Errorlets

Developers: Derek Coley, Grant Magdanz, Austin Bisharat

Errorlets is a library in pure Javascript that aims to make event driven programming simpler with support for streams and error handling. The library supports sequential, event-based operations that may commonly produce errors. It allows for communicating with a server, gracefully handling errors in an async, event-based manner, and asynchronously reading a file from disk while handling errors appropriately.

• Proposal: https://docs.google.com/document/d/1f9NyP0QyaPp0DWdDv_708tJy-we8FWIsw9b1YF42riI/
• Tutorial: https://docs.google.com/document/d/1RG-mfuAG0UG-DF4MtX1yPyotlD9Dy2Vqh-zomR6r9RY/
• Design Doc: https://docs.google.com/document/d/1mVWaFXrAYqPPvODa06BykwZuDAMMqoszpCqiDo3kOzw/
• Poster: https://docs.google.com/presentation/d/1pfmNLMkmqx7cYZAaKnc9kAkINxQKvLbkpNHD3mIwrW0/
• Screencast: https://www.dropbox.com/s/myx6oqkhkl3gfll/errorlets-screencast.mov?dl=0

Usage

Lets set up an event handler on a button in the DOM with the id "target", and log Hello World! to the console. We build a simple StateMachine and immediately call run() on it to invoke it.

Event("click", document.getElementById("target"))
	.next(function() { console.log("Hello world!"))
	.run()

Now, lets demonstrate the simplicity of using a Stream object to deal with GET requests to a web server. This specific scenario gets a server name from a text-area with the id "server".

var request = {
  type: 'GET',
  url: document.getElementById("server").value,
  headers: []
}

Begin()
  .request(request)
  .next(handleServerIsUp)
  .error(handleServerIsDown)
  .done()

function handleServerIsUp() {
    console.log("Server " + document.getElementById("server").value + " is up :)"); 
}

function handleServerIsDown() {
    console.log("Server " + document.getElementById("server").value + " is down :("); 
}

To get a little more complicated, suppose we wanted to repeatedly request the server until we get a successful response back. ErrorLets provides an until construct that stops a stream when a function f evaluates to true, or a passed in event happens on a target.

var request = {
  type: 'GET',
  url: document.getElementById("server").value,
  headers: []
}

Begin()
  .stream(request)
  .next(function() { return "up!"; })
  .error(function () { return "down!"; })
  .until(function(res) { return res === "up!" })
  .done(handleServerIsUp)

Note that in this example, the function passed into done is invoked after the StateMachine has finished execution.

Thanks

UW CSE401 and Arrowlets for inspiration