Google Developer Day Japan 2008 is being held on June 10th at Google’s offices in Shibuya and I’ve registered to attend this year. There were a number of sessions that people could take part in but I decided to register for a Google appengine hackathon. I’m pretty curious about appengine since I’ve been working at becoming more familiar with really newly evolving technologies and not necessarily ones that have been around a while. Newly evolving technologies is something I’ve always felt I’ve had to catch up on since starting programming in high school. Going to high school with folks like Bob Ippolito (Mochikit, simplejson) and Konrad Rokicki who started coding stuff when they were in early middle school didn’t help my self esteem.

Anyway, in the spirit of learning about Appengine I took a dive into the documentation and learned a few of appengines silly limitations but I came up with a simple application that utilizes the simple python library I created for prefix back in college. I put it up in my mercurial repository under /hg/prefix-appengine/ (Update (2023-10-02): Link removed as I seem to have removed it long ago) if you care to take a look.

The main work is done in two handlers which are essentially the controller part of the MVC pattern. One simply renders the page as a template, which is really simple since there isn’t any template code, and the other implements a simple rest API that I use for an AJAX call to evaluate an expression given by the user. Using JSON seemed like a waste since there was only one returned value.

class PrefixHandler(webapp.RequestHandler):
    def get(self):
        self.response.out.write(template.render("main.tpl", {}))

    # def post(self):
    #     self.redirect('/')

class EvalHandler(webapp.RequestHandler):
    def get(self):
        expression = self.request.get("exp")
        values = {}
        try:
            output = prefix.parser.parse(expression).evaluate()
            values = {
                "value": output
            }
        except ValueError, arg:
            output = "ERROR: " + str(arg)
            values = {
                "error": output
            }
        self.response.out.write(simplejson.dumps(values))

The rest of the code is in the javascript which I just wrote strait into the template file because I was lazy. The javascript uses jquery to do an AJAX call when the button is pressed and update the HTML DOM.

var lastvalue = "";

$(document).ready(function() {
  $("#eval").click(function() {
    expression = $("#exp").val();
    $("#output").html("Loading..");
    uri = "eval?exp=";
    uri += encodeURIComponent(expression.replace("Ans", lastvalue));
    uri = uri.replace(/%20/g|>, '+');

    $.getJSON(uri,
      // Callback
      function (data) {
        output = "<font color='#FF0000'>ERROR: Invalid response from server</font>";
        if (data.value != null) {
          output = expression + " = <font color='#00FF00'>" + data.value + "</font>";
          lastvalue = data.value;
        } else {
          if (data.error &amp;&amp; data.error.length>0) {
            output = "<font color='#FF0000'>"+ data.error +"</font>";
          }
        }
        $("#output").html(output);
      }
    );
  });
});