There's a difference between "an operating system as a platform for developing applications" and "an operating system as a system-level REPL". As a platform fo

There's a difference between "an operating system as a platform for developing applications" and "an operating system as a system-level REPL". As a platform for developing apps: not sure about plan9. As a hypothetically richer system-level REPL: having a larger subset of "everything" accessible through a filesystem-like interface allowed Plan9 to offer at least the possibility of more possibilities here. Fuse and the contributed filesystems are pretty close in spirit I think: eg, the grabfs filesystem for macfuse (which gives you access to the per-window images via a file system interface).