expression.str

interface IHashtable
{
    double lookup(string i);
    void set(string i, double d);        
}

class Hashtable()
{
    var o = {};

    public double lookup(string i) 
    {
        return this.o[i];
    }
    public void set(string i, double d)
    {
        this.o[i] = d;
    }
}

interface IExpression {
    double evaluate(IHashtable vars);
}

class Constant(double value) : IExpression {
    public double evaluate(IHashtable vars) {
        return value;
    }
}

class VariableReference(string name) : IExpression {
    public double evaluate(IHashtable vars) {
        var value = vars.lookup(name);
        return value;
    }
}

class Operation(IExpression left, string op, IExpression right) : IExpression {
    public double evaluate(IHashtable vars) {
        double x = this.left.evaluate(vars);
        double y = this.right.evaluate(vars);
        switch (this.op) {
            case '+': return x + y;
            case '-': return x - y;
            case '*': return x * y;
            case '/': return x / y;
        }
    }
}

IExpression e = new Operation(new VariableReference("x"),'+',new Constant(3));

function f() { 
        IExpression e = new Operation( new VariableReference("x"), 
            '*', new Operation( new VariableReference("y"), '+', new Constant(2) )); 
        IHashtable vars = new Hashtable(); 
        vars.set("x", 3); 
        vars.set("y", 5);
        e.evaluate(vars); // Outputs "21" 
        vars.set("x", 1.5);
        vars.set("y", 9); 
        return e.evaluate(vars); // Outputs "16.5" 
    } 

f();