Mapping and Visiting in Functional and Object-Oriented Programming

class Exp{public abstract T Accept(Visitor v, D x);}class PlusExp : Exp {private Exp e1, e2;public PlusExp(Exp a, Exp b){e1 = a;e2 = b;}public Exp LeftOperand{get {return e1;}}public Exp RightOperand{get {return e2;}}public override T Accept(Visitor v, D x){return v.Visit(this, x);}} class TimesExp : Exp {private Exp e1, e2; public TimesExp(Exp a, Exp b){e1 = a;e2 = b;} public Exp LeftOperand{get {return e1;}}public Exp RightOperand{get {return e2;}} public override T Accept(Visitor v, D x){return v.Visit(this, x);}} class Identifier : Exp {private string name; public Identifier(string s) {name = s;}public string Name{get {return name;} }public override T Accept(Visitor v, D x){return v.Visit(this, x);}}class Literal : Exp {private string litString; public string LiteralString{get {return litString;}}public Literal(string s) {litString = s; } 98JOURNAL OF OBJECT TECHNOLOGYVOL 7, NO. 7 B THE VISITOR DESIGN PATTERN public override T Accept(Visitor v, D x){return v.Visit(this, x);}} class Environment: Dictionary{}; class Interpreter: Visitor {public int Visit(PlusExp e, Environment env){return (e.LeftOperand.Accept(this, env) + e.RightOperand.Accept(this, env));}public int Visit(TimesExp e, Environment env){return (e.LeftOperand.Accept(this, env) * e.RightOperand.Accept(this, env));}public int Visit(Identifier e, Environment env){return env[e.Name];} public int Visit(Literal e, Environment env){return Int32.Parse(e.LiteralString);}} class ReversePolishConverter: Visitor { private const string SPACE = " "; public string Visit(PlusExp e, None x){return e.LeftOperand.Accept(this, x) + SPACE + e.RightOperand.Accept(this, x) +SPACE + "+";}public string Visit(TimesExp e, None x){return e.LeftOperand.Accept(this, x) + SPACE + e.RightOperand.Accept(this, x) +SPACE + "*";}public string Visit(Identifier e, None x){return e.Name;} public string Visit(Literal e, None x){return e.LiteralString;}} class None {};class Program { VOL 7, NO. 7JOURNAL OF OBJECT TECHNOLOGY99 MAPPING AND VISITING IN FUNCTIONAL AND OBJECT-ORIENTED PROGRAMMING public static void Main() {TimesExp ast = new TimesExp(new PlusExp (new Literal("1"), new Identifier("x")),new Literal("2")); // (1 + x) * 2Environment env = new Environment(); env.Add("x",9);Interpreter interpreter = new Interpreter();ReversePolishConverter reversPolishConverter = new ReversePolishConverter();None nothing = new None();Console.WriteLine(interpreter.Visit(ast, env));Console.WriteLine(reversPolishConverter.Visit(ast, nothing));}}} C DOUBLE DISPATCH VISITING IN CLOS ;;; Visitor in CLOS Common Lisp Object System. ;; Expression Classes (defclass Expression () ()) (defclass PlusExpression (Expression)((a1 :initarg :firstAddend :accessor firstAddend)(a2 :initarg :secondAddend :accessor secondAddend))) (defclass MinusExpression (Expression)((m :initarg :minuend :accessor minuend)(s :initarg :subtrahend :accessor subtrahend))) (defclass TimesExpression (Expression)((f1 :initarg :firstFactor :accessor firstFactor)(f2 :initarg :secondFactor :accessor secondFactor))) (defclass DivideExpression (Expression)((dividend :initarg :dividend :accessor dividend)(divisor :initarg :divisor :accessor divisor)))(defclass Identifier (Expression)((name :initarg :name :accessor name))) (defclass IntegerLiteral (Expression)((literal :initarg :literal :accessor literal))) ;; Visitor classes(defclass Visitor () ()) 100JOURNAL OF OBJECT TECHNOLOGYVOL 7, NO. 7 C DOUBLE DISPATCH VISITING IN CLOS (defclass Interpreter (Visitor) ())(defclass ReversePolish (Visitor) ()) ;; Visit multi methods.;; Notice that the visit method specializes on both an Expression;; and a (kind of) of visitor. (defmethod visit ((e PlusExpression) (v Interpreter) &optional; extra)(+ (visit (firstAddend e) v extra) (visit (secondAddend e) v extra)))(defmethod visit ((e MinusExpression) (v Interpreter) &optional; extra)((visit (minuend e) v extra) (visit (subtrahend e) v extra))) (defmethod visit ((e TimesExpression) (v Interpreter) &optional; extra)(* (visit (firstFactor e) v extra) (visit (secondFactor e) v extra))) (defmethod visit ((e DivideExpression) (v Interpreter) &optional; extra)(/ (visit (dividend e) v extra) (visit (divisor e) v extra))) (defmethod visit ((e Identifier) (v Interpreter) &optional; extra)(lookup-identifier extra (name e))) (defmethod visit ((e IntegerLiteral) (v Interpreter) &optional; extra)(convert-string-to-integer (literal e))) (defmethod visit ((e PlusExpression) (v ReversePolish) &optional; extra)(concatenate ’string (visit (firstAddend e) v extra) " "(visit (SecondAddend e) v extra) " " "+")) (defmethod visit ((e MinusExpression) (v ReversePolish) &optional; extra)(concatenate ’string (visit (minuend e) v extra) " "(visit (subtrahend e) v extra) " " "-")) (defmethod visit ((e TimesExpression) (v ReversePolish) &optional; extra)(concatenate ’string (visit (firstFactor e) v extra) " "(visit (SecondFactor e) v extra) " " "*"))(defmethod visit ((e DivideExpression) (v ReversePolish) &optional; extra)(concatenate ’string (visit (dividend e) v extra) " "(visit (divisor e) v extra) " " "/")) (defmethod visit ((e Identifier) (v ReversePolish) &optional; extra)(name e)) (defmethod visit ((e IntegerLiteral) (v ReversePolish) &optional; extra)(literal e)) VOL 7, NO. 7JOURNAL OF OBJECT TECHNOLOGY101 MAPPING AND VISITING IN FUNCTIONAL AND OBJECT-ORIENTED PROGRAMMING ;; Auxiliary stuff.(defun convert-string-to-integer (str &optional; (radix 10))"Given a digit string and optional radix, return an integer."; Details not relevant for this paper)(defun lookup-identifier (env name)(cdr (assoc name env :test (function equal)))) ;; Some sample visiting:(defun main ()(let* ((expr1 (make-instance ’PlusExpression:firstAddend (make-instance ’IntegerLiteral :literal "3"):secondAddend (make-instance ’IntegerLiteral :literal "2")))(expr2 (make-instance ’TimesExpression:firstFactor (make-instance ’IntegerLiteral :literal "3"):secondFactor (make-instance ’Identifier :name "var"))) (expr3 (make-instance ’MinusExpression:minuend expr1:subtrahend expr2)) (interpretation (make-instance ’Interpreter))(reverse-polish (make-instance ’ReversePolish))(env ’(("x" . 5) ("var" . 7))))(list (visit expr1 interpretation env)(visit expr2 interpretation env)(visit expr3 interpretation env) (visit expr1 reverse-polish)(visit expr2 reverse-polish)(visit expr3 reverse-polish))))