How to use :phase power command in REPL?

Question

Power mode of Scala REPL's enables :phase command:

:phase <phase>           set the implicit phase for power commands

I cannot seem to find documentation on how to use this command. For example, say I set parser compiler phase

scala> :phase parser
Active phase is now: Parser

What power command has been affected by it? How do I call these affected power commands? How does it differ from :settings -Xprint:parser?

Answer 1

The command causes your code to be evaluated enteringPhase, which is accomplished using intp.setExecutionWrapper. You can witness the wrapping with -Dscala.repl.debug.

It wraps the "print" of REPL:

lazy val $print: _root_.java.lang.String = $line4.$read.$iw.$iw.$r.phased.atCurrent {

The phased object just adapts the call to enteringPhase.

Here is an example where you see the erased type at a later phase:

scala> global.rootMirror.staticClass("scala.Option").typeSignature
res0: $r.global.Type =
[+A <: <?>]AnyRef
        with IterableOnce[A]
        with Product
        with Serializable {
  def <init>(): Option[A]
  final def isEmpty: Boolean
  final def isDefined: Boolean
  final override def knownSize: Int
  def get: A
  final def getOrElse[B >: A](default: => B): B
  final def orNull[A1 >: A](implicit ev: Null <:< A1): A1
  final def map[B](f: A => B): Option[B]
  final def fold[B](ifEmpty: => B)(f: A => B): B
  final def flatMap[B](f: A => Option[B]): Option[B]
  def flatten[B](implicit ev: A <:< Option[B]): Option[B]
  final def filter(p: A => Boolean): Option[A]
  final def filterNot(p: A => Boolean): Option[A]
  final def nonEmpty: Boolean
  final def withFilter(p: A => Boolean): Option.this.WithFilter
  class WithFilter extends AnyRef
  final def...

scala> global.rootMirror.staticClass("scala.Option").typeSignature.getClass
res1: Class[_ <: $r.global.Type] = class scala.reflect.internal.Types$PolyType

scala> :phase cleanup
Active phase is now: Cleanup

scala> global.rootMirror.staticClass("scala.Option").typeSignature.getClass
res2: Class[_ <: $r.global.Type] = class scala.reflect.internal.Types$ClassInfoType

scala> global.rootMirror.staticClass("scala.Option").typeSignature
res3: $r.global.Type =
Object
        with scala.collection.IterableOnce
        with Product
        with java.io.Serializable {
  def <init>(): Option
  final def isEmpty(): Boolean
  final def isDefined(): Boolean
  final override def knownSize(): Int
  def get(): Object
[snip]

The :type command in REPL is not affected because it calls exitingTyper.