Interpreter.js

function interpret(form, env, ctx){
	if(form instanceof Array){
		switch(form[0]){
			case 'lambda': {
				var params = form[1];
				var body = form[2];
				return ctx(function(ctx){ return function() {
					var e = Object.create(env);
					for(var j = 0; j < params.length; j++)
						e[params[j]] = arguments[j];
					return interpret(body, e, ctx)
				}})
			};
			case 'if': {
				var test = form[1];
				var consequent = form[2];
				var alternate = form[3];
				return interpret(form[1], env, function(c){
					if(c) return interpret(consequent, env, ctx)
					else  return interpret(alternate, env, ctx)
				})
			};
			case 'callcc': {
				return interpret(form[1], env, function(f){
					var fctx = function(){
						return function(x){ return ctx(x) }};
					return f(fctx)(fctx)
				})
			};
			case 'quote': {
				return ctx(form[1])
			};
			default: {
				return interpretCall(form, env, ctx);
			};
		}
	} else if(typeof form === 'string') {
		return ctx(env[form])
	} else {
		return ctx(form)
	}
}

function interpretCall(form, env, ctx){
	return interpret(form[0], env, function(callee){
		return interpret$(form.slice(1), env, function(args){
			return callee(ctx).apply(null, args)
		})	
	})
}
function interpret$(form, env, ctx){
	if(!form.length) return ctx(null);
	else return interpret(form[0], env, function(x0){
		return interpret$(form.slice(1), env, function(x$){
			if(x$) return ctx([x0])
			else return ctx([x0].concat(x$))
		})
	})
}

function id(x){ return x }

var env0 = {
	trace: function(ctx){ 
		return function(x) { return ctx(console.log(x)) }},
	begin: function(ctx){
		return function(x$) { return ctx(arguments[arguments.length - 1]) }}
};

interpret(['trace', ['callcc', ['lambda', ['return'], ['begin', 
	['trace', ['quote', 1]],  // traces
	['return', ['quote', 2]], // returns, sent to 'trace'
	['trace', ['quote', 3]]   // not executed
]]]], env0, id);