ZipperFS
Haskell is the coolest bloody thing in programming EVAR.
Check this: a superficially Unix-like filesystem with amazing capabilities, and perfect concurrency behavior. No threads required. In 540 lines of Haskell.
... 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.
(Emphasis mine)
Somebody turn this into a FUSE module already!
(Hat Tip: Lambda the Ultimate)