[Sintaxis] Interpretador

[Kuk]

❞ Cobol does not have the ability to evaluate expressions dynamically, i.e. defining an expression in a string and having it evaluated during the runtime. OO Cobol is a whole different story. There is no eval command, but we can create our own way to evaluate things using the Interpreter Pattern!

That sounds great, but what is an evalute command anyways? Good question. Language elements stored in a string are nothing, but a string. An eval command take that string and interpret its content, processing the elements accordling. That means power, the ability to define code blocks and have it processed when needed. The Interpreter Pattern allows you to define your own grammar, and process it in order to do some useful task.

The Interpreter Pattern allows much more than simple arithmetic expressions evaluation, thought this is exactly what we are going to show here. The classes in this solutions are:

• IExpression: Interface that declares an operation
• Context: Global information used by the expression
• TerminalExpression: Represent elements in the grammar that do no to get replace, such as symbols
• NonTerminalExpression: Represents elements that will be replaced during the evaluation such as variables or even rules
• ComputeFormula: Client class that also contains the parser used to break expression's elements.

The ComputeFormula class deals with a lot of concepts not covered in this article, but I recommend you to understand that class in order to get yourself used to things like stacks, collections, string handling and much more.

OO Cobol code

Everything starts with an Interface definition. IExpression declares the contract to which classes must adhere in order to participate in this solution.

 INTERFACE-ID. IExpression as "InterpreterPattern.IExpression".
   environment division.
     configuration section.
       repository.
         class  ClassContext  as  "InterpreterPattern.Context".
         
   procedure division.
   
     method-id. EvaluateExpression as "Evaluate".
        
        data division.
          linkage section.
          
          01  inContext   object reference ClassContext.
          01  outValue    usage comp-2.
         
         procedure division using inContext returning outValue.
      end method EvaluateExpression.
 END INTERFACE IExpression.       

The following classes implements the IExpression interface: AddExpression.cob, SubtractExpression.cob, , MultiplyExpression.cob, DivideExpression.cob, and PowerExpression.cob. These are also the classes that contains the actual calculation performed over operands.

Below the code for PowerExpression code:

 CLASS-ID. PowerExpression AS "InterpreterPattern.PowerExpression" 
           inherits ClassNonTerminalExpression.
   environment division.
     configuration section.
       repository.
         class ClassContext                as "InterpreterPattern.Context"
         class SystemString                as "System.String"
         class ClassNonTerminalExpression  as "InterpreterPattern.NonTerminalExpression"
         
          interface IExpression             as "InterpreterPattern.IExpression".
          
   *> Instance's data and methods
    object. 
      data division.
        working-storage section.
        copy "types.book".
        
        01  variable  object reference SystemString private.
        
      procedure division.
      
        method-id. NEW.
           data division.
             linkage section.
             01   inLeftExpression    object reference IExpression.
             01   inRightExpression   object reference IExpression.
             
           procedure division using inLeftExpression, inRightExpression.
             
             invoke super "NEW" using inLeftExpression, inRightExpression.
         
        end method NEW.
        
        method-id. EvaluateExpresion as "Evaluate" override.
          data division.
            working-storage section.
            
            01 anExpression   object reference IExpression.
            
            linkage section.
            
            01 inContext  object reference ClassContext.
            01 outValue   type SystemDouble.
          
          procedure division using inContext returning outValue.
          
            invoke super "GetLeftNode" returning anExpression
            move  anExpression::"Evaluate"(inContext)  to  outValue
            invoke super "GetRightNode" returning anExpression
             
            *> This is the real "PowerExpression"
             compute outValue = outValue ** anExpression::"Evaluate"(inContext) 
               
        end method EvaluateExpresion.
    end object.
  END CLASS PowerExpression.

Other operations' classes such as AddExpression or MultiplyExpression have pretty much basically the same code structure.

The TerminalExpression class that defines symbols behavior:

 CLASS-ID. TerminalExpression AS "InterpreterPattern.TerminalExpression".
   environment division.
     configuration section.
       repository.
         class ClassContext        as "InterpreterPattern.Context"
         class SystemCollectionHashTable as "System.Collections.Hashtable"
         class SystemString        as "System.String"
         
         interface IExpression       as "InterpreterPattern.IExpression".
          
   *> Instance's data and methods
    object. implements IExpression.
      data division.
        working-storage section.
        copy "types.book".
        
        01  variable  object reference SystemString private.
        
      procedure division.
      
        method-id. NEW.
           data division.
             linkage section.
             01   inName    object reference SystemString.
             
           procedure division using inName.
           
             set   variable     to  inName
         
        end method NEW.
        
        method-id. EvaluateExpression as "Evaluate".
          data division.
            linkage section.
            
            01 inContext  object reference ClassContext.
            01 outValue   type SystemDouble.
          
          procedure division using inContext returning outValue.
          
            move inContext::"GetValue"(variable)   to  outValue
               
        end method EvaluateExpression.
    end object.
  END CLASS TerminalExpression.

The NonTerminalExpression class that defines rules:

 CLASS-ID. NonTerminalExpression AS "InterpreterPattern.NonTerminalExpression".
   environment division.
     configuration section.
       repository.
         class ClassContext        as "InterpreterPattern.Context"
         class SystemCollectionHashTable as "System.Collections.Hashtable"
         class SystemString        as "System.String"
         
         interface IExpression       as "InterpreterPattern.IExpression".
          
   *> Instance's data and methods
    object. implements IExpression.
      data division.
        working-storage section.
        copy "types.book".
     
        01  leftNode    object reference IExpression private.      
        01  rightNode   object reference IExpression private.      
        
      procedure division.
      
        method-id. NEW.
           data division.
             linkage section.
             01   inLeftExpression  object reference IExpression.
             01   inRightExpression object reference IExpression.
           procedure division using inLeftExpression inRightExpression.
             
             invoke self "SetLeftNode"  using inLeftExpression
             invoke self "SetRightNode"   using inRightExpression
           
        end method NEW.
        
         method-id. SetLeftNode.
          data division.
            linkage section.
            
            01 inNode   object reference IExpression.
          
          procedure division using inNode.
          
            set  leftNode     to  inNode
               
        end method SetLeftNode.   
         method-id. SetRightNode.
          data division.
            linkage section.
            
            01 inNode   object reference IExpression.
          
          procedure division using inNode.
          
            set  rightNode     to  inNode
              
        end method SetRightNode.   
         method-id. GetLeftNode.
          data division.
            linkage section.
            
            01 outNode   object reference IExpression.
          
          procedure division returning outNode.
          
            set  outNode      to  leftNode
               
        end method GetLeftNode.   
         method-id. GetRightNode.
          data division.
            linkage section.
            
            01 outNode   object reference IExpression.
          
          procedure division returning outNode.
          
            set  outNode       to  rightNode
               
        end method GetRightNode.   
        method-id. EvaluateExpression as "Evaluate".
          data division.
            linkage section.
            
            01 inContext  object reference ClassContext.
            01 outValue   type SystemDouble.
          
          procedure division using inContext returning outValue.
          
            *> Implemented because NetCobol.Net does not supports Abstract classes
               
        end method EvaluateExpression.
    end object.
  END CLASS NonTerminalExpression.

[Kuk]

The ComputeFormula class is the "client" class of this example. It contains a parser, the expression tree builder and, of course, uses Interpreter pattern to arrange things.

 CLASS-ID. ComputeFormula AS "InterpreterPattern.ComputeFormula".
   environment division.
     configuration section.
       repository.
         class SystemCollectionHashTable   as "System.Collections.Hashtable"
         class SystemString                as "System.String"
         class SystemCollectionStack       as "System.Collections.Stack"
         class SystemDoubleWrapper         as "System.Double"
         class SystemObject                as "System.Object"
          
          class ClassAddExpression          as "InterpreterPattern.AddExpression"
          class ClassSubtractExpression     as "InterpreterPattern.SubtractExpression"
          class ClassMultiplyExpression     as "InterpreterPattern.MultiplyExpression"
          class ClassDivideExpression       as "InterpreterPattern.DivideExpression"
          class ClassPowerExpression        as "InterpreterPattern.PowerExpression"
          class ClassTerminalExpression     as "InterpreterPattern.TerminalExpression"
          class ClassNonTerminalExpression  as "InterpreterPattern.NonTerminalExpression"
          class ClassContext                as "InterpreterPattern.Context"
          
          interface IExpression             as "InterpreterPattern.IExpression"
          
          property StringNull               as "Empty"
          property StringLength             as "Length"
          property StackCount               as  "Count".
          
   *> Instance's data and methods
    object.
      data division.
        working-storage section.
        copy "types.book".
        01 booleanTrue  pic 1 value b"1".
        01 booleanFalse   pic 1 value b"0".
        01 expression   object reference SystemString.
        01 ctx      object reference ClassContext.      
        01 operators    object reference SystemCollectionHashTable private.
        01 tempObject   object reference SystemObject.
        
      procedure division.
      
        method-id. NEW.
         
           procedure division.
             invoke SystemCollectionHashTable "NEW" returning operators
             
             invoke operators "Add" using by value "+" by value "1"
             invoke operators "Add" using by value "-" by value "1"
             invoke operators "Add" using by value "/" by value "2"
             invoke operators "Add" using by value "*" by value "2"
             invoke operators "Add" using by value "^" by value "2"
             invoke operators "Add" using by value "(" by value "0"
        end method NEW.
        
        method-id. SetContext   as "SetContext".
           data division.
             linkage section.
             
             01  inContext  object reference ClassContext.
           
           procedure division using inContext.
           
             set ctx        to  inContext
                      
        end method SetContext.
        method-id. SetExpression   as "SetExpression".
           data division.
             linkage section.
             
             01  inExpression  object reference SystemString.
           
           procedure division using inExpression.
           
             set expression       to  inExpression
                      
        end method SetExpression.
        
        method-id. EvaluateExpression as "Evaluate".
          
          data division.
          
            working-storage section.
            
            01 postFixExp   object reference SystemString.
            01 rootNode   object reference IExpression.
            
            linkage section.
            
            01 outValue   type SystemDouble.
            
          procedure division returning outValue.
            
            set   postFixExp    to  self::"infixToPostFix"(expression)
            set   rootNode      to  self::"BuildTree"(postFixExp)
            set   outValue      to  rootNode::"Evaluate"(ctx)
            
        end method EvaluateExpression.
    
        method-id. GetNonTerminalExpression as "GetNonTerminalExpression" private.
          
          data division.
           
             working-storage section.
             
             01 postFixExp    object reference SystemString.
             01 rootNode    object reference IExpression.
             01 singleChar    pic x.
             
             linkage section.
             
             01 operation     object reference SystemString.
             01 leftExpression  object reference IExpression.
             01 rightExpression object reference IExpression.
             01 outValue    object reference ClassNonTerminalExpression.
             
           procedure division using operation leftExpression 
                                              rightExpression returning outValue.
             set  outValue       to  null
             set  singleChar       to  operation::"Trim"()
             
             evaluate singleChar
             
                  when "+"
                    invoke ClassAddExpression "NEW" using leftExpression, 
                                                          rightExpression 
                           returning outValue
              
                  when "-"
                    invoke ClassSubtractExpression "NEW" using leftExpression, 
                                                               rightExpression 
                           returning outValue
                  when "*"
                    invoke ClassMultiplyExpression "NEW" using leftExpression, 
                                                               rightExpression 
                           returning outValue
                  when "/"
                    invoke ClassDivideExpression "NEW" using leftExpression, 
                                                             rightExpression 
                           returning outValue
             
                  when "^"
                    invoke ClassPowerExpression "NEW" using leftExpression, 
                                                            rightExpression 
                           returning outValue
             end-evaluate
                    
         end method GetNonTerminalExpression.
         
         method-id. BuildTree as "BuildTree" private.
           
           data division.
           
             working-storage section.
             
             01 currentChar   object reference SystemString.
             01 leftOperand   object reference IExpression.
             01 rightOperand  object reference IExpression.
             01 exp       object reference IExpression.
             01 stack       object reference SystemCollectionStack.  
             01 rootNode    object reference IExpression.
             01 indexer     pic s9(09) value zeros.
             linkage section.
             
             01 inFormula     object reference SystemString.
             01 outExpression   object reference IExpression.
             
           procedure division using inFormula returning outExpression.
             invoke SystemCollectionStack "NEW" returning stack
             
             move   zeros         to  indexer
             perform until indexer = StringLength of inFormula
               set  currentChar     to  inFormula::"Substring"(indexer, 1)
               
               if   (self::"IsOperator"(currentChar) = booleanFalse)
                  set   exp       to  ClassTerminalExpression::"NEW"(currentChar)
                  
                  invoke stack "Push" using by value exp
               else
                  set  tempObject   to  stack::"Pop"()
                  set  rightOperand   to  tempObject as IExpression
                  set  tempObject   to  stack::"Pop"()
                  set  leftOperand  to  tempObject as IExpression
                  set  exp  to  self::"GetNonTerminalExpression"
                               (currentChar, leftOperand, rightOperand)
                  
                  invoke stack "Push" using by value exp
               end-if
               
               add  1           to  indexer
               
             end-perform
             
             set  tempObject        to  stack::"Pop"()
             set  outExpression     to  tempObject as IExpression
                    
         end method BuildTree.
         
         method-id. IsOperator as "IsOperator" private.
           data division.
           
             working-storage section.
             01  singleChar   pic x. 
             
             linkage section.
             
             01  inCharacter  object reference SystemString.
             01  outBool    pic 1.
             
           procedure division using inCharacter returning outBool.
             set   singleChar      to  inCharacter
           
             if (singleChar = "+" or "-" or "*" or "/" or "^")
               move  booleanTrue     to  outBool
             else
               move  booleanFalse    to  outBool
             end-if
             
         end method IsOperator.
         
         method-id. infixToPostFix as "InFixToPostFix".
           data division.
             working-storage section.
             01 stack       object reference SystemCollectionStack.  
             01 pfExpr      object reference SystemString.
             01 tempString    object reference SystemString.
             01 expr      object reference SystemString.
             01 indexer     type SystemInt32.
             01 stringVal1    object reference SystemString.
             01 stringVal2    object reference SystemString.
             01 val1      type SystemDouble.
             01 val2      type SystemDouble. 
             01 currentChar   object reference SystemString.
             
             linkage section.
             01 inString    object reference SystemString.
             01 outString     object reference SystemString.
    
           procedure division using inString returning outString.
             invoke SystemCollectionStack "NEW" returning stack
     
             set  pfExpr        to  StringNull of SystemString
             set  tempString      to  StringNull of SystemString
             set  expr        to  inString::"Trim"()
             
             move   zeros         to  indexer
             perform until indexer = StringLength of inString
               set currentChar     to  inString::"Substring"(indexer, 1)
              
               if  (currentChar::"Equals"(" ") = booleanTrue)
                 add  1        to  indexer
                 exit to test of perform 
               end-if
               if (self::"IsOperator"(currentChar) = booleanFalse) 
               and (currentChar::"Equals"("(") = booleanFalse) 
               and (currentChar::"Equals"(")") = booleanFalse) 
                  set  pfExpr   to  SystemString::"Concat"(pfExpr, currentChar)
               end-if
           
               if (currentChar::"Equals"("(") = booleanTrue)
                  invoke stack "Push" using currentChar
               end-if
                  
               if (currentChar::"Equals"(")") = booleanTrue)
                  set  tempObject      to  stack::"Pop"() 
                  set  tempString      to  tempObject as SystemString
                  perform until tempString::"Equals"("(") = booleanTrue
                      set  pfExpr    to  SystemString::"Concat"(pfExpr, tempString)
                      set  tempObject  to  stack::"Pop"() 
                      set  tempString  to  tempObject as SystemString
                  end-perform
            
                  set   tempString     to  StringNull of SystemString
               end-if
         
               if (self::"IsOperator"(currentChar) = booleanTrue)
                  if (StackCount of stack >; 0)
                   set  tempObject     to  stack::"Pop"() 
                   set  tempString     to  tempObject as SystemString
                   set  tempObject     to  operators::"Get_Item"(tempString)
                   set  stringVal1     to  tempObject as SystemString
                   set  tempObject     to  operators::"Get_Item"(currentChar)
                   set  stringVal2     to  tempObject as SystemString
                   
                   move SystemDoubleWrapper::"Parse"(stringVal1)  to  val1
                   move SystemDoubleWrapper::"Parse"(stringVal2)  to  val2
                   
                   perform until val1 <; val2
                       set   pfExpr       to  SystemString::"Concat"(pfExpr, tempString)
                       
                       move  -100         to  val1
                       
                       if (StackCount of stack >; 0)
                        set  tempObject     to  stack::"Pop"()
                        set  tempString     to  tempObject as SystemString 
                        set  tempObject     to  operators::"Get_Item"(tempString) 
                        set  stringVal1     to  tempObject as SystemString
                        set  val1       to  SystemDoubleWrapper::"Parse"(stringVal1)
                       end-if
                       
                   end-perform    
                    
                   if (val1 <; val2) and (val1 > -100)
                    invoke stack "Push" using tempString
                   end-if
                  end-if
               
                  invoke stack "Push" using currentChar
               end-if
               
               add   1       to  indexer
             end-perform
            
             perform until StackCount of stack = 0
                 set   tempObject  to  stack::"Pop"()
                 set   tempString  to  tempObject as SystemString
                 set   pfExpr    to  SystemString::"Concat"(pfExpr, tempString)
             end-perform
             
             set   outString     to  pfExpr
            
            end method infixToPostFix.
          
     end object.
   END CLASS ComputeFormula.

Testing our expression evaluator

The little application below shows how to use our expression evaluator:

 IDENTIFICATION DIVISION.
 PROGRAM-ID. MAIN AS "ConsoleApplication1.Main".
 ENVIRONMENT DIVISION.
 CONFIGURATION SECTION.
 SPECIAL-NAMES.
 REPOSITORY.
     class ClassComputeFormula   as "InterpreterPattern.ComputeFormula"
     class ClassContext          as "InterpreterPattern.Context".
     
  DATA DIVISION.
  WORKING-STORAGE SECTION.
  01  computeFormula    object reference ClassComputeFormula.
  01  context           object reference ClassContext.
  01  result            comp-2.
  01  displayResult     pic s9(09)v99.
  
  PROCEDURE DIVISION.
      invoke  ClassComputeFormula "NEW" returning computeFormula
      invoke  ClassContext        "NEW" returning context
      
      invoke  computeFormula "SetExpression" using "((a ^ b) + c) * d"
      
      invoke  context "CreateVariable" using "a", 10
      invoke  context "CreateVariable" using "b", 2
      invoke  context "CreateVariable" using "c", 2
      invoke  context "CreateVariable" using "d", 8.5
      
      invoke  computeFormula "SetContext" using context
      invoke  computeFormula "Evaluate" returning result
      
      move    result                 to  displayResult
      
      display displayResult
      
  END PROGRAM MAIN.

What else can I do with that Pattern?

You name it! Maybe a script language to provide some automation support in your application. Maybe you could create basic Cobol evaluators. That would be something really cool! There are no limits!

Download the source

Cool things with OO Cobol - Source Code

Some references in the web

Interpreter .NET Design Pattern in C# and VB - dofactory.com