Multi-file ADTs

[joan38]

Hi,

Here is a proposal to add the permits keyword on sealed traits and classes to allow the implementation of multi file ADTs:

sealed class Shape permits Circle, Square, Rectangle

This would work similarly to Java 17: https://docs.oracle.com/en/java/javase/17/language/sealed-classes-and-interfaces.html

We have multiple instances were we dropped the sealed keyword just because the file was too big.
Some other are using a trick with traits: https://github.com/BalmungSan/scala-multifile-adt

Thanks

[som-snytt]

The use case falls under "prefer composition to inheritance". Delegation to private implementation classes is made trivial by export:

package shapely

sealed trait Shape:
  def area: Double

case class Circle(x: Int, y: Int, r: Int) extends Shape:
  private val circled = Circled(x, y, r)
  export circled.*

case class Square(x: Int, y: Int, s: Int) extends Shape:
  private val squared = Squared(x, y, s)
  export squared.*

Instead of "multifile ADT", I think a more generally useful feature would be "multifile compilation unit".

That would allow a huge class with its huge companion object to be in two files. (That has been requested previously.)

No doubt this could be implemented in a build tool, since the compiler is nominally agnostic about how a compilation unit is sourced.

[joan38]

I like this idea

[joan38]

However I wonder how are the imports going to be interpreted. The order these files in a compilation unit will change if the code is compilable or even its the behavior potentially