用类型别名描述递归语法

时间:2017-03-02 06:36:22

标签: scala recursive-datastructures recursive-descent type-alias fastparse

如何使用类型别名来描述此递归语法:

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

就目前而言,Scala编译器(2.12.1)给了我:

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。

编辑(回复@ OlivierBlanvillain'以下答案)

这个答案真的很美,而且正是我想要的,我会记住它的未来。

然而,由于其他原因,在这种特殊情况下,我不得不采用这些定义:

  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

另见: Instantiate types from recursive type grammar

1 个答案:

答案 0 :(得分:3)

使用修正点类型。例如:

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

让你写:

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]