C is declarative, kind of

Inconsequential musing of the day:

If you look at say, a Python file — or Bash or Ruby or almost any other scripting language — you can’t find any information1 about the names defined in a program without actually running the script. This is because in Python, the only way to associate a name with a value is to execute a line of code that causes it to get some value (maybe an assignment statement, maybe a function definition, maybe an eval that contains one of these things). The association of top-level names to values is inherently a procedural process, defined only by the language’s execution model.

Contrast this with the situation in C. C has actual declarations, which associate certain information to a name completely independently of the execution model, and in fact the execution model usually doesn’t2 cause values to become associated with top-level names at all. C’s top-level namespace is specified declaratively: it states directly which names are associated with which values.

Does this matter? In practice, probably not much. In theory, a declarative program layout should be easier to manipulate through automated tooling (for example, to list out the documentation for all of the top-level names defined in a module/file without running any of its code). But most reasonable Python programs are pretty declarative at the top level anyway: you rarely see things like

# webserver.py
turing_machines = [...]

if does_it_halt(turing_machines[0]):
    def post(blah, bleh):
        blee
if does_it_halt(turing_machines[2]):
    def get(bloo):
        bluh

globals()[running_time(turing_machines[1])].__doc__ =
  ''.join(accepted_language(turing_machines[1]))

On the other hand, languages like C do make it a lot easier to catch typos, which I consider to be an appreciable benefit. Not that typos are a particularly difficult to track down class of bug.

But mostly I just found it interesting to think about how C’s model is, in a way, more declarative than lots of more modern languages.


  1. In general. Of course most specific scripts aren’t quite so horrible. 
  2. I think there are a couple exceptions to this having to do with extern and/or static top level variables and perhaps also function pointers (can those be defined at the top level?). But the top level of a basic C program consists entirely of function declarations and no assignment. 
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Project Serpent: Day 8

Somehow Day 7’s post got lost amid my move and starting my first day of work1. My accomplishments for that day are probably smooshed into the post for Days 5 and 6, but I haven’t been paying close enough attention.

Needless to say, I’ve decided to extend my deadline to a second week. This isn’t really a problem, because the point was never to finish something fast, it was just to stick with it until it got finished at all. I think I’m pretty well on track for this (but I do think that this next deadline is going to be stricter: I can’t just pat myself on the back and be content that I’m making progress, so I want to be careful not to let this drag on longer than it needs to).

Anyway, a brief recap. At the end of days 5 and 6 (and maybe 7, I guess?), I had implemented my NONBLOCKING effect and its yield command, which allowed me to write much more natural-looking code. I felt very satisfied with my work, and proud for thinking of it and fighting my way through the type errors until it actually worked the way I wanted it to.

And then I called it a day, because it was late, but that meant that I ended that day of development without actually using my triumph to write a natural event loop to evolve and draw the game.

So on Day 8, I spent my time doing that. First, since I was now writing my entire game loop within the Effects DSL, I needed to write an effect that provides drawing commands rather than just using IO actions. I did this.

But I soon discovered that whenever I used these drawing commands, they would grind my game loop to a halt: I had previously put in a console.log every time my yield code fired, and whenever I used my drawing effect I’d get exactly one line of output from it. I worried that perhaps my yield effect wasn’t as composable as I had thought, that my entire architecture was flawed and that if I’d just thought about it more rigorously before spending a day implementing it I would have realized that it couldn’t possibly work, that it was too good to be true.

Actually it turned out to be an embarrassing example of the kind of mistake that doesn’t get caught by a type system. Interpreters for Effects programs are defined in continuations, and the thing that makes this powerful is that you can define when, how, and how many times the continuation is called. For instance, you can call the continuation under a lambda abstraction and pass it values that don’t actually exist. You can call the continuation many times and build up a list of the results.

Or you can call the continuation zero times, simulating a nonlocal exit. The code following a command that doesn’t call its continuation… doesn’t get executed.

Needless to say, this is exactly what I did.

Despite all those problems, I soon ended up with a simple program that just makes the snake run off the edge of the screen. But the program was written entirely in my little Effects DSL, and looks very natural, and I’m confident that it will be easy to write many different variations when it comes time to add more features.

Day 8 was well spent.


  1. I am so excited about this. So much that trying to find the words would distract me unacceptably from Project Serpent, so unfortunately I won’t. 

Project Serpent: Days 5 and 6

I didn’t have time to write a blog post yesterday, because I was moving to San Francisco1. I was still doing work though, and today I’ll write a catch up post (in addition to my day 7 post later).

Yesterday (was that the earliest day I haven’t written about? I think so) an enticing thought occurred to me, about how to integrate my [GAME phase, 'GameClock ::: FPS 5] effectful programs into the Javascript model of “compute only in small chunks, so as not to monopolize control of the browser”. The idea was simple and glorious: just write an Effect that provides a “yield” command, and have the handler for that command register the continuation as a JS callback rather than applying it directly!

I spent most of the day thinking about various aspects of this — what type it would need to have and the finer details of how it would work. When I finally got off the plane and arrived at my new home for the next few months, I sat down and started writing some code, and things became much clearer.

Except… they also became a bit less clear in other ways. I don’t recall all the specifics now, but as my ideas started taking shape in the code I was engulfed in a hail of bewildering errors that came about from the specific way that Idris’s Effects library works rather than anything about the semantics of my code. Rather than deal with them late at night, I finally decided to just go to bed and figure out how to massage Idris into accepting my definitions tomorrow.

Likewise, yesterday was mostly spent scrutinizing strange type and inference errors and trying to figure out what I could tell the compiler to make it understand why my code is acceptable. This process was a bit frustrating and took a few long hours, but finally at the end of it all I had shut down the worst of these errors for good, and was able to write this simple program that ticks the gamestate at the correct rate by always sending the snake forward:

dummyGame : { [GAME (Playing False rules), 'GameClock ::: FPS 5, NONBLOCKING] } Eff ()
dummyGame = do
  delta <- yield
  newFrame <- 'GameClock :- tick delta
  if newFrame
     then do collision <- turn Straight
             if collision then playAgain startState else value ()
     else value ()

This piece of code does everything correctly — respecting the frame rate and returning control to the browser after every frame — without having to manually break up the control flow at all. I’m pretty happy with how it turned out!

I still don’t have a way of drawing the game state yet, but that can be solved pretty simply: all I need to do is write an effect that provides drawing commands! I’ll be working on that later today.


  1. actually Berkeley, but I am enough a stranger to the area that I consider them the same. 

Project Serpent: Day 4

Halfway through my first week, and it looks like unless things pick up I’m going to be taking a second. That’s fine though, just as long as I keep working.

Right now I have a very preliminary “step game” function that allows the snake to move around, maintaining its length, but doesn’t do any collision detection (including whether it has eaten any food). The obvious thing to do here would be to detect collisions and remove food (code is already in place to grow the snake if the collision detection signals that food has been eaten), but instead I’ve decided to take a little break to switch to working on the front end.

What have I been doing on the front end? Well, as in everything, there are degrees to this, and I’ve only been working on the “middle” front end so far — actual rendering and user input aren’t coming yet, but I’m one abstraction level closer to that. I am writing code that is “aware” that user input and frame rates exist and must be taken into account.

I’ve been handling this as follows. There are a few rough “parts” to the Effects library: the specification itself (defined in Spec.idr), the “resources” that are required to back those effects (which I define in GameState.idr), and “effectful programs” that make use of the operations defined in the specification to manipulate the resources. The great thing about effectful programs is that they can have a list of effects available, and can use operations from any of them. So for example, I’m writing programs right now with access to [GAME (Playing False rules), FPS 5]: I can manipulate a game that’s unpaused, and… hey, this FPS thing is new!

FPS is an effect I wrote yesterday night when I realized that if I’m writing code that will run on every browser frame, I shouldn’t necessarily have that code run a simulation tick every time. The coordinates I’m planning to use in Serpent are relatively low resolution, and moving forward by an entire “square” every frame would be way too much. So the FPS effect manages a little counter that can be queried to say “Hey, this much time has passed since I last talked to you. Should I run another simulation tick yet?”. Currently I have it set up to run 5 ticks per second, but that can be easily adjusted later.

Of course, 5 ticks per second is fine for the game clock, but a UI that runs at 5 FPS is going to be annoying. I plan to solve this using tagged effects: [FPS 5] represents the ability to respect a clock that runs at 5 ticks per second, but ['GameClock ::: FPS 5, 'UIClock ::: FPS 60] represents the ability to respect both a game clock that runs at 5 frames and a UI clock that runs at 60 frames per second. This is similar to how many RTS games run on a coarse-grained clock used for the simulation and a finer-grained clock so that nice animations can be shown between the game’s relatively abrupt state transitions.

And finally, right now I’m just writing a function from user inputs to effectful programs with access to [GAME (Playing False rules), 'GameClock ::: FPS 5, 'UIClock ::: FPS 60], but I plan to soon write an effect representing input as well. After that, I hope to be able to write code like this:

if !('GameClock :- tick delta)
  then turn !getDirectionCommand
  else pure False
if !('UIClock :- tick delta)
  then {- draw a new frame somehow -}
  else pure False

Of course, I’m not quite there yet. But I think I should be by the end of today!

Project Serpent: Day 3

Yesterday I spent some time improving my MenuInput data type. I realized I haven’t mentioned that at all in this series of posts, so I’ll talk about it a little.

A MenuInput basically describes an editable entry in an options menu. Simple menu inputs might be things like NatBox "Number of heads" 3. This represents an input box that can contain a natural number, labeled “Number of heads”, and whose default value is 3. Similarly, FloatBox "Game speed" 1 is a box called “Game speed” that contains a floating point number defaulting to 1. There are also selection boxes, like Options "Hair color" ["blue", "green", "red"] "green", which means that there are three choices for hair color defaulting to green. And finally, there are toggle options with additional configuration available if they’re toggled “on”: Toggle "Gravity" [FloatBox "Strength" 9.8, Options "Direction" ["up", "down", "left", "right"] "down"] means that gravity can be either on or off, and if it’s on you can also configure its strength and select one of the four cardinal directions for it.

Every kind of menu input has a type of values that it accepts. A NatBox accepts natural numbers, a FloatBox accepts floating point numbers, an Option accepts one of the available strings from the list of choices1, and a Toggle can either be “off” or it can be “on” with a list of values for the extra configuration options. Because Idris is a dependently typed language, we can actually encode this statement as a function from MenuInputs to Types, and that’s exactly what valueFor does in Spec.idr.

So part of my time yesterday was spent writing additional utilities for working with lists of menu inputs and the values they can take. For example, I have a function to apply an update2 to a particular entry in a menu, or to get the value of a certain entry.

Anyway, that is one system I have developed that I hope to get a lot of use out of when I get to programming the menus. So far, I’m not there yet, but I’m getting there: the other thing I spent time on yesterday was my skeleton implementation of the game state, with a focus on states in the “in-game” phase. Right now I’m working on what you might call the meat of the game: stepping a game state by moving the snake, doing collision detection, and possibly growing it or ending the game.

Today I’ll finish writing functions for stepping the game state, and hopefully by tomorrow I’ll be ready to start hooking those functions up to some kind of UI!


  1. we represent these in Idris as strings paired with proofs that they’re in the list of available options. 
  2. I also have a function mapping MenuInputs to the kinds of updates you can perform on them: for example, a Toggle can be toggled and a NatBox can be incremented or decremented, and any input can be set to an exact value. See updateFor in Spec.idr for details. 

Project Serpent: Day 2

Yesterday I only barely got my four hours in — a reminder, I suppose, that even in the wake of my accomplishment, finishing larger projects will still be difficult for me. I’ll have to be more careful about starting to work a bit earlier in the day.

Once I did get started working, I think I got a reasonable amount done. I started by defining a type for the concrete game state (parameterized by the “phase” category I talked about yesterday), which should store everything that’s necessary to represent and evolve the game. After that I wrote a skeleton implementation of a handler for every command I defined in Spec.idr. But while working on this, I realized that my types were looser than they needed to be.

I took a break to think about this and ended up storing in all Phases the set of active mutators and their parameters. For now, this mostly lets me specify the menus more tightly: the Reset command, for example, now states in the type that it resets all mutators to their default states, and the SaveMenu command states that it does some validation checking and then either stays at the menu without changing anything or returns to the main menu with the new parameters in effect. All the other commands also state that they don’t change the set of active mutators.

Making the mutator state available to this set of rules also opens the way for a much more dramatic change later: I’ll be able to add to the Playing phase parameters like the snake’s length and score, so that rules about how the snake grows can be encoded in types too!

Of course, with all this type fun, I’ve still done little work on the game’s implementation. I’ll have to pick up the pace soon.

What I Gained from Hacker School

I spent the last two and a half months at Hacker School, which has been described as a “writer’s retreat for programmers”. You fly out to New York to their space, where you join a bunch of other programmers, providing social pressure to do some programming every day and a community of people with whom you can talk about programming. For a solitary person such as myself, both of these are extremely helpful and novel.

Hacker School is also completely free to attend and even has cost of living grants for a steadily growing set of demographics. Their business model is like that of a recruiting agency: they help out any Hacker Schooler who is looking for a job and charge companies for hiring these people. For me, this sounded almost too good to be true: having never had a job, I was desperate for help breaking into the technology industry through the wall of college degree and years-of-experience requirements. But there was no way my parents could afford to fly me all the way out to New York and put me up in an apartment for two and a half months.

So I went to Hacker School for two reasons: to experience and environment where programming is a normal part of life, and to get a programming job. What did I actually get from Hacker School?

First and most exciting: I did get a job. The wonderful Hacker School facilitators, especially Sonali Sridhar, guided me through the process of writing a resume that is impressive despite its lack of experience, and of reaching out to Hacker School’s various partner companies. I got more interviews in a couple weeks than in the six months I had spent optimistically looking for a programming job on my own. And while most of these leads eventually turned me down, one company seemed consistently impressed by me and kept inviting me back… and on December 1st I start with Prismatic. When I got accepted to Hacker School I hardly dared hope for an outcome this good, and it definitely wouldn’t have happened if not for Hacker School.

There are also some things I gained that I hadn’t really anticipated.

For example: I have been wanting to start a blog for at least a year, probably longer, but never been able to commit to writing regular posts. There might even be a handful of dead WordPress or Blogspot blogs of mine scattered around the web — I don’t remember any of them, and certainly there are none with more than one post.

But at Hacker School I started to realize that people like hearing what I have to say. I even gave a little 20 minute talk and the handful of people who wanted to hear it seemed to enjoy it. And at the same time, I got together with some other Hacker Schoolers who were also trying to commit to writing one blog post a week. At first I thought this attempt would be like all the others, that I’d make a handful of posts and then forget again. But it’s been three months and I’m still posting… and people are even reading.

And I didn’t produce anything at Hacker School that I’d call finished, but that’s normal: I hadn’t really finished any projects before Hacker School either. What isn’t normal is that after Hacker School, I noticed this deficiency and corrected it. I can’t help but think that Hacker School’s atmosphere of programming every day had something to do with this sudden burst of productivity.

Finally, there’s something I feel like I could have gained from Hacker School that I didn’t. I met a lot of people there that I had fun interacting with, people who I would have been happy to get to know better and who probably would have been happy to know me… but I didn’t come away with any new close relationships, mainly because I have difficulty initiating and sustaining interactions. But even here it’s not too late: the Hacker School community is broad, and there are resources in place for helping alums connect with other alums and with current hacker schoolers. I hope to make a bit more effort along these lines as soon as I can work myself up to it!


Finally, did I mention that Hacker School is always looking for new applicants? If anything I’ve talked about sounds interesting, check out their site — it could cost you nothing at all!