scala extending trait with methods returing various types, types mismatch

Question

I do something like this:

sealed trait Foo[T] { def getA: Decoder[T] def getB: Either[Type1, Type2] } trait Hello[T] extends Foo[T] { def getA: Decoder[T] def getB: Type1 } trait World[T] extends Foo[T] { def getA: Decoder[T] def getB: Type2 }

Currently, Either is wrong way to define all types. How to solve it?

Dmytro Mitin · Accepted Answer · 2020-10-22 13:39:17Z

You can try to narrow return type in child traits

trait Hello[T] extends Foo[T] { def getA: Decoder[T] def getB: Left[Type1, Type2] } trait World[T] extends Foo[T] { def getA: Decoder[T] def getB: Right[Type1, Type2] }

Or you can try to introduce a type member

sealed trait Foo[T] { def getA: Decoder[T] type Out def getB: Out } trait Hello[T] extends Foo[T] { def getA: Decoder[T] type Out = Type1 def getB: Type1 } trait World[T] extends Foo[T] { def getA: Decoder[T] type Out = Type2 def getB: Type2 }

Or you can try a type class

sealed trait Foo[T] { type This <: Foo[T] def getA: Decoder[T] def getB(implicit tc: TC[T, This]): tc.Out } trait Hello[T] extends Foo[T] { type This <: Hello[T] def getA: Decoder[T] } trait World[T] extends Foo[T] { type This <: World[T] def getA: Decoder[T] } trait TC[T, F <: Foo[T]] { type Out } object TC { implicit def hello[T, F <: Hello[T]]: TC[T, F] { type Out = Type1 } = null implicit def world[T, F <: World[T]]: TC[T, F] { type Out = Type2 } = null }

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