Why doesn't scala infer the type members of an inherited trait?

问题

I have a group of types that each have their own type member:

sealed trait FieldType {
    type Data
    def parse(in: String): Option[Data]
}
object Name extends FieldType { 
    type Data = String 
    def parse(in: String) = Some(in)
}
object Age extends FieldType { 
    type Data = Int 
    def parse(in: String) = try { Some(in.toInt) } catch { case _ => None }
}

And I have a group of types that operate on sets of the FieldTypes (using boilerplate rather than abstracting over arity):

sealed trait Schema {
    type Schema <: Product
    type Data <: Product
    val schema: Schema
    def read(in: Seq[String]): Option[Data]
}
trait Schema1 extends Schema {
    type D1
    type FT1 <: FieldType { type Data = D1 }
    type Schema = Tuple1[FT1]
    type Data = Tuple1[D1]
    def read(in: Seq[String]) = schema._1.parse(in(0)).map(Tuple1.apply)
}
trait Schema2 extends Schema {
    type D1
    type D2
    type FT1 <: FieldType { type Data = D1 }
    type FT2 <: FieldType { type Data = D2 }
    type Schema = (FT1, FT2)
    type Data = (D1, D2)
    def read(in: Seq[String]) = {
        for {
            f <- schema._1.parse(in(0))
            s <- schema._2.parse(in(1))
        } yield (f, s)
    }
}

I thought I could use this system to elegantly define sets of fields that are meaningful because scala would be able to infer the type members:

class Person extends Schema2 {
    val schema = (Name, Age)
}

However, this doesn't compile! I have to include definitions for all the type members:

class Person extends Schema2 {
    type D1 = String; type D2 = Int
    type FT1 = Name.type; type FT2 = Age.type
    val schema = (Name, Age)
}

How come scala can't infer D1,... and FT1,...? How can I refactor this so I don't have to specify the type variables in Person?

Note: Once I have a better understanding of macros, I plan to use them for the Schema types. Also, I'd rather not use shapeless. It's a great library, but I don't want to pull it in to solve this one problem.

回答1:

By declaring this:

val schema: Schema

you specify that schema must of type Schema or any of its subtypes. Hence, knowing the type of schema, you cannot infer Schema because it could be any supertype of schema.type.

You can solve your problem by reversing completely the thing: define the type alias in terms of schema.type:

trait Schema2 extends Schema {
    type Schema = (FieldType, FieldType)
    type FT1 = schema._1.type
    type FT2 = schema._2.type
    type D1 = FT1#Data
    type D2 = FT2#Data
    type Data = (D1, D2)
    def read(in: Seq[String]) = {
        for {
            f <- schema._1.parse(in(0))
            s <- schema._2.parse(in(1))
        } yield (f, s)
    }
}

(Not sure it will actually work, but in theory this should typecheck.)

来源：https://stackoverflow.com/questions/24983882/why-doesnt-scala-infer-the-type-members-of-an-inherited-trait

标签

scala

scala-2.10