Add withErrorMessage and withFailureMessage to Parsers.Parser,

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

@@ -443,6 +443,62 @@ trait Parsers {
      *  @return opt(this)
      */
     def ? = opt(this)
+
+    /** Changes the failure message produced by a parser.
+     *
+     *  This doesn't change the behavior of a parser on neither
+     *  success nor error, just on failure. The semantics are
+     *  slightly different than those obtained by doing `| failure(msg)`,
+     *  in that the message produced by this method will always
+     *  replace the message produced, which is not guaranteed
+     *  by that idiom.
+     *
+     *  For example, parser `p` below will always produce the
+     *  designated failure message, while `q` will not produce
+     *  it if `sign` is parsed but `number` is not.
+     *
+     *  {{{
+     *  def p = sign.? ~ number withFailureMessage  "Number expected!"
+     *  def q = sign.? ~ number | failure("Number expected!")
+     *  }}}
+     *
+     *  @param msg The message that will replace the default failure message.
+     *  @return    A parser with the same properties and different failure message.
+     */
+    def withFailureMessage(msg: String) = Parser{ in =>
+      this(in) match {
+        case Failure(_, next) => Failure(msg, next)
+        case other            => other
+      }
+    }
+
+    /** Changes the error message produced by a parser.
+     * 
+     *  This doesn't change the behavior of a parser on neither
+     *  success nor failure, just on error. The semantics are
+     *  slightly different than those obtained by doing `| error(msg)`,
+     *  in that the message produced by this method will always
+     *  replace the message produced, which is not guaranteed
+     *  by that idiom.
+     *
+     *  For example, parser `p` below will always produce the
+     *  designated error message, while `q` will not produce
+     *  it if `sign` is parsed but `number` is not.
+     *
+     *  {{{
+     *  def p = sign.? ~ number withErrorMessage  "Number expected!"
+     *  def q = sign.? ~ number | error("Number expected!")
+     *  }}}
+     *
+     *  @param msg The message that will replace the default error message.
+     *  @return    A parser with the same properties and different error message.
+     */
+    def withErrorMessage(msg: String) = Parser{ in =>
+      this(in) match {
+        case Error(_, next) => Error(msg, next)
+        case other          => other
+      }
+    }
   }
 
   /** Wrap a parser so that its failures become errors (the `|` combinator

test/files/run/parserNoSuccessMessage.check

@@ -0,0 +1,20 @@
+[1.2] failure: string matching regex `\d+' expected but `x' found
+
+-x
+ ^
+[1.1] failure: string matching regex `\d+' expected but `x' found
+
+x
+^
+[1.3] parsed: (Some(-)~5)
+[1.2] parsed: (None~5)
+[1.2] error: Number expected!
+
+-x
+ ^
+[1.1] error: Number expected!
+
+x
+^
+[1.3] parsed: (Some(-)~5)
+[1.2] parsed: (None~5)

test/files/run/parserNoSuccessMessage.scala

@@ -0,0 +1,19 @@
+object Test extends scala.util.parsing.combinator.RegexParsers {
+  def sign = "-"
+  def number = "\\d+".r
+  def p = sign.? ~ number withErrorMessage  "Number expected!"
+  def q = sign.? ~! number withErrorMessage  "Number expected!"
+
+  def main(args: Array[String]) {
+    println(parseAll(p, "-x"))
+    println(parseAll(p, "x"))
+    println(parseAll(p, "-5"))
+    println(parseAll(p, "5"))
+    println(parseAll(q, "-x"))
+    println(parseAll(q, "x"))
+    println(parseAll(q, "-5"))
+    println(parseAll(q, "5"))
+  }
+}
+
+