Re: master 1e3b0f2: Improve doc strings of project.el

[Dmitry Gutov]

On 18.07.2020 13:10, Eli Zaretskii wrote:
> > Date: Sat, 18 Jul 2020 11:05:46 +0200
> > From: tomas@tuxteam.de
> > Cc: emacs-devel@gnu.org
> >
> > > I could be wrong in some details here, but a file descriptor seems
> > > like a prime example of an abstraction.
> >
> > Well, that's at the Mother of Abstraction, aka the user space/kernel
> > space barrier :-)
>
> IMO, there's nothing opaque about a file descriptor.  It is just a
> number.  The abstraction here isn't in the descriptor itself, it's in
> what it _represents_.

Potato, pot-ah-to.

> The descriptor is actually a _handle_ for an
> object that itself is not exposed at all; you can only manipulate it
> via the handle.

So it's actually even more opaque.

> > Yes, this is some kind of OO. But it leaks a couple of things:
> > first, it's a "small number" (i.e. not some random 64 bit
> > address, but the pattern of counting up from zero and of building
> > bitmaps of FDs is considered --mostly-- viable), i.e. the mental
> > model around is that the kernel "has" a table somewhere indexed
> > by FD. This detail wasn't probably intended to leak in the first
> > place, but was just "the obvious thing to do" in a world in which
> > machines with a 32 bit address space were considered serious iron.
>
> That detail is not a general trait of a handle.  Another popular
> example of a handle is a FILE pointer in C, where the details of the
> object it points to are generally not public.  Here, the numerical
> value of the handle is no longer small.

A FILE pointer is less interesting, given that the structure on the 
other end is usually more or less the same. Even if it's concealed.

> Yet another similar abstraction, much more close to our domain, is the
> Lisp_Object representation on the C level: with a couple of
> exceptions, each object is actually a handle of a C struct;

Exactly.

"There's nothing opaque about a project instance.  It is just a 
Lisp_Object."

> P.S. The interpretation of a file descriptor as an index into some
> small table doesn't hold in modern OSes anyway.  It was true for
> MS-DOS, but modern OSes have moved away of that.

Meaning that it's an abstraction that stood the test of time.