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No One Just Designs a Programming Language (featuring Thorsten Ball)


Marianne ponders the consequences of different design decisions and how to direct her research through an enormous amount of information and choices. Thorsten Ball, author of Writing a Compiler in Go, talks about his experiences designing Monkey and some of his regrets in retrospect. More from Thorsten Ball's can be found at Writing an Interpreter in Go can be purchased here. Writing a Compiler in Go can be found here.

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MB: So let's get one question out of the way, does the world really need another programming language? Probably not, no.

MB: You're listening to Marianne Writes a Programming Language, and I'm Marianne.

MB: I have always wanted to write a programming language.

MB: I figured I would learn so much from the challenge. Even 15 years into my career in technology, I feel like there are all these weird holes in my knowledge. I know a bit about abstracts syntax trees, a bit about tokens, a bit about compilers and bytecode, even a bit about logic gates. But I don't have a clear sense of how all those things work together. One thing my career so far has convinced me of is that no knowledge is useless, things I learned in the past become valuable in the most unexpected ways.

MB: The other reason why I want to write a programming language is because I can't find a language that does exactly what I want to do. If you're familiar with my work, you probably already know that I'm very interested in complex systems and models. I spent a year or two exploring formal specification, a set of tools that I still use, to be honest, before I realized that languages like TLA+ and Alloy are looking for mathematical correctness. And that wasn't what I wanted. That wasn't what I was interested in. I didn't want to know if certain problems are impossible. I wanted to know how likely certain types of problems are in a given system.

MB: I wanted to model resilience, but one thing at a time. We'll get to all of that in more detail later. So how exactly does one write a programming language? When I started this project, I thought finding a jumping off point would be easy. I typed How do you design a programming language into Google and expected to get back at least a few medium posts laying out the basic decisions. Even knowing very little upfront I had a sense that in order for programming language to work, there had to be some sense of cohesion in its design. You couldn't just pick and choose things you liked in other languages and just think it would be OK. Some things would not work with other things, but what were those critical choices? I was surprised to find not much information. I expected someone to have developed a menu of some kind--even a primitive one--to serve as training wheels for beginners, even a complete bullshit one drafted to start fights on Hacker News. I expected to be able to skim a few opinions and have a good idea of where I wanted to start researching, except I couldn't find anything like that.

MB: A theme that's going to come up a lot in this series is the incredible divide that exists between the academic world of programming and the commercial world of programming as we go down this rabbit hole together. We'll talk about completely different styles of programming, strange data types. Jesus, there's a whole world of obscure experimental languages that appear in research papers, rack up a host of citations, and never touch an actual computer other than their inventor's. At one point I asked a researcher I interviewed for this series, why do professors keep publishing descriptions of implementations without actual code? Why do I see demos of things but no source code repositories?

MB: And he told me: because they're ashamed. If they share their code, other people will find all their programming mistakes. And to me, this is a really fascinating problem. One side of this divide is developing really interesting stuff, but doesn't know how to produce mature production ready code. The other side has those skills, but finds the language of academia to be inaccessible and difficult to parse.

MB: In my search for an overview of the theory behind designing programming languages, I kept getting referred back to these thick computer science textbooks, the Wizard book, the Dragon book. You've seen them referenced as well, I'm sure. I felt like this was probably a bad place to start. Perhaps I'm reckless or just intellectually lazy, but adding a few hundred page college textbooks to the top of my reading list seemed like a good way to destroy my momentum really fast, even though without question, that was the proper way to learn. It would do me no good at all if it killed my interests. I can't remember how I found Writing an Interpreter in Go. Just one of those things, I guess. But I once I found it, I knew immediately this is how I wanted to start. I recently had started learning Go for work and I can't tell you what a relief it is to have books about general programming concepts not written in Java. Thank God. Even better, Writing an Interpreter in Go implements a programming language called Monkey, which was developed specifically for the book, which meant the author was someone who had actually gone through the process of designing a full programming language.

TB: Umm... I think you're overestimating how novel monkey is

MB: And that is Thorsten Ball, creator of Monkey, author of Writing an Interpreter in Go, and also its follow up, Writing a Compiler in Go.

TB: So I didn't set out to create a completely new programming language ... I couldn't like... no, I don't-- I wouldn't even know what it should do, but what I realized and I'm sure you've seen it now that you've dug into this, there's a ton of material on how to create program languages. And in my experience, it falls into two categories. The one category is overly academic textbook kind of stuff where it says, like, you know, it doesn't even show you what the programming looks like. It shows you only snippets and or it's glosses over the syntax and says, you know, we don't have to worry about syntax. Let's just use this lisp as a stand in whatever.

TB: And then on the other hand, you have these tutorials, posts that show how to build a programming language and they mostly show really small toy examples. And they stripped down so much. And it's often just a calculator like, you know, like a repl where you can type in one plus two when you get out three. And what I wanted was something in the middle fully documented, including tests with the real syntax, like no parentheses lisp stuff, which I love-- like, for the record, I'm a fan.

TB: But what I wanted was like the real thing, like all of the programming languages, the real ones, they have curly braces and they have keywords and whatever. And I wanted that, like, how does that work? You know? So what I basically ended up doing is writing a lisp and then putting the monkey outfit on top with the curly braces and the fn keywords and let whatever, but it's it's really similar to JavaScript in a way, in the best possible way, I guess. And JavaScript famously is also, you know, it started as a Scheme implementation, like a really small Scheme implementation .. you have-- it's immutable by default. You have first class functions. That means you can pass the lambdas around, you have higher level functions. You can return functions from other functions. Not a lot of crazy data types, no static type system.

TB: So I just took... a minimal Scheme added the curly braces and arrays and hashes and the syntax for literals, so you can like literal types, so you can type array literal or like a hash map or whatever, and that's basically it.

MB: So what were the kind of design choices that you made when you were thinking about what Monkey should look like? I mean, like not in terms of curly braces or no curly braces, but in terms of like, should it be a strict typing or like dynamic typing or should be this or should it be that? Like, how did you think about was it like you had a design phase with Monkey or was it just like I'm just going to build things and we'll see where we end up? Right.

TB: The second one, yeah, definitely. And honestly, I feel like my my brain is not built for the first type of step. It's if you start out and want to design a program language from the ground up without, you know, building it at least in a prototype-y way, I cannot think that far ahead. It's that the layer we're talking about here-- the programming language itself and the built in functions and the keywords and you know how they work together. They are so deep down.... That every little change you make has a lot of ramifications further up.

MB: Yeah,

TB: And that is the most impressive thing about programming language design. Like if you, for example, look at the Go programming language, you know, there's this whole debate about should Go have generics or not, and they Go authors famously pushed that decision in front of them and didn't want to make a decision for years or said, no, Go doesn't need them, and there's too many ramifications and effects that that would have, and people would just say, oh, can you just add this and kind of just add that? And they experimented around and had these prototypes and design concepts. And I was so impressed by.... Whenever they present it, like, oh, we make this attempt and turns out that this would result in code that looks like this or we made this and that would change Go code to be written like that. I was so impressed by their understanding of basically the power they wield in a way, because, you know, look at Go or any other language the idioms you have in a language, they just come from all of the pieces lying around, basically, like people go, oh, Go's error handling works like this. So we going to use it like that. We're going to this. This is what an error looks like. This is how we're going to use it. Ba ba ba ba ba. And if you add more pieces into that mix, suddenly the whole game changes and suddenly... you know...

TB: If you add the wrong piece, you can change how code is written in that language completely, you know? I guess Java is famous, like if you look at Java in the pre generic days when they didn't have generic looks completely different than modern Java. And the same goes to C++. Right. They added a lot of stuff. And people-- people say, you know, C++ 11 is not the same as the one from the nineties or early two thousands, whatever. All of this has ramifications.

MB: That's the thing that I think I find the most intimidating about.... like, even undertaking this, is understanding that tiny little design decisions I don't even realize I'm making could have dramatic impacts. Like I think Java six that is like the point where they changed something fundamental in how things works. And like everybody's struggling to migrate off of. Like the the Python 2 to Python 3 story like, oh, we did this wrong. We're going to do it differently so that we can grow and continue to thrive as a language. But it's going to be really, really hard to make the transition. Like, I worry probably unnecessarily, because to reach that point of success is in-and-of itself unlikely.

TB: Right.

MB: But still as a creative undertaking, you always want your work to represent the best that you can do and you never want to go "If I had only thought a little bit harder about it, I could have seen that problem coming." Is there anything in Monkey that in retrospect, if you were to go back, you would think I would have done that differently in the way it's designed.

TB: ...Yeah..... um.... I added return statements to Monkey and, you know. There's two ways to return a value from a function in Monkey. Either you add a return statement and that returns the value or it uses the last value of the last expression. Right. So if you have a small function and it doesn't take parameters, but you just put in one plus two and it returns three, this function.

TB: Or you can add, you know, an early exit and say if A > 3 return a five, closing brace and then out six or whatever, so you have an early exit and.... Those-- like those return statements, I added these like on a whim, I was like, I like the implicit return because I was used to it from Ruby, I really like it because it allows you to write these really dense small functions and pass them around... Scheme again. Return statements are not Scheme. So suddenly, once you add return statements, you break... you kind of jump into this whole paradigm and go like "Here I am!" return statements and we're just going to exit from this function early. But it doesn't really fit a lot of the other stuff and it breaks kind of control flow. For example, if-conditionals are also expressions.

TB: So if you say if true, then one else five, that whole thing evaluates to one. Right. But you can now add a return statement in the consequence order of the alternative arm of the if-conditional. And that suddenly changes to control flow because now this is not an expression anymore, but it's an expression with like a hidden control flow statement in it. And that needs to go up. Right. And then as I said, like I have first class functions, I have higher level functions. That means you can return functions from other functions. That means you can have two functions in another function and return from those two functions. Right. And this is..... I had a few bugs in there. And it's just like... it's horrible. Like it's you can make it work and it's working now. I guess there's still some bugs that nobody has found, but it's... That there was one thing where I was short sighted and I was like, "OK, I'm kind of trying to shoehorn something into this, but but I don't know what I'm doing. And now it's really hard to implement."

MB: I'm seeing more and more parallels between the task of writing a programming language and working with complex systems, I can't figure out if that's because I focus on complex systems and therefore I'm predisposed to see them everywhere. The same way a cardiologist will think of problems with your heart and a neurologist will think of problems with your brain. But it does seem to be the case that programmers create languages without being able to fully anticipate exactly how they will be used or how technology will change around them. Is the solution to this dilemma the foresight earned through experience? Should a student's first programming language be treated like the first pancake off the griddle and just thrown away? I think about the stories I've heard about the programming language C and its development, like the conventional origin story about C is that Dennis Ritchie just walked into work one day and decided to create it. In reality, the development of C started with the development of BCPL and B. Computer scientists involved in those languages would build things, then change the implementation of those languages to clear blockers they had discovered and in fact could only have discovered through using the language. C then started from all of those lessons learned and then itself had many different conflicting and undocumented implementations before it was standardized in 1982, thirteen years after its development had started.

MB: I guess, like, from the perspective of somebody who has now done this for, like... what sounds like two or three years, right? Like the first book took a year. The second book took a year...

TB: Yeah, the first book was released in 2016.

MB: Oh so like... way more than that then.

TB: I started digging into it in '15, I guess. Yeah.

MB: So like a five year journey, like if you could highlight almost like the.... the decision tree, if I can call it that, like I think there is a certain baseline level of decisions that once you have gone that direction, other things are taken off the table, like what you were saying with return statements when you decided that you wanted it to be kind of a lisp Scheme flavored language, return statements just kind of didn't fit and didn't kind of work. Like, how would you think about that in retrospect? Like, were there core decisions that stand out in your mind as "I went down this road and that took all of these things off the table" logically.

TB: I don't think I can name all of them, but a few stand out and that it won't be in chronological order.

MB: That's all right.

TB: But so as I said, like the big one was Monkey is a Scheme in disguise, right? It is a pretty simple language. It has only a few data types, first class functions. And I added the syntax on top of it with the braces and, you know, the short keywords and let statements and stuff like that.

TB: So this set. The basics, and then I realized....

TB: Once I implement that and actually, you know, wrote it down, I realized, oh, that's not really useful, you know, so I added more data types like hashes and arrays. So to make it more practical and make it more relatable or interesting, I guess, because this was always missing and once you implement an interpreter that can do binary expressions with numbers and booleans. It's cool, Yes. But it's-- it's not five hours of fun, you know.

TB: So I added these and then.... Somewhere around that time, I guess I also made a decision to not add assignments or mutability to the language. So everything's basically immutable, you cannot say that A equals 8 and then A equals 5 or whatever that is right now, that's undefined behavior it's not implemented. And the reason why I did it was because that, you know. A equals eight or five is simple, sounds simple, right, but once you add mutability and you can assign stuff or reassign stuff. You also need to incorporate that in your closures which which captures state, right? Like you have a closure is a function that wraps or closes around state. So what if it closes around state like a binding of a name like A equals five? Right. When the closure is created, A equals five. And after the closure is created, you say equals eight now. You know, what does the closure return? That is a decision you have to make in different languages, have different answers to this, right?

MB: Right.

TB: That has effects like your decision. There is.. it's a simple decision to make, but it has a lot of consequences. And I kind of, you know, I avoided that decision or said it's immutable, but it also means.... It's not easy to implement loops right? Because a loop is based on a condition that changes, I guess,

MB: Right, got to increment the i.

TB: Exactly, yeah, and if you don't have a way to change something, that's not straightforward to implement. Right, like a for loop, like i = 0; i < 10; i++... That includes assignment. That includes mutability.

MB: Right.

TB: So if you don't have assignment then that's off the table.

MB: That's interesting because I would not have made that connection until you pointed it out. And like "Oh yeah. You really can't, can you?"

TB: Yeah.

TB: So I made a livestream in winter last year where I, you know, let's try out this live streaming thing. And I said, let's add loops to Monkey. And I'm not sure whether you can see it in the stream, but towards the end of it, I was like, Goddamn it, I need assignment for this. The problem is you can. You can.... you can of course, you can write like while true and then have it executed for all of eternity, right? Or you can say while My blah, blah, blah function and where blah, blah, blah function as a built in function that returns whatever, but as you know in the books everything is test driven. So we write the test first and you want to make sure that it works. And there's no good way to write a test for loops without basically-- Do like a dependency injection or mocking of the whole system, which breaks this whole idea of really short and easy to read tests

MB: Right. Right.

TB: So, yeah.

TB: And then. Yes, so there was assignments and mutability, and I also I didn't want to add classes, just aesthetic reasons, I guess, like the Scheme/Lisp py-whores were like, you know, objects are just poor man's closure and closures are a poor man's objects. So if we have closures we don't need objects, of course, readers are interested in.... how would you implement classes? But such that, again, is such a seemingly small decision that has incredibly huge consequences, like look at look at JavaScript. Like a couple of years ago to added classes and people did classes, classes in air quotes by, you know, returning objects with methods from other functions. But now that you have the class keyword and like this whole syntactic structure, for them, it changed how people write code. So it's, again, like huge consequences. And, you know, I didn't want to have classes. I'm I don't think I'm a huge OOP fan anymore, I just didn't want to have them. And also everything that I just said, I had to fit in in a reasonable amount of pages in a book.

MB: It's not a five part series then?

TB: Yeah and as you know: everything is test driven. So it's not just implementation, but it's also a test for the implementation that you need to show in the book and every bit of code. I think only two helper functions. I'm not sure.

MB: This is the place for something clicked for me. Immutable data, classes, loops, implicit returns. What we're discussing are the conflicts that arise when mixing functional programming styles with procedural* programming styles. So maybe a key part of that menu I've been looking for is about the assumptions that ground those styles. How do we want to handle state? Are functions first class? Is inheritance important?

MB: ....I think I need to get a few more books.

  • This should actually be imperative, not procedural but I didn't feel like rerecording that voice over just to correct that error ;)

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