As additional support for this thesis, the interface to the logical (sometimes physical) file system inside many database kernels is almost exactly this. All fo

As additional support for this thesis, the interface to the logical (sometimes physical) file system inside many database kernels is almost exactly this. All four file types have a concrete use case that requires their existence. One of the things that makes them easier to use in a database kernel context is that even though they are all built on top of the same low-level storage primitives, each file type usually has its own purpose-built API instead of a generic overloaded file API, which would get messy very quickly. A file's type never changes and the set of types is enumerable, so this is clean. The only thing keeping this from being a user space thing is that there is no practical way of making this POSIX compliant. POSIX has all but permanently frozen what is feasible in a file system. Many database kernels either work off raw block devices or emulated raw blocks device on top of a POSIX file system, on which they build a file system, but it is never exposed to the world which eliminates any requirement for compliance with external standards. Imagine how much easier it would be to build things like databases if this was a first-class API.