Pre-SIP: export, dual* of import

Scala Improvement Process
21

My 2 penith.

I repeatedly hit the need for some “import for export” semantics. The most common thing is if something is declared within a companion object, but needs to be visible at the package level. The second most common is when something is declared deep down in a nested package and needs to be provided as a default when you import the top level package – this happens a lot in the Scala collections.

Let’s look at a sketch of an example:

package bioinf

opaque type DNA = String
object DNA {
   def (s: String) toDNA: DNA = {
    assert(s.forAll("agct" contains _))
    s
  }
}

// pulls DNA.toDNA to be visible in the top-level scope
import for export DNA.toDNA

So we can now write things like:

// import everything in the bioinf top-level scope, including the exports
// so equivalent to import {bioinf._, bioinf.DNA.toDNA}
import bioinf._

"gattaca".toDNA

Currently to make this work we either need a bunch of imports to pull things out of the various objects, or we have to put something top-level in the package e.g . fake types and companion objects. This causes havoc with implied instances, and is a maintenance pain. Adding aliases alters the semantics, as you have to include extra vals/types to shadow in the underlying types.

1 Like
22

As fommil noted a year ago, Andy Scott’s initiative landed in 2.13 as -Yimports:bioinf.DNA where you define packages and “predef” objects as “root imports”. That’s a slightly larger hammer.