Support command prefixes in REPL

e.g. `:q` or `:qu` instead of `:quit`.

If exactly one command matching the prefix is found, it is executed.

If multiple matching commands are found, an error is shown. (Currently
this is not possible, as each command starts with a different letter.)

Author: cb372

compiler/src/dotty/tools/repl/ParseResult.scala

@@ -41,6 +41,9 @@ sealed trait Command extends ParseResult
 /** An unknown command that will not be handled by the REPL */
 case class UnknownCommand(cmd: String) extends Command
 
+/** An ambiguous prefix that matches multiple commands */
+case class AmbiguousCommand(cmd: String) extends Command
+
 /** `:load <path>` interprets a scala file as if entered line-by-line into
  *  the REPL
  */
@@ -116,19 +119,29 @@ object ParseResult {
     stats
   }
 
+  private val commands: List[(String, String => ParseResult)] = List(
+    Quit.command -> (_ => Quit),
+    Help.command -> (_  => Help),
+    Reset.command -> (_  => Reset),
+    Imports.command -> (_  => Imports),
+    Load.command -> (arg => Load(arg)),
+    TypeOf.command -> (arg => TypeOf(arg)),
+    DocOf.command -> (arg => DocOf(arg))
+  )
+
   def apply(source: SourceFile)(implicit state: State): ParseResult = {
     val sourceCode = source.content().mkString
     sourceCode match {
       case "" => Newline
-      case CommandExtract(cmd, arg) => cmd match {
-        case Quit.command => Quit
-        case Help.command => Help
-        case Reset.command => Reset
-        case Imports.command => Imports
-        case Load.command => Load(arg)
-        case TypeOf.command => TypeOf(arg)
-        case DocOf.command => DocOf(arg)
-        case _ => UnknownCommand(cmd)
+      case CommandExtract(cmd, arg) => {
+        val matchingParsers = commands.collect {
+          case (command, f) if command.startsWith(cmd) => f
+        }
+        matchingParsers match {
+          case Nil => UnknownCommand(cmd)
+          case f :: Nil => f(arg)
+          case _ => AmbiguousCommand(cmd)
+        }
       }
       case _ =>
         implicit val ctx: Context = state.context

compiler/src/dotty/tools/repl/ReplDriver.scala

@@ -329,6 +329,10 @@ class ReplDriver(settings: Array[String],
       out.println(s"""Unknown command: "$cmd", run ":help" for a list of commands""")
       state
 
+    case AmbiguousCommand(cmd) =>
+      out.println(s""""$cmd" matches more than one command. Try typing a few more characters. Run ":help" for a list of commands""")
+      state
+
     case Help =>
       out.println(Help.text)
       state