(This post is part of Crystal Advent Calendar 2015)

This Christmas eve something curious happened: we were happily coding in Crystal when, one moment when we took our eyes away from the screen, a translucid figure appeared nearby. The entity approached and said: “I’m the Ghost of Christmas Past. Come with me.”

We saw ourselves coding a new language that would resemble Ruby but be compiled and type safe. At that moment the language was really like Ruby: to create an empty Array you would write [] , or Set.new to create an empty set. And we were happy, until we realized compile times were huge, exponential, unbearable, and got sad.

We spent an awful time trying to make it work with no avail. Finally, we decided to make a change: speficy the types of empty generic types, for example [] of Int32 or Set(Int32).new . Compile times were back to normal. And we were kind of happy again, but at the same time felt that we were leaving behind some of Ruby’s feeling. The language diverged.

We looked back at the Ghost of Christmas Past to ask him what did all of that mean, but we found a similar but different figure in its place. She said: “I’m the Ghost of Christmas Present. Join me.”

Around us, a small but vibrant community was programming in Crystal. They were happy. There was no mention of the annoyance of having to specify types for generic types. Everyone was feeling that Ruby’s spirit was somehow still present: in the familiar API and classes, in the syntax, in the powerful blocks. Additionally, the increased performance, both in terms of CPU and concurrency, coupled with better type safety, really paid off, so having to specify a type now and then didn’t feel like bothersome.

Again, we looked at the Ghost to ask for a meaning: it seemed that we took a good decision in the past, right? But, just as before, there was something else in its place, a mechanical crystalline figure. It spoke: “I’m the Ghost of Christmas Yet to Come. Follow me.”

The small community was still programming in Crystal, though most didn’t seem to be as happy as before. We tried to ask them why, but nobody noticed our presence. We tried to use a computer and search for Crystal on the internet, but our hands couldn’t touch anything. We turned to the Ghost with an inquisitive face, and noticed it had a keyboard and a small screen in its chest. We searched “Crystal sucks”, which would hopefully show posts of complaints about the language. And indeed there were quite a few of them. Most were about huge compile times and memory usage. “Huge compile times?”, we thought. “We solved that years ago!”, we shouted to the Ghost. The only reply we got from it was “compiling…”, the vision faded and we were back at the office, alone.

Back to the present

“Let’s do some math”, we said. The biggest program we have in Crystal right now is the compiler, which has about 40K lines of code. It takes about 10 seconds to compile, and it takes 940MB of memory to do so. One of our Rails apps, counting the total number of lines in its gems and the code in the “app” directory, has about 320K line of code, 8 times bigger than the compiler. If we rewrite it in Crystal, or at least do an application with a similar functionality, it would take 80 seconds to compile it, each time, and 8GB of memory to do so. That’s a lot of time to wait after each change, and an awful lot of memory too.

Can we improve this situation, with the current language? Can we introduce incremental compilation? We spent some time thinking about how to cache a previous compilation’s semantic result (inferred types) and use that for the next compilation. An observation is that a method’s type depends exclusively on the type of the arguments, the type of the methods it invokes, and the types of instance, class, and global variables.

So, one idea is to cache the inferred instance variables types of all the types in a program, together with the types of method instantiations and its dependencies (on which types that method depends, and specifically which other methods it calls). If instance variables types remain the same, a method’s code didn’t change, and the dependencies (invoked methods) didn’t change, we can safely reuse the result (types and generated code) from the previous compilation.

Note that the above “if” starts with “if instance variables types remain the same”. But how can we know that? The problem is that the compiler determines their type by traversing the program, instantiating methods, and checking what gets assigned to them. So we can’t really reuse the cache because we can’t know the final types until we type the whole program! It’s a chicken and egg problem.

The solution seems to be having to specify types of instance, class and global variables. With this, once we type a method its type can never change (because everything that’s non-local to a method, like instance variables, can’t change anymore). We would be able to cache that information and reuse it for next compilations. Not only that, but type inference becomes much simpler and faster, even without a cache.

Is this the right thing to do? We will once again diverge a bit more from Ruby. What future do we want? Do we want to stick with the current approach at the cost of having to wait a lot of time between each compilation? Or is it better to specify some more types but have a more agile development cycle?

What we really want is a language that’s fun to use, and efficient. Having to wait a lot of time for compilation to finish isn’t fun at all, even less fun that having to annotate a few types now and then. And these types are just for generic, instance, class and global variables: no types annotations are required in local variables and method arguments. Considering how rarely these types change, compared to how many times you are going to be writing new methods and compiling your program, it feels it’s something worth of a change.

We already started working on this new compiler, because we want to do this as soon as possible as a lot of code out there will break. While the current compiler works directly on the AST, in the new compiler we work with a flow graph, which will allow us to have a simpler compiler (one which anyone could understand and jump right into it and contribute) and easier to understand and optimize code. It will also make it possible to introduce new features like Ruby’s retry with minimal effort, because the flow graph allows for cycles and “goto”-like jumps.

If you’d like to know more about this change, there’s a tracking issue about it.

Questions and Answers