Magpie Wants You!

June 06, 2011 code language magpie oscon

Update 2021/09/12: Magpie is very old and long-dead at this point. I learned a lot from it, but I’m no longer maintaining it.

I’ve been very fortunate to get lots of feedback and encouragement so far while I’ve worked on my little language Magpie, but I haven’t actually asked for people to get directly involved. I still had lots of basic syntax and semantic decisions to make and it would have been an exercise in frustration to drag anyone else along through that.

I may be entirely wrong, but I think I’m past the worst of that. Now that classes, patterns, and multimethods are implemented and working, I believe Magpie may be at a point where adventurous souls could try to play with it. Multimethods were the big missing piece, and I’m super excited that they’re working now. To give you an idea why, here’s some fun stuff they enable:

Overloading methods

Magpie is a dynamically typed language (flirtations with optional static typing notwithstanding). But in Magpie, every method is a multimethod, which means you can overload them.

def (this is String) split()
  this split(" ")

def (this is String) split(separator is String)

Here we’ve defined two split methods on strings. The first takes no arguments, and the second takes a separator. That means you can do both:

"eenie-meenie-miney-moe" split("-")
"eenie-meenie-miney-moe" split()

In most dynamic languages, you’d have to implement that by doing some manual instanceof or !== undefined checks in the body of a single split() method. In Magpie, it just works. You can overload by arity, or type, or both:

def (this is String) split(maxResults is Int)

def (this is String) split(separator is String, maxResults is Int)

Pretty much any kind of argument list can be overloaded and Magpie will pick the right one at runtime based on what you actually pass it.

And just to clarify, you don’t have to define all of these methods in one place or even in one module. They can be anywhere you want. In fact, you’re free to define methods on types you didn’t create, which leads us to…

Sane monkey-patching

One of the most powerful features of Ruby is that you can extend existing classes with new methods. This lets you define libraries which make the most of Ruby’s object.method syntax and can lead to beautifully readable APIs.

The problem, though, is that if two libraries happen to cram a method with the same name on the same class, all hell breaks loose. Very high on my list of goals for Magpie was to solve this problem, which is why classes don’t own methods. Instead, they are lexically scoped like variables.

This means that when you define a method “on” a class like our split method up there, you aren’t touching the class at all. Instead, you’re defining a function in your local scope that happens to take a String as its left-hand argument. Outside of the scope where you defined it, that method doesn’t exist.

If two different modules define methods on the same class with the same name, there’s no collision, as long as they don’t import each other. You can even use both of those modules in the same program without collision. You should be free to define methods on any classes you like if it makes your code easier to read.

Do actual stuff

Of course, all of the neat language features in the world are pretty useless if the language can’t solve actual problems. For almost all of its history, the only thing you could actually do with Magpie was print to standard out. I know, not very inspiring.

Fortunately, I’m finally getting to the fun part of building real APIs for file IO, concurrency, networking, etc. I have a ton of work to do here (hopefully with your help!) but at least now you can read files and spawn threads. To prove it, here’s a toy asynchronous web server:

import io
import net
import async

val server = ServerSocket new(8080)
while true do
  val socket = server accept()
  // Start a new thread to respond to the request.
  run with
    // Process the request (assume its a GET).
    val path = socket readLine() split(" ")[1]

    // Open the file being served.
    open("." + path) use with
      // Read it and write it to the socket.
      socket write(it read())
    socket close()

Where you come in

If you’ve ever had the desire to help build a programming language and its ecosystem, and what you see here looks cool, I’d love to have you involved. It will be a lot of work, and it’s likely that more things are broken than working, but you’ll have the chance to get in on the ground floor. Hell, you may be building the ground floor. If that sounds like fun, here’s what you can do:

That’s really about it. You might want to follow me on twitter or point your RSS reader at my blog too. Beyond that, we’ll have to see where it goes from here.

What comes next

Of course, it’s entirely possible that no one will sign the charter for this little voyage but me, and that’s OK. I’ll keep hacking on Magpie regardless, but for the next few weeks, I’ve got something much more fun to focus on: preparing for my talk on Magpie at OSCON.

I feel like I didn’t stress that enough. OMG I’M GIVING A TALK ON MAGPIE AT OSCON. It’s hard to describe how that makes me feel. Kind of like kissing the prettiest girl in middle school behind the bleachers but also I just drank a huge cherry Slurpy and I kind of feel like I need to throw up. Something like that.

I will be speaking on Thursday July 28th at 1:40 PM. I’m not sure if that means that I’ll be drinking before 1:40 or after. Perhaps a little bit of both just to be safe.

So, if you’re going to be at OSCON this year (and you should be because OMG I’m giving a talk at it!), come see my talk. At the very worst, I promise to give you a hug afterwards if you ask nicely. So even if the talk bombs, you get a free hug.