏，我是Scala的新手，这对我来说相当复杂。

Answer 1

The compiler supposes tipe has the type you provide: PackratParser[Type]. This means tipe ~("->" ~> tipe) is a Parser[Type ~ Type], while "bool" etc. are converted to Parser[String]. Combining Parser[String] and Parser[Type ~ Type] using | gives you a Parser[Object] (as the common supertype of String and Type ~ Type). To solve this, you need to make sure every alternative (argument of |) is a Parser[Type]. Typically it should look like

lazy val tipe : PackratParser[Type] =
 "bool" ^^^ BoolType |
 ...
 tipe ~("->" ~> tipe) ^^ { case (t1, t2) => someFunctionOf(t1, t2) } |
 ("(" ~> tipe <~")")

using ^^ and ^^^ combinators.

(Note: if you are unfamiliar with { case (t1, t2) => ... } syntax, I recommend starting with something more basic.)

键入missmatch with packrat scala / kiama error

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