Add all the A tasks

Task/Arithmetic-evaluation/Scala/arithmetic-evaluation.scala

@@ -0,0 +1,46 @@
+package org.rosetta.arithmetic_evaluator.scala
+
+object ArithmeticParser extends scala.util.parsing.combinator.RegexParsers {
+
+  def readExpression(input: String) : Option[()=>Int] = {
+    parseAll(expr, input) match {
+      case Success(result, _) =>
+        Some(result)
+      case other =>
+        println(other)
+        None
+    }
+  }
+
+  private def expr : Parser[()=>Int] = {
+    (term<~"+")~expr ^^ { case l~r => () => l() + r() } |
+    (term<~"-")~expr ^^ { case l~r => () => l() - r() } |
+    term
+  }
+
+  private def term : Parser[()=>Int] = {
+    (factor<~"*")~term ^^ { case l~r => () => l() * r() } |
+    (factor<~"/")~term ^^ { case l~r => () => l() / r() } |
+    factor
+  }
+
+  private def factor : Parser[()=>Int] = {
+    "("~>expr<~")" |
+    "\\d+".r ^^ { x => () => x.toInt } |
+    failure("Expected a value")
+  }
+}
+
+object Main {
+  def main(args: Array[String]) {
+    println("""Please input the expressions. Type "q" to quit.""")
+    var input: String = ""
+
+    do {
+      input = readLine("> ")
+      if (input != "q") {
+        ArithmeticParser.readExpression(input).foreach(f => println(f()))
+      }
+    } while (input != "q")
+  }
+}