Scala example source code file (MatchWarnings.scala)

This example Scala source code file (MatchWarnings.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

bind, classinfotype, collection, compiler, list, nosymbol, nsc, polytype, reflection, string, symbol, tree, treemakerwarnings, type, utilities

The MatchWarnings.scala Scala example source code

/* NSC -- new Scala compiler
 *
 * Copyright 2011-2013 LAMP/EPFL
 * @author Adriaan Moors
 */

package scala.tools.nsc.transform.patmat

import scala.language.postfixOps
import scala.collection.mutable
import scala.reflect.internal.util.Statistics

trait MatchWarnings {
  self: PatternMatching =>

  import global._

  trait TreeMakerWarnings {
    self: MatchTranslator =>

    import typer.context

    // Why is it so difficult to say "here's a name and a context, give me any
    // matching symbol in scope" ? I am sure this code is wrong, but attempts to
    // use the scopes of the contexts in the enclosing context chain discover
    // nothing. How to associate a name with a symbol would would be a wonderful
    // linkage for which to establish a canonical acquisition mechanism.
    private def matchingSymbolInScope(pat: Tree): Symbol = {
      def declarationOfName(tpe: Type, name: Name): Symbol = tpe match {
        case PolyType(tparams, restpe)  => tparams find (_.name == name) getOrElse declarationOfName(restpe, name)
        case MethodType(params, restpe) => params find (_.name == name) getOrElse declarationOfName(restpe, name)
        case ClassInfoType(_, _, clazz) => clazz.rawInfo member name
        case _                          => NoSymbol
      }
      pat match {
        case Bind(name, _) =>
          context.enclosingContextChain.foldLeft(NoSymbol: Symbol)((res, ctx) =>
            res orElse declarationOfName(ctx.owner.rawInfo, name))
        case _ => NoSymbol
      }
    }

    // Issue better warnings than "unreachable code" when people mis-use
    // variable patterns thinking they bind to existing identifiers.
    //
    // Possible TODO: more deeply nested variable patterns, like
    //   case (a, b) => 1 ; case (c, d) => 2
    // However this is a pain (at least the way I'm going about it)
    // and I have to think these detailed errors are primarily useful
    // for beginners, not people writing nested pattern matches.
    def checkMatchVariablePatterns(cases: List[CaseDef]) {
      // A string describing the first variable pattern
      var vpat: String = null
      // Using an iterator so we can recognize the last case
      val it = cases.iterator

      def addendum(pat: Tree) = {
        matchingSymbolInScope(pat) match {
          case NoSymbol   => ""
          case sym        =>
            val desc = if (sym.isParameter) s"parameter ${sym.nameString} of" else sym + " in"
            s"\nIf you intended to match against $desc ${sym.owner}, you must use backticks, like: case `${sym.nameString}` =>"
        }
      }

      while (it.hasNext) {
        val cdef = it.next()
        // If a default case has been seen, then every succeeding case is unreachable.
        if (vpat != null)
          context.unit./*error*/warning(cdef.body.pos, "unreachable code due to " + vpat + addendum(cdef.pat))
        // If this is a default case and more cases follow, warn about this one so
        // we have a reason to mention its pattern variable name and any corresponding
        // symbol in scope.  Errors will follow from the remaining cases, at least
        // once we make the above warning an error.
        else if (it.hasNext && (treeInfo isDefaultCase cdef)) {
          val vpatName = cdef.pat match {
            case Bind(name, _)   => s" '$name'"
            case _               => ""
          }
          vpat = s"variable pattern$vpatName on line ${cdef.pat.pos.line}"
          context.unit.warning(cdef.pos, s"patterns after a variable pattern cannot match (SLS 8.1.1)" + addendum(cdef.pat))
        }
      }
    }
  }
}