Arithmetic evaluation: Difference between revisions

Content added Content deleted

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@@ Line 1,998: / Line 1,998: @@
 is practically non-existent, to avoid obscuring the code.
-<lang scala>package org.rosetta.arithmetic_evaluator.scala
+<lang scala>
+package org.rosetta.arithmetic_evaluator.scala
 object ArithmeticParser extends scala.util.parsing.combinator.RegexParsers {
-  def readExpression(input: String) : Option[()=>_] = {
+  def readExpression(input: String) : Option[()=>Int] = {
-      parseAll(expr, input) match {
+    parseAll(expr, input) match {
-        case Success(result, _) =>
+      case Success(result, _) =>
-          Some(result)
+        Some(result)
-        case other =>
+      case other =>
-          println(other)
+        println(other)
-          None
+        None
     }
   }
-  private type AST = ()=>Int
+  private def expr : Parser[()=>Int] = {
+    (term<~"+")~expr ^^ { case l~r => () => l() + r() } |
+    (term<~"-")~expr ^^ { case l~r => () => l() - r() } |
+    term
+  }
-  private def expr : Parser[AST] =
+  private def term : Parser[()=>Int] = {
-    (term~("+" | "-")~expr ^^ toExpr) | term
+    (factor<~"*")~term ^^ { case l~r => () => l() * r() } |
+    (factor<~"/")~term ^^ { case l~r => () => l() / r() } |
+    factor
+  }
-  private def term : Parser[AST] =
+  private def factor : Parser[()=>Int] = {
+    "("~>expr<~")" |
-    (factor~("*" | "/")~term ^^ toExpr) | factor
+    "\\d+".r ^^ { x => () => x.toInt } |
+    failure("Expected a value")
-  private def factor : Parser[AST] =
-    "("~> expr <~")" | digits ^^ toLiteral | failure("Expected a value")
-  private def digits =
-    "\\d+".r
-  private def toLiteral(partialResult: String) =
-    () => partialResult.toInt
-  private def toExpr(partialResult: ~[~[AST, String], AST]) = {
-    partialResult match {
-      case (l~op)~r =>
-        op match {
-          case "+" => () => l() + r()
-          case "-" => () => l() - r()
-          case "*" => () => l() * r()
-          case "/" => () => l() / r()
-          case x => error("Unknown operation " + x + ".")
-        }
-    }
   }
 }
@@ Line 2,055: / Line 2,044: @@
     } while (input != "q")
   }
+}
-}</lang>
+</lang>
 Example: