Emily programming language: Status update one

About a month ago, I put up some plans for a programming language. It actually got some interest!, so I thought I’d go over some things that have happened since then.

First off, I decided the name of the language is definitely Emily, unless I’m somehow legally forced to call it something else. I made a website for it and an announcement-feed Twitter, at emilylang.org and @emilylanguage, respectively.

Second off, I now have a prototype implementation! It’s a very, very minimal implementation– you seriously couldn’t develop anything in it, and it’s missing most of the language’s better planned elements– but it does demonstrate a couple of the language’s striking features, and it’s turing-complete. (And the turing completeness part can be demonstrated in an amusing way: It turns out to be possible to embed Unlambda programs in it.)

Programs written in the language don’t look very pretty right now. Most of the “operators” in Emily, like = or +, are eventually intended to be defined as macros, and those macros aren’t implemented in the prototype yet, so setting a variable looks a little funny:

# Prints "3.0"
set .test 3
print test

And adding looks a little funny:

# Prints "7.0"
print (3 .plus 4)

In the final version of Emily, writing “3 + 4” will do the same thing as writing “3 .plus 4”– the only difference will be that + has operator precedence, and .plus does not.

There’s one macro that *is* in already– ^, or “lambda”, which turns whatever comes right after it into a function:

# Prints "4.0"
^x( print x ) 4

This defines an “anonymous function”– if you aren’t familiar with that idea, it’s a function that doesn’t have a name but just shows up in an expression, like a literal– that takes one argument and prints it. It then feeds the anonymous function the argument “4”.

With these couple things explained, here’s a more complicated Emily program that works in the current interpreter:

set .countup ^arg {
    set .count (arg.from)
    loop ^(
        print count
        print "\n"
        set .count ( count .plus (arg.step) )
        count.lt (arg.to)
    )
}

countup[ set .from 10; set .to 20; set .step 2 ]

Again, not very pretty, but this demonstrates a couple of interesting things that Emily tries to do.

To recap my last blog post a bit, everything in Emily is formally a function– everything. Objects are functions pretending to be objects, functions which take key names (in the form of strings like .plus— assume this is just a funny way of saying “plus”) and map them to values. Numbers are objects, which are functions. Scopes are objects (which are functions); the language sets up and shuffles around the scope functions silently, and it forwards any bare symbols it finds (like “set” or “count” in the examples) to the current scope function as a string argument. A lot of the time these various functions don’t really *act* like functions, of the kind we normally recognize (for this reason I’ve been calling them “blocks”, after Smalltalk) but the fact the language doesn’t distinguish the different kinds of functions semantically opens up a lot of possibilities for useful abstraction. The only “verb” in Emily is “apply”– apply function A to argument B. Everything else is just built on top of that.

Not a lot happens in this example, but a couple things do stand out: assigning a variable value just means invoking the “set” method on the current scope (i.e., passing .set to the current scope block, doing which returns a function that can be used to alter that scope block’s mappings). There are no special flow control constructs– “loop” is just a function that takes a closure as argument and executes it until it returns false. The thing that really stands out to me here though are the appearance of {} and []– the countup language body is wrapped in a {} to give it its own variable scope, and when we invoke countup the argument list is written as an “object literal”, denoted by []. What’s interesting here is how these two things are implemented by the language.

Parenthesis in Emily create “groups”– I have to call them that because I used the word “block” for something else already, maybe a mistake. Any parenthesis in Emily can contain multiple statements, separated by semicolons. A trick here is that group (), scoped group {}, and object literal [] are all the same thing, distinguished only by what the scope is within the group and what is returned at the end. More interestingly, there’s no “magic” to the group operators– effectively, what defines the different group types are a couple of lines of setup/teardown code that run at the beginning and end of the group, and that setup/teardown could in principle be written in Emily. In other words, {} and [] could in the final version just be implemented as macros! In the current implementation, all three group types are handled by the same code in the interpreter, with the difference being:

    (Assume any time a statement executes, its evaluated value is assigned to a variable named “last”)
    Plain groups: Set [scope] to the enclosing scope; after final statement return “last”.
    Scoped groups: Create a block [newscope], which prototype-inherits from the enclosing scope, and set [scope] to it. After final statement return “last”.
    Object literals: Create a block [object]. Create a block [newscope], which prototype-inherits from the enclosing scope. Assign [newscope].set = [object].set and [newscope].this = [object]. After final statement, return [object].

Did I get too language wonky there? The point I’m trying to make here is that “an object literal syntax” is a really basic thing to a language, but this language kind of doesn’t have one, because it doesn’t need one. That’s just normal code running inside of the [], and the thing that makes [] “special”– the setup/teardown– could have been implemented by an end user. This is exciting to me because it implies end users can implement other language extensions, things as complicated as an object literal syntax, on their own.

The implementation

The prototype implementation of Emily is hosted on BitBucket, although it will not do you a lot of good right now. The main problem is that it is written in Objective-C, which means that it will only easily build if you have OS X and XCode. Compiling on other operating systems is possible but will require setting up something like GNUStep. I haven’t investigated this myself, mostly because I intend to port to something-other-than-Objective-C rather than bother with GNUStep. If you do have a mac, or do have GNUStep set up, you’ll probably find it pretty easy to set up and extend. The current implementation is only about five or six source files. (Sorry if the BitBucket page is a little confusing, they just changed their layout and now it’s all weird– the cloud with the arrow in it is the “download source tarball” button.)

Remember, this is literally the very first thing I got to run at all– as of this exact writing there’s no code documentation, no documentation of the exact language as implemented so far (the closest is the sample code files), and there are known memory leaks. I’m not even giving this a version number yet. Still, it’s a start! If you want to try actually using or extending it, feel free to contact me and I’ll help you get set up.

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