The Unison platform

Unison is a new programming platform, currently under active development. This repo contains the code for the Unison node backend (written in Haskell, lives in the node directory, with source in src), and the Unison editor (currently written in Elm, found in the folder editor-elm).

If you're wondering what the project is about, you can get a glimpse with this video and post discussing the Unison semantic editor. The editor is just one piece of the overall platform, and there's updates and more info at unisonweb.org as well as background posts with additional context.

Since Unison isn't terribly useful in its current form, the rest of this README will focus on stuff that will be of interest for potential contributors, namely, how to build the code, and a brief tour of the (fairly small but action-packed) codebase. If you're just interested in the project and want to follow along with the progress, unisonweb.org is the place to go, or you can also say hello or lurk in the chat room.

Still here? All right then! Let's get to it.

A brief code tour

First, a bit of orientation. Here's the directory structure:

shared/
node/
editor/
editor-elm/

The editor-elm/ directory is the current Elm implementation of the Unison editor. It's being phased out, in favor of a Unison editor written in Haskell and compiled to Javascript via GHCJS. Thus, we can share code between the editor and Unison node backend. So what's with the directory structure? . The shared/ directory has Haskell code that will be shared between the editor and node, the node/ directory has code specific to the node, and editor/ is the currently-in-development Haskell version of the editor, which also depends on shared/.

The dependencies are what you'd expect---shared/ has minimal external dependencies, and node/ and editor/ depend on shared. Thus, it should be very obvious and explicit what code and external dependencies are going to be compiled to JS.

Build intructions

Building the project is easy. You do not need to have anything installed or set up in advance. Here are complete instructions:

$ git clone https://github.com/unisonweb/platform.git unisonweb
$ cd unisonweb
$ # If you don't have Nix 1.8 or later (see note below)
$ ./install-nix.sh

After install completes, you'll see instructions about starting a fresh terminal session or sourcing the given command. Do that, then:

$ cd node
$ ./shell.sh

Each subproject has a shell.sh file to download and build any needed dependencies, and drop you into a shell where you can use cabal as normal. So there's node/shell.nix, shared/shell.nix, and editor/shell.nix. You'll see lots of output the first time you run one of these, as they use Nix to download and build dependencies, and you may see some warnings about Haddock documentation that you can ignore. Subsequent launches will be snappy since you'll have all the dependencies in your Nix store.

Once that's done, you'll be in a Nix shell and can use cabal as normal:

$ cabal run node # cabal repl, cabal test also work
Running node...
Setting phasers to stun... (port 8080) (ctrl-c to quit)

That last line is a message from Scotty telling you that the node HTTP server is running.

A brief code tour of the Haskell code

The Unison Haskell code, which has the language, its typechecker, and the node implementation, is split between shared/ and node/. It's not actually much code right now, only about 3k lines!

Obviously, this number is going to go up over time, but right now, it's pretty bite-sized and (hopefully) easy enough to follow. Certainly not of the scale of something like GHC, which clocks in at over 135k LOC!

One brief note for orientation. Because of the split between shared/ and node/, a module like Unison.Term (in shared/), which has the basic type and instances for JSON encoding/decoding, has a counterpart in shared/, Unison.Term.Extra with things like binary serialization or hashing of Unison.Term values. Logically, it would be nice to put all functionality in the Unison.Term module, but binary serialization and hashing code isn't needed by the editor and we don't want to accidentally compile those libraries and code to JS or rely on tree-shaking to hopefully trim it out. Other .Extra modules are analogous.

Now, where to begin? Everyone learns differently. You might prefer to read the code 'inside out' (or perhaps 'bottom up'), starting from the core language syntax tree and typechecker, then expanding out to where these get exposed to the outside world. If this route sounds appealing, here's a reasonable path:

• Unison.Term in shared/ is the module containing the definition for Unison language terms and Unison.Type is the module containing the definition for Unison language types. Eventually, we'll add Unison.TypeDeclaration.
• In both Term and Type, the same pattern is used. Each defines a 'base functor' type, F a, which is nonrecursive, and the actual thing we use is an abstract binding tree over this base functor, an ABT F. ABT (for 'abstract binding tree') is defined in Unison.ABT. If you aren't familiar with abstract binding trees, here is a nice blog post explaining one formulation of the idea, which inspired the Unison.ABT module. A lot of operations on terms and types just delegate to generic ABT operations. Also see Unison.ABT.Extra.
• The main interface to the typechecker is in node/ in Unison.Typechecker, and the implementation is in Unison.Typechecker.Context. There isn't a lot of code here (about 500 LOC presently), since the typechecking algorithm is pretty simple. Unlike a unification-based typechecker, where the typechecking state is an unordered bag of unification constraints and higher-rank polymorphism is usually bolted on awkwardly later, Dunfield and Krishnaswami's algorithm keeps the typechecking state as a nicely tidy ordered context, represented as a regular list manipulated in a stack-like fashion, and the algorithm handles higher-rank polymorphism very cleanly. They've also extended this work to include features like GADTs, though this new algorithm hasn't been incorporated into Unison yet.
• From here, you can move to Unison.Node, which defines the interface satisfied by the node, Unison.Node.Implementation, containing a simple implementation of that interface, and Unison.NodeServer, which just wraps the node API in an HTTP+JSON interface.
• Lastly, node/src/Node.hs has the code which creates an instance of a Unison.NodeServer. The src/node/Node.hs file also has the definition of the current Unison 'standard library'. The node logic is agnostic to the "standard library" chosen, so whatever creates an instance of Unison.Node has to supply it with the standard library it should use.

If instead, you'd rather work from the 'outside in' (or perhaps 'top down'), you could start with Unison.NodeServer and work your way back the other direction to modules like Term, Type, and ABT. Since the entire point of the node codebase is to expose an API over HTTP, Unison.NodeServer will end up referencing directly or indirectly all the code in the node, and all the Unison language and typechecker.

That's all for now!