Call for collaboration: Disco web UI

Disco

For the past nine (!) years I have been developing Disco, a functional teaching language for use in a discrete mathematics course. It features first-class functions, polymorphism, and recursive algebraic data types, along with various built-in mathematical niceties and syntax that is designed to be close to mathematical practice.

As a simple example, here’s a recursive function in Disco to compute the hyperbinary numbers:

h : N -> N
h(0)      = 1
h(2n)     = h(n) + h(n .- 1)
h(2n + 1) = h(n)

Since recursive functions in Disco are automatically memoized, this evaluates almost instantly even on very large numbers:

Disco> h(1000000)
1287

If you want to know more about Disco, you can read a paper about it here, or read the language documentation.

The problem

I want students in my Discrete Mathematics course (or any Discrete Mathematics course) to be able to use Disco: to play around at a REPL, write and test their own code, and so on.

However, getting students to install Haskell and build Disco from source is a total non-starter, for multiple reasons. Some students in the course are not all that tech-savvy. Some students don’t even have their own computer, or the computer they do have chokes when trying to compile a large Haskell project (e.g. because of limited memory). Even aside from those issues, I simply don’t want to spend time and effort helping students install stuff at the beginning of the semester (at least not in this class).

Old solution: Replit

For a couple years, students were able to use Disco via a site called Replit, which provided free educational accounts, and supported arbitrary Nix configurations. Since the disco package on Hackage was picked up by nixpkgs, it was possible to spin up a Disco interpreter on Replit in just a few seconds. Replit provided a virtual file system, an editor, and, of course, a REPL.

This was a great solution while it lasted, but sadly it is no longer viable:

• Disco has been broken in nixpkgs for a while now, and I have no idea how to fix it.
• Even if I could fix it, Replit has stopped supporting free education accounts, and started trying to shove LLMs into everything, making it no longer a viable teaching platform.

Solution criteria

There are a few non-negotiable criteria that any solution must meet:

• It must either run on existing cloud infrastructure, or run completely client-side. I do not want to be in the business of running a server, or of worrying about mitigating DOS attacks (by which I mean students accidentally running infinite loops generating infinite amounts of memory). I also do not want to be in the business of managing accounts and logins.

• Students should not have to install anything. As I mentioned before, this is a nonstarter for some students.

• It should allow students to work at a Disco REPL, and also test their own Disco code.

Potential solutions

• Of course, a really nice solution would be to find an existing site which replicates some of the functionality we used to have with Replit and supports educational uses. I kind of doubt such a thing exists, though I would be happy to be wrong about this.

• Another possibility is to have students use VSCode in their browsers via GitHub; to make this work we would have to add LSP support to Disco and package it up appropriately. I’m not really excited about using GitHub for this, although implementing LSP for Disco is independently a great idea.

• The other solution I can think of is to compile Disco to WASM, and build a web UI on top of it, so that the whole thing runs in the student’s browser. I spent some time on this last year and successfully got Disco to compile to WASM, but didn’t make it any farther than that. Honestly, I simply don’t know anything about web development, and I am not very motivated to learn.

UI levels

Running with the last idea above (WASM + a custom web UI), what could such a thing look like? What features would we want? Here are some different solutions I could imagine, in increasing order of effort.

• Level 0 would be to have just a web-based REPL. Students can edit Disco code on their own device, upload it to the website, then evaluate expressions at the REPL. Having to reupload their code every time they make changes would be annoying, but this would still be better than nothing.

• Level 1 would be to have a web-based editor and REPL. Students can type code in the editor, then push a button to load it into the REPL and play with it.

• Level 2 would be to have a web-based filesystem + editor + REPL. Students can see a list of multiple files, edit them individually, and load any of them into the REPL. The filesystem must live on their own computer, not on a remote server; but it could be their real filesystem, or a virtual filesystem.

I don’t know how difficult any of these are; I would assume that at least some of them can be achieved by gluing together some existing Javascript components (e.g. CodeMirror?). I’m sure it will require extending Disco itself with some new APIs, but I can definitely work on that part once I know what would be needed.

If you know how to build web UIs like this and are interested in helping, get in touch!