pantless.js

load(["lib/underscore.js"]);
load(["lib/json2.js"]);

var code = ['def', 'add', ['fn', ['x','y'], ['+','x','y']]];

var fns = {
  'quote':    function(body) {
     return JSON.stringify(body);
   },
  'def':      function(name, body) {
     return name + " = " + compile(body) + ";";
   },
  'fn':       function(arglist, body) {
     return "function("+arglist.join(",")+") {\n return " + compile(body) + ";\n}";
   },
  '+':        function() {
     return _.reduce(arguments, "", function(xs,y) { return xs + "+" + y}).substring(1);
   }
};

function compile(exp) {
  if(_.isArray(exp)) {
    return fns[_.first(exp)].apply(this, _.rest(exp));
  } else {
    return exp;
  }
}

print(compile(code));