Interpreter - Design Pattern

Objective 🎯

Provide a way to interpret/evaluate a language grammar or expression.

Type ✅

✔️Behavioral: Describes how objects interact/communicate between themselves.

❌Creational: Describes how to instantiate an object without large and complex.

❌Structural: Describes how objects/classes are composed to form larger structures.

UML 📐

Participants 🔗

• AbstractExpression:

• Declares an Evaluate() operation that all nodes (leaf and non-leaf) must override

• Terminal Expression:

• Implements a concrete operation

• Nonterminal Expression:

• Implements an operation and execute the next expression

• Interpreter:

• Defines an interface to receive the Context
• Based on the Context (user’s input) will create the Expression classes
• Will interpret the user’s input (Context) by executing the Evaluate method of the Expression classes

• Client:

• Will receive the user’s input data
• Responsible to create and maintain the Context
• Will instantiate an Interpreter object and call the method Interpret passing the Context as argument

Notes 📝

• This pattern is recommended for relatively simple grammar interpretation, which doesn't need to evolve and extend much.

• Terminal Expressions are considered the leaf nodes

• Non-Terminal Expressions are considered the non-leaf nodes

• The example below illustrates how this pattern can be used to interpret SQL statements

Sample Code 🎮

Structural Example 🏛️

public static class InterpreterStructural
    {
        // Client
        public static void Execute()
        {
            Context lContext = new Context();
            lContext._UserInput = "AABBCC";

            Interpreter lInterpreter = new Interpreter();
            lInterpreter.Interprete(lContext);
        }
    }

    public class Interpreter
    {
        public void Interprete(Context prContext)
        {
            List<Expression> _Expressions = new List<Expression>();
            _Expressions.Add(new NonTerminalExpression());
            _Expressions.Add(new TerminalExpression());

            foreach (Expression lExpressionCurrent in _Expressions)
            {
                lExpressionCurrent.Evaluate(prContext);
            }
        }
    }

    public abstract class Expression
    {
        public abstract void Evaluate(Context prContext);
    }

    public class NonTerminalExpression : Expression
    {
        public override void Evaluate(Context prContext)
        {
            Console.WriteLine($"--Executing a Non-Terminal Operation with the input \"{prContext._UserInput}\"");
        }
    }

    public class TerminalExpression : Expression
    {
        public override void Evaluate(Context prContext)
        {
            Console.WriteLine($"----Executing a Terminal Operation with the input \"{prContext._UserInput}\"");
        }
    }

    public class Context
    {
        public string _UserInput { get; set; }
    }

Output

Real-world Example 🔥

public static class InterpreterPractical
    {
        // Client
        public static void Execute()
        {
            CommandLineContext lCommandLineContext = new CommandLineContext();
            lCommandLineContext._UserExpression = " Lorem Ipsum Dolor Sit Amet, Consectetur -u -t";

            CommandLineInterpreter lCommandLineInterpreter = new CommandLineInterpreter();
            lCommandLineInterpreter.Interprete(lCommandLineContext);

            lCommandLineContext._UserExpression = " Lorem Ipsum Dolor Sit Amet, Consectetur -l";
            lCommandLineInterpreter.Interprete(lCommandLineContext);
        }
    }

    public class CommandLineInterpreter
    {
        public void Interprete(CommandLineContext prCommandLineContext)
        {
            List<CommandLineExpression> _CommandLineExpressions = new List<CommandLineExpression>();

            // GET EXPRESSIONS
            List<string> lExpressions = prCommandLineContext._UserExpression.Substring(prCommandLineContext._UserExpression.IndexOf("-")).Split(" ").ToList();

            prCommandLineContext._Result = prCommandLineContext._UserExpression.Substring(0, prCommandLineContext._UserExpression.IndexOf("-"));

            foreach (string lExpressionCUrrent in lExpressions)
            {
                switch (lExpressionCUrrent)
                {
                    case ("-u"): _CommandLineExpressions.Add(new ToUpperCaseExpression()); break;
                    case ("-l"): _CommandLineExpressions.Add(new ToLowerCaseExpression()); break;
                    case ("-t"): _CommandLineExpressions.Add(new TrimExpression()); break;
                    default: break;
                }
            }

            // EXECUTE EXPRESSIONS
            Console.WriteLine($"\n\r- INTERPRETING INPUT: \"{prCommandLineContext._Result}\"");
            foreach (CommandLineExpression lCommandLineExpressionCurrent in _CommandLineExpressions)
            {
                lCommandLineExpressionCurrent.Evaluate(prCommandLineContext);
                Console.WriteLine($"-- {lCommandLineExpressionCurrent.GetType().Name} Executed - Result: \"{prCommandLineContext._Result}\"");
            }
            Console.WriteLine($"- INTERPRETATION RESULT: \"{prCommandLineContext._Result}\"");
        }
    }

    public abstract class CommandLineExpression
    {
        public abstract void Evaluate(CommandLineContext prCommandLineContext);
    }

    public class ToUpperCaseExpression : CommandLineExpression
    {
        public override void Evaluate(CommandLineContext prCommandLineContext)
        {
            prCommandLineContext._Result = prCommandLineContext._Result.ToUpper();
        }
    }

    public class ToLowerCaseExpression : CommandLineExpression
    {
        public override void Evaluate(CommandLineContext prCommandLineContext)
        {
            prCommandLineContext._Result = prCommandLineContext._Result.ToLower();
        }
    }

    public class TrimExpression : CommandLineExpression
    {
        public override void Evaluate(CommandLineContext prCommandLineContext)
        {
            prCommandLineContext._Result = prCommandLineContext._Result.Trim();
        }
    }

    public class CommandLineContext
    {
        public string _UserExpression { get; set; }
        public string _Result { get; set; }
    }

Output

Source Code 🎲

https://github.com/VictorLins/DesignPatterns