Angel \”Java\” Lopez on Blog

September 9, 2010

Writing an Interpreter in .NET (Part 5)

Filed under: .NET, Programming Languages, Test-Driven Development — ajlopez @ 9:45 am

Continuing with this series, this time I added a simple parser. The new solution:

You can download from

The new class is:

The parser can be constructed using a TextReader or a string:

    public class Parser
        private Lexer lexer;
        public Parser(Lexer lexer)
            this.lexer = lexer;
        public Parser(TextReader reader)
            : this(new Lexer(reader))
        public Parser(string text)
            : this(new Lexer(text))

I have only two defined expressions (ConstantExpression and VariableExpression). The public method ParseExpression now recognize integers and names (the lexer still does not recognize delimeted strings):

        public IExpression ParseExpression()
            Token token = this.NextToken();
            if (token == null)
                return null;
            if (token.TokenType == TokenType.Integer)
                return new ConstantExpression(token.Value);
            if (token.TokenType == TokenType.String)
                return new ConstantExpression(token.Value);
            if (token.TokenType == TokenType.Name)
                return new VariableExpression((string) token.Value);
            throw new InvalidDataException(string.Format("Unexpected token '{0}'",

The parser uses the lexer defined in the previous post. As usual, the new Parser and ParseExpression were written using TDD. Some tests:

        public void ParseIntegerExpression()
            Parser parser = new Parser("1");
            IExpression expression = parser.ParseExpression();
            Assert.IsInstanceOfType(expression, typeof(ConstantExpression));
            Assert.AreEqual(1, expression.Evaluate(null));
        public void ParseVariableExpression()
            Parser parser = new Parser("one");
            IExpression expression = parser.ParseExpression();
            Assert.IsInstanceOfType(expression, typeof(VariableExpression));
            VariableExpression varexpr = (VariableExpression)expression;
            Assert.AreEqual("one", varexpr.Name);

All tests in green:

Good code coverage:

Note that using TDD, I can add features in “baby steps”, avoiding debugging times and big chunks of added code. And I have the “green” feedback: each new functionality is tested and it doesn’t broke the previous code.

Next steps: add more expressions (binary arithmetic expressions, operators, …) and commands (if, for, …), “training” parser to process them, and add to lexer the process of delimited strings.

Keep tuned!

Angel “Java” Lopez

1 Comment »

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    Comment by teak gartenmöbel — March 7, 2011 @ 10:25 am

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