So the library deciding how to load and possibly when to evict sounds like it was kind of the point. Simulating what might be the job of mmap() on a modern system.
Resources also provided the programmer with a way of working with the data in a structured way. Unix is of course remarkably primitive here, you'll get bytes and like it.
It's always a little funny to see the scorn Unix minimalists have for the old Mac OS. It seems very hard for some people to understand that rather than technical purity or whatever, the Mac developers were working from the interface on in, and the interface was meant to make the computer approachable for people who'd never touched one before and weren't interested in computers for their own sake.
The article hints at how files work. On MFS you would refer to a file as a (volume ID, filename) pair. On HFS, you would refer to a file as a (volume ID, directory ID, filename) triplet. A bunch of toolbox calls (syscalls) got duplicate versions for HFS—but if you were working with legacy code, you could create a fake volume ID called a “working directory” that could be used as if it were a volume ID in a (volume ID, filename) pair. These “working directories” are just awful. The working directory table is global to the entire system and they are not reference-counted—if you open the same working directory twice, you get the same ID both times, and you only have to close it once.
As a funny note—the original filesystem, MFS, had a maximum file name length of 255 characters. HFS, its successor, had a 31-character limit when it first appeared. As a compromise, the new FSSpec file APIs that appeared in System 7 used a 63-character limit filename, because 63 characters was the maximum filename length that the Finder supported.
You could create files with names longer than 63 characters on MFS volumes, it’s just that if you browsed these volumes in the Finder, the Finder would crash!
I’ve got a blog post in the work that is going through some of the wackiness I’ve seen in the old Mac OS filesystem API, and I’ve recently been on the RetroDev Discord helping some people write file handling code for classic Mac OS programs.
I remember putting FKEY resources into the System resource file so the user could type Shift-Command-<digit> anywhere to automate some common task.
