pantless.js

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

var fact = ['def', 'fact', ['fn', ['n'], 
             ['if', ['<=', 'n', 2],
               'n',
               ['*', 'n', ['fact', ['-', 'n', 1]]]]]]

var infix = ['+', '-', '*', '/', '<', '>', '<=', '>='];

var forms = {
  'quote':    function(body) {
    return JSON.stringify(body);
  },
  'cond':     function() {
    return _.map(_.partition(arguments, 2), function(pair) {
               return compile(pair[0]) + " ? " + compile(pair[1]) + " : \n";
             }).join('') + "null";
  },
  'def':      function(name, body) {
    return name + " = " + compile(body) + ";";
  },
  'fn':       function(arglist, body) {
    return "function("+arglist.join(",")+") {\n return " + compile(body) + ";\n}";
  },
  'if':       function(test, if_true, if_false) {
    return compile(test) + " ? " + compile(if_true) + " : " + compile(if_false);
  }
};

function compile(exp) {
  if(_.isArray(exp)) {
    var first = _.first(exp);
    var rest = _.rest(exp);
    if(_.include(infix, first)) {
      return _.map(rest, compile).join(first);
    } else if(_.include(_.keys(forms), first)) {
      return forms[first].apply(this, rest);
    } else {
      return first + "(" + _.map(rest, compile).join(',') + ")";
    }
  } else {
    return exp;
  }
}

print(JSON.stringify(fact));
print(compile(fact));
eval(compile(fact));
print(fact(5)) //120

  /*["def","fact",["fn",["n"],["if",["<=","n",2],"n",["*","n",["fact",["-","n",1]]]]]]*/
  /*fact = function(n) {*/
  /*return n<=2 ? n : n*fact(n-1);*/
  /*};*/
  /*120*/