[Unison](http://unisonweb.org) is a new programming language, currently under active development. It's a modern, statically-typed purely functional language, similar to Haskell, but with a unique ability to describe entire distributed systems with a single program. Here's a simple example:
* At each transfer, any required definitions (such as `foo` and `x`) will be dynamically deployed to the `alice` node and cached for future use.
* The `Remote.transfer bob` transfers control of the rest of the computation to the `bob` node.
* The `bar x y` computation happens on the `bob` node. Again, any needed definitions (`bar`, `x`, and `y`) will be dynamically deployed to the `bob` node.
This dynamic transfer / deployment of arbitrary computations is possible because definitions in Unison are identified by a cryptographic hash of their content, _including the hashes of all dependencies_ (the hash is also "nameless" as it isn't affected by naming of variables). To transfer a computation, we send it to the recipient, and the recipient checks to see if the computation references any unknown hashes. Any unknown hashes are synced to the recipient before the transfer completes and the computation proceeds.
If you'd like to learn more about the project, the talk [How to write a search engine in 15 lines of code](http://unisonweb.org/2016-10-12/search.html) has more of an introduction to the language. For a more complete overview of the syntax look at the [unison language reference](/unison-src/tests/language-reference.u).
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](http://unisonweb.org) is the place to go, or you can also say hello or lurk [in the chat room](https://gitter.im/unisonweb/unison).
*`yaks/` has subprojects for various utilties not specific to Unison (the result of ["yak-shaving"](https://en.wiktionary.org/wiki/yak_shaving)). Once mature, each of these might be moved to independent projects and published on Hackage.
The build uses [Stack](http://docs.haskellstack.org/). If you don't already have it installed, [follow the install instructions](http://docs.haskellstack.org/en/stable/README.html#how-to-install) for your platform. (Hint: `brew update && brew install stack`)
*`Unison.Term` and `Unison.Type` have the syntax trees for terms and types. 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](http://semantic-domain.blogspot.com/2015/03/abstract-binding-trees.html), which inspired the `Unison.ABT` module. A lot of operations on terms and types just delegate to generic `ABT` operations.
*`Unison.Parsers` has the main entry point for the parser.
*`Unison.Typechecker.Context` is the implementation of the typechecker, and `Unison.Typechecker` has the "public interface to the typechecker" and some convenience functions. There isn't a lot of code here (about 700 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](http://www.mpi-sws.org/~neelk/bidir.pdf) 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](http://semantic-domain.blogspot.com/2015/03/new-draft-sound-and-complete.html), though this new algorithm hasn't been incorporated into Unison yet.
