We present a file server/OS where threading and exceptions are all realized via delimited continuations. We use zipper to navigate within any term. If the term in question is a finite map whose keys are strings and values are either strings or other finite maps, the zipper-based file system looks almost the same as the Unix file system. Unlike the latter, however, we offer: transactional semantics; undo of any file and directory operation; snapshots; statically guaranteed the strongest, repeatable read, isolation mode for clients; built-in traversal facility; and just the right behavior for cyclic directory references.
We can easily change our file server to support NFS or 9P or other distributed file system protocol. We can traverse richer terms than mere finite maps with string keys. In particular, we can use lambda-terms as our file system: one can cd into a lambda-term in bash.
The implementation of ZFS has no unsafe operations, no GHC let alone Unix threads, and no concurrency problems. Our threads cannot even do IO and cannot mutate any global state --- and the type system sees to it. We thus demonstrate how delimited continuations let us statically isolate effects even if the whole program eventually runs in an IO monad.
zfs-talk: Expanded talk with the demo. It was originally presented as an extra demo at the Haskell Workshop 2005
ZFS.tar.gz: The complete source code, with comments. It was tested with GHC 6.4 on FreeBSD and Linux
From: Zipper-based file server/OS
Neat, a referentially transparent filesystem with transactional semantics in 540 lines of Haskell. I can think of some interesting uses for this.