Testing DSL idea: Use string interpolation to mark positions in source code with strong typing.

[retronym]

I've been toying with an idea to use string interpolation to embed have strongly typed positioned assertions in fragments of source code within JUnit tests. Documenting this here for discussion.

scala>  class TestContext() {
     |     val markers = scala.collection.mutable.Buffer[Pos]()
     |     final class Pos() {
     |       markers += this
     |       var pos: Int = -1
     |       def hasCompletions(alts: String*): this.type = {
     |         // this.assertions += () => this.global.completionsAt(pos).containsAll(alts)
     |         this
     |       }
     |       def check(): Unit = ()
     |     }
     |     def check() = markers.foreach(_.check)
     |     implicit class StringContext_code(private val value: StringContext) {
     |       def code(names: Pos*): String = {
     |         var pos = 0
     |         value.parts.zipWithIndex.foreach { case (part, i) =>
     |           pos += part.length
     |           if (i > 0) names(i - 1).pos = pos
     |         }
     |         value.parts.mkString
     |       }
     |     }
     |   }
defined class TestContext

scala>   def mkContext = new TestContext()
mkContext: TestContext

scala>

scala>   val c = mkContext; import c._
c: TestContext = TestContext@14c99bf6
import c._

scala> val m1, m2, m3 = new Pos
m1: c.Pos = TestContext$Pos@661e1399
m2: c.Pos = TestContext$Pos@3ffd4b12
m3: c.Pos = TestContext$Pos@424f02b8

scala> val code: String =
     |     code"""
     |     class C {
     |       def foo = "".reverse${m1}.reverse$m2.rev${m3.hasCompletions("reverse")}
     |     }
     |     """
code: String =
"
    class C {
      def foo = "".reverse.reverse.rev
    }
    "

scala>   m2.hasCompletions("charAt")
res0: m2.type = TestContext$Pos@3ffd4b12

scala>   c.markers.foreach(m => println(m.pos))
49
53
64

scala>   c.check()

This is an alternative to the "magic comment markers" used in the presentation compiler tests, and is more general than the approach taken in https://github.com/scala/scala/blob/2.12.x/test/junit/scala/tools/nsc/interpreter/CompletionTest.scala#L31

Different test use cases could add extension methods to Pos to work with different fragments of testing DSL.

[retronym]

/cc

@fommil (author of https://github.com/ensime/pcplod) @alefas / @pavelfatin who've written a variety of test DSL for IntellIJs typechecker @smarter @DarkDimius for a perspective from dotty

[dwijnand]

Pretty cool, IMO.