Skip to content
This repository

HTTPS clone URL

Subversion checkout URL

You can clone with HTTPS or Subversion.

Download ZIP

Worlds in Clojure (support for scoped side-effects and speculative computations)

tree: f7bb9351eb

Fetching latest commit…

Cannot retrieve the latest commit at this time

README.md

clj-worlds is an implementation of Alex Warth's "worlds" in Clojure. The idea is explained nicely in this paper.

Worlds support scoped side-effects. In Clojure, they can be conveniently modelled as a special kind of ref, called a "world-ref" or w-ref for short.

A w-ref always has a value that is relative to the current world. Worlds form a single-rooted hierarchy, and a world can "commit" all of its changes to its parent world.

The API of clj-worlds is heavily inspired by Clojure's own ref API. Like refs, w-refs can be created (w-ref), dereferenced (w-deref), set (w-ref-set) and altered (w-alter).

Example

(let [w (sprout (this-world)) ; w is a child of the top-level world
      r (w-ref 0)]   ; r is a world-ref
  (w-deref r)        ; in top-level world, r is 0
  (in-world w        ; in world w...
    (w-deref r)      ; ... r is also 0
    (w-ref-set r 1)) ; ... we set r to 1
  (w-deref r)        ; back in top-level world, r is still 0!
  (commit w)         ; commit all of w's changes to its parent world
  (w-deref r))       ; now, at top-level, r is 1

Comparison with refs

What is the difference with Clojure's refs and STM, you ask? Like transactions, worlds isolate and group side-effects. Unlike transactions:

  • Worlds are first-class entities: worlds can be created, passed around, and "opened" and "closed" at will. Committing a world's changes is an explicit operation and is not tied to the control flow of a block.

  • Worlds are not a concurrency control mechanism. World commits are not atomic, and while individual world operations are thread-safe, worlds do not support multiple atomic updates and thread isolation.

Next Steps

  1. Integrate w-refs with regular refs. Make commit atomic.
  2. Experiment with stronger consistency checks. Using the default semantics of worlds, worlds can see inconsistent world-lines when other worlds commit to their parent. A possible fix is to introduce snapshot isolation, as for example employed by MVCC (which also happens to lie at the basis of Clojure's STM).
Something went wrong with that request. Please try again.