Dylan complained about the lack of testability with file system APIs, which reminded me of a neat looking library I stumbled across while looking at build systems: EntityFS. Among a bunch of other useful sounding things it provides:

File system APIs are backend-independent. File systems can be created in RAM memory, on Zip files, on file system directories, etcetera Sadly it’s under the LGPL which makes it difficult if you happen to sell to big enterprises that are still somewhat scared of GPL type licenses, but they’re nowhere near as common as they used to be. I should note though that Holocene Software are offering commercial licenses for an undisclosed price.