Angel \”Java\” Lopez on Blog

September 13, 2011

AjScript: Javascript-Alike interpreter in C# (2) Expressions

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As in other interpreters, one key piece is the Expression, something to evaluate during the execution of a program. In AjScript (repository) I have an IExpression:

public interface IExpression
    object Evaluate(IContext context);

These are the implemented expression in current solution:

A simple expression, ConstantExpression:

public class ConstantExpression : IExpression
    private object value;
    public ConstantExpression(object value)
        this.value = value;
    public object Value { get { return this.value; } }
    public object Evaluate(IContext context)
        return this.value;

A bit more interesting, VariableExpression, given a name, and a context, returns the value of a named variable:

public class VariableExpression : IExpression
    private string name;
    public VariableExpression(string name)
    { = name;
    public string Name { get { return; } }
    public object Evaluate(IContext context)
        return context.GetValue(;

Sometimes, an expression has one or two parameters. I have UnaryExpression, BinaryExpression as abstract class. The BinaryExpression code:

public abstract class BinaryExpression : IExpression
    private IExpression leftExpression;
    private IExpression rigthExpression;
    public BinaryExpression(IExpression left, IExpression right)
        this.leftExpression = left;
        this.rigthExpression = right;
    public IExpression LeftExpression { get { return this.leftExpression; } }
    public IExpression RightExpression { get { return this.rigthExpression; } }
    public abstract object Apply(object leftValue, object rightValue);
    public object Evaluate(IContext context)
        object leftValue = this.leftExpression.Evaluate(context);
        object rightValue = this.rigthExpression.Evaluate(context);
        return this.Apply(leftValue, rightValue);

Note that the Apply method is abstract. A concrete implementation:

using Microsoft.VisualBasic.CompilerServices;
public class ConcatenateExpression : BinaryExpression
    public ConcatenateExpression(IExpression left, IExpression right)
        : base(left, right)
    public override object Apply(object leftValue, object rightValue)
        if (leftValue == null)
            leftValue = string.Empty;
        if (rightValue == null)
            rightValue = string.Empty;
        return Operators.ConcatenateObject(leftValue, rightValue);

I’m a lazy programmer: note the use of Visual Basic Operators. I could write a custom implementation. I have all the tests in place to refactor this expression.

I didn’t write all expressions at once. An example: I wrote Context class using tests, and then the VariableExpression class using tests. Then I added variable support in Parser, and evaluates simple expressions that use constants and variables. I use this pattern many times: add a new base class (Context, Function…), add a new expression, improve lexer/parser to support the new expression, use the expression in code tests, etc.

Upcoming topics: commands, function implementation, closures, hoisting, native objects. And AjScript examples.

Keep tuned!

Angel “Java” Lopez

1 Comment »

  1. […] by AjScript: Javascript-Alike interpreter in C# (2) Expressions « Angel “Java” Lopez … — September 13, 2011 @ 9:15 am […]

    Pingback by AjScript: Javascript-Alike interpreter in C# (1) Base Interface « Angel “Java” Lopez on Blog — September 13, 2011 @ 9:24 am

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