Add non backtracking versions of ~> and <~

Author: JanBessai

src/main/scala/scala/util/parsing/combinator/Parsers.scala

@@ -314,6 +314,36 @@ trait Parsers {
     def ~! [U](p: => Parser[U]): Parser[~[T, U]]
       = OnceParser{ (for(a <- this; b <- commit(p)) yield new ~(a,b)).named("~!") }
 
+
+    /** A parser combinator for non-back-tracking sequential composition which only keeps the right result.
+     *
+     * `p ~>! q` succeeds if `p` succeds and `q` succeds on the input left over by `p`.
+     * In case of failure, no back-tracking is performed (in an earlier parser produced by the `|` combinator).
+     *
+     * @param q a parser that will be executed after `p` (this parser) succeeds -- evaluated at most once, and only when necessary
+     * @return a `Parser` that -- on success -- reutrns the result of `q`.
+     *         The resulting parser fails if either `p` or `q` fails, this failure is fatal.
+     */
+    @migration("The call-by-name argument is evaluated at most once per constructed Parser object, instead of on every need that arises during parsing.", "2.9.0")
+    def ~>! [U](q: => Parser[U]): Parser[U] = { lazy val p = q // lazy argument
+      OnceParser { (for(a <- this; b <- commit(p)) yield b).named("~>!") }
+    }
+
+    /** A parser combinator for non-back-tracking sequential composition which only keeps the left result.
+     *
+     * `p <~! q` succeeds if `p` succeds and `q` succeds on the input left over by `p`.
+     * In case of failure, no back-tracking is performed (in an earlier parser produced by the `|` combinator).
+     *
+     * @param q a parser that will be executed after `p` (this parser) succeeds -- evaluated at most once, and only when necessary
+     * @return a `Parser` that -- on success -- reutrns the result of `p`.
+     *         The resulting parser fails if either `p` or `q` fails, this failure is fatal.
+     */
+    @migration("The call-by-name argument is evaluated at most once per constructed Parser object, instead of on every need that arises during parsing.", "2.9.0")
+    def <~! [U](q: => Parser[U]): Parser[T] = { lazy val p = q // lazy argument
+      OnceParser { (for(a <- this; b <- commit(p)) yield a).named("<~!") }
+    }
+
+
     /** A parser combinator for alternative composition.
      *
      *  `p | q` succeeds if `p` succeeds or `q` succeeds.

src/test/scala/scala/util/parsing/combinator/RegexParsersTest.scala

@@ -38,6 +38,33 @@ class RegexParsersTest {
     assertEquals(result(5), extractResult(parseAll(q, "5")))
   }
 
+  @Test
+  def parserSkippingResult: Unit = {
+    object parser extends RegexParsers {
+      def quote = "\""
+      def string = """[a-zA-Z]*""".r
+      type ResultType = String
+      def p: Parser[ResultType] = quote ~> string <~ quote
+      def q: Parser[ResultType] = quote ~>! string <~! quote
+      def halfQuoted = quote ~ string ^^ { case q ~ s => q + s }
+    }
+    import parser._
+    val failureLq = parseAll(p, "\"asdf").asInstanceOf[Failure]
+    val failureRq = parseAll(p, "asdf\"").asInstanceOf[Failure]
+    val failureQBacktrackL = parseAll(q | quote, "\"").asInstanceOf[Error]
+    val failureQBacktrackR = parseAll(q | halfQuoted, "\"asdf").asInstanceOf[Error]
+
+    val successP = parseAll(p, "\"asdf\"").get
+    assertEquals(successP, "asdf")
+    val successPBacktrackL = parseAll(p | quote, "\"").get
+    assertEquals(successPBacktrackL, "\"")
+    val successPBacktrackR = parseAll(p | halfQuoted, "\"asdf").get
+    assertEquals(successPBacktrackR, "\"asdf")
+
+    val successQ = parseAll(q, "\"asdf\"").get
+    assertEquals(successQ, "asdf")
+  }
+
   @Test
   def parserFilter: Unit = {
     object parser extends RegexParsers {