Click here to Skip to main content
15,861,168 members
Articles / Programming Languages / Javascript

Implementing Programming Languages Using C# 4.0

Rate me:
Please Sign up or sign in to vote.
4.92/5 (115 votes)
12 Jul 2012MIT17 min read 229.2K   3.6K   249  
An introduction to creating programming language tools using C# 4.0.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;

namespace Diggins.Jigsaw
{
    /// <summary>
    /// Simplifies the parse tree for evaluation.
    /// </summary>
    public class JavaScriptTransformer : TreeTransformer
    {
        public static Node Transform(Node n)
        {
            return new JavaScriptTransformer().TransformAlNodes(n);
        }

        Node ToAssignment(Node left, string op, Node right)
        {
            var assignOp = new Node("AssignOp", "=");
            var binOp = new Node("Binaryop", op);
            var binExpr = new Node("BinaryExpr", left, binOp, right);
            return new Node("AssignExpr", left, assignOp, binExpr);
        }

        protected override Node InternalTransform(Node n)
        {
            switch (n.Label)
            {
                case "Literal":
                    return n[0];
                case "LeafExpr":
                    return n[0];
                case "ParenExpr":
                    return n[0];
                case "Expr":
                    return n[0];
                case "Statement":
                    return n[0];
                case "ExprStatement":
                    return n[0];
                case "While":
                    // While loops are a special case of for loops.
                    return new Node("For", new Node("Empty"), n[0], new Node("Empty"), n[1]);
                case "PostfixExpr":
                    {
                        Debug.Assert(n.Count != 0);
                        // Is it really a postfix expression? If not return the sub-expression
                        if (n.Count == 1)
                            return n.Nodes[0];
                        var last = n.Nodes.Last();
                        switch (last.Label)
                        {
                            case "Field":
                                return LeftGroup(n, "FieldExpr");
                            case "Index":
                                return LeftGroup(n, "IndexExpr");
                            case "ArgList":
                                {
                                    var call = LeftGroup(n, "CallExpr");

                                    // Method calls have special semantics.
                                    // You know it is a method if the left side of a call is a field express 
                                    if (call[0].Label == "FieldExpr")
                                    {
                                        var obj = call[0][0];
                                        var field = call[0][1];
                                        var args = call[1];
                                        return new Node("MethodCallExpr", obj, field, args);
                                    }
                                    else
                                    {
                                        return call;
                                    }
                                }
                            default: throw new Exception("Unexpected node in postfix-expression " + last.Label);
                        }
                    }
                case "NamedFunc":
                    return new Node("VarDecl", n[0], new Node("AnonFunc", n[1], n[2]));
                case "TertiaryExpr":
                    // Is not really a tertiary expression return the sub-expression
                    return (n.Count == 1) ? n[0] : n;
                case "BinaryExpr":
                    // Is not really a binary expression return the sub-expression
                    return (n.Count == 1) 
                        ? n[0] 
                        : SplitBinaryExpression(n);
                case "AssignExpr":
                    {
                        // Is it really an assignment expression? If not return the sub-expressions
                        if (n.Count == 1)
                            return n[0];
                        // Transform special assignement operators into plain assignment and binary operation
                        switch (n[1].Text)
                        {
                            case "=": return n;
                            case "+=": return ToAssignment(n[0], "+", n[2]);
                            case "-=": return ToAssignment(n[0], "-", n[2]);
                            case "*=": return ToAssignment(n[0], "*", n[2]);
                            case "/=": return ToAssignment(n[0], "/", n[2]);
                            case "%=": return ToAssignment(n[0], "%", n[2]);
                            case "|=": return ToAssignment(n[0], "|", n[2]);
                            case "&=": return ToAssignment(n[0], "&", n[2]);
                            case "^=": return ToAssignment(n[0], "^", n[2]);
                            case "||=": return ToAssignment(n[0], "||", n[2]);
                            case "&&=": return ToAssignment(n[0], "&&", n[2]);
                            case ">>=": return ToAssignment(n[0], ">>", n[2]);
                            case "<<=": return ToAssignment(n[0], "<<", n[2]);
                            default:
                                throw new Exception("Unexpected assignment operator " + n[1].Text);
                        }
                    }
            }
            return n;
        }

        public static int Precendence(string op)
        {
            switch (op)
            {
                case "=":
                case "+=":
                case "-=":
                case "*=":
                case "/=":
                case "%=":
                case ">>=":
                case "<<=":
                case "|=":
                case "&=":
                case "^=":
                case "||=":
                case "&&=":
                    return 10;
                case "||":
                    return 20;
                case "&&":
                    return 30;
                case "|":
                    return 40;
                case "^":
                    return 50;
                case "&":
                    return 60;
                case "==":
                case "!=":
                    return 70;
                case ">=":
                case "<=":
                case ">":
                case "<":
                    return 80;
                case ">>":
                case "<<":
                    return 90;
                case "+":
                case "-":
                    return 100;
                case "*":
                case "/":
                case "%":
                    return 110;
                default:
                    throw new Exception("Not a recognized operator");
            }
        }

        public static Node SplitBinaryExpression(Node node)
        {
            if (node.Count <= 3) return node;

            // Long expressions should always have an odd number of nodes.
            Debug.Assert(node.Count % 2 == 1);

            // Find the lowest priority operation 
            int pivot = 1;
            for (int i = 3; i < node.Count; i += 2)
                if (Precendence(node[i].Text) < Precendence(node[pivot].Text))
                    pivot = i;

            Node left = new Node("BinaryExpr", node.Nodes.Take(pivot).ToArray());
            Node right = new Node("BinaryExpr", node.Nodes.Skip(pivot + 1).ToArray());
            return new Node("BinaryExpr", SplitBinaryExpression(left), node[pivot], SplitBinaryExpression(right));
        }
    }
}

By viewing downloads associated with this article you agree to the Terms of Service and the article's licence.

If a file you wish to view isn't highlighted, and is a text file (not binary), please let us know and we'll add colourisation support for it.

License

This article, along with any associated source code and files, is licensed under The MIT License


Written By
Software Developer Ara 3D
Canada Canada
I am the designer of the Plato programming language and I teach computer science at Bishop's University. I can be reached via email at cdiggins@gmail.com

Comments and Discussions