How to compile output of a compiler phase?

Question

Using -Xprint flag in scalac we get output of different compiler phases, for example given the following Foo.scala

object Foo {
  val x = 42
}

then scalac -Xprint:jvm Foo.scala outputs

package <empty> {
  object Foo extends Object {
    <static> private[this] val x: Int = _;
    <stable> <accessor> def x(): Int = Foo.this.x;
    def <init>(): Foo.type = {
      Foo.super.<init>();
      Foo.this.x = 42;
      ()
    }
  }
}

How to compile the phase itself, that is, say we have source file jvmphase.scala like so

package <empty> { ...

containing the phase source code instead of the original vanilla Scala source code, then how to achieve something similar to scalac jvmphase.scala?

I guess you can redirect the output to a file and then compile that file? — Luis Miguel Mejía Suárez, Oct 16 '19 at 21:23
@LuisMiguelMejíaSuárez I tried that, it does not seem to work. For example, it does not recognise `` giving `identifier expected but $XMLSTART$< found`. — Mario Galic, Oct 16 '19 at 21:24
@DmytroMitin Just for learning and experimentation purposes I would like to make changes to phase code and see what happens. Perhaps learning how to write the "lowest-level" de-sugared Scala code would make me understand "sugared" Scala better. — Mario Galic, Oct 16 '19 at 21:44
@MarioGalic If you want to make changes between phases maybe you should create compiler plugin. — Dmytro Mitin, Oct 16 '19 at 21:54
@MarioGalic "compiling output of a compiler phase" sounds strange. Literally compiler compiles source file. Output of a compiler phase is not a source any more. For example `jvm` phase is after `erasure` phase. And it's not clear what you expect to get while "compiling output of a compiler phase" besides result of compilation of the original source. — Dmytro Mitin, Oct 16 '19 at 22:09
@DmytroMitin Ah, it seems I miss understood what phase output is. I assumed because it looks a bit like source code it is something I could write. Consider putting your comments in an answer and I can accept it. — Mario Galic, Oct 16 '19 at 23:51

score 3 · Accepted Answer · answered Oct 17 '19 at 07:17

"compiling output of a compiler phase" sounds strange. Literally compiler compiles source file. Output of a compiler phase is not a source any more (although scalac -Xprint:... tries to print it similarly). For example jvm phase is after erasure phase. And it's not clear what you expect to get while "compiling output of a compiler phase" besides result of compilation of the original source.

If you want to make changes between phases maybe you should create compiler plugin.

https://docs.scala-lang.org/overviews/plugins/index.html

https://dotty.epfl.ch/docs/reference/changed-features/compiler-plugins.html

How to compile output of a compiler phase?

1 Answers1