用类型别名描述递归语法 [英] Describe recursive grammar with type aliases

问题描述

如何用类型别名描述此递归语法:

How can I describe this recursive grammar with type aliases:

type FieldValue = Seq[String] :+: String :+: Int :+: Long :+: CNil
type FieldLeaf = FieldValue :+: SubField :+: CNil
type SubField = Seq[Field]
type Field = (String, FieldLeaf)

按现状，Scala编译器(2.12.1)给我:

As it stands, the Scala compiler (2.12.1) gives me:

Error:(14, 25) illegal cyclic reference involving type FieldLeaf
  type Field = (String, FieldLeaf)

PS的上下文是使用 fastparse 解析递归语法.

PS the context of this is parsing a recursive grammar with fastparse.

编辑(以回应下面@OlivierBlanvillain的回答)

这个答案确实是一件很美的事情，而且正是我一直在寻找的东西，我会为将来记住它.

That answer was really a thing of beauty and exactly what I was looking for, I'll remember it for the future.

但是，由于其他原因，在这种情况下，我不得不采用以下定义:

However, for other reasons, in this particular case I had to go with these definitions instead:

  case class Field(name: String, leaf: FieldLeaf)
  sealed trait FieldLeaf
  sealed trait FieldValue extends FieldLeaf
  case class StringsFieldValue(value: Seq[String]) extends FieldValue
  case class StringFieldValue(value: String) extends FieldValue
  case class IntFieldValue(value: Int) extends FieldValue
  case class LongFieldValue(value: Long) extends FieldValue
  case class SubField(value: Seq[Field]) extends FieldLeaf

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另请参阅: 从递归类型语法实例化

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See also: Instantiate types from recursive type grammar

推荐答案

使用定位点类型.例如:

Use a fix point type. For example:

case class Fix[F[_]](out: F[Fix[F]])

让您写信:

type FieldValue = Seq[String] :+: String :+: Int :+: Long :+: CNil
type FieldLeaf[F] = FieldValue :+: SubField[F] :+: CNil
type SubField[F] = Seq[F]
type Field0[F] = (String, FieldLeaf[F])

type Field = Fix[Field0]

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