I no longer remember why we designed this whole synonym stream thing.
(Where by "we" I really mean Dan Weinreb and Dave Moon, I was mostly
just in the room when they did this.) I _think_ it is mimicking
something that the Multics I/O system does.

I'm not aware of any modern system where I/O has this feature. It is
true that modern streams are often wrapped around other streams to
compose features (a text string may wrap a binary stream which may in
turn wrap an unbuffered stream, etc.), but the idea of one stream
wrapping another as an _indirection_ mechanism, so that you can
dynamically replace the wrapped stream with a different one, seems
unique to Common Lisp.

I'm not sure what problem it solves, since I don't remember ever wanting
to use it in Common Lisp (or wishing I had something like it when
programming in Java, Python, or C). Buy maybe I'm forgetting something.

I would be interested to know of any place that _does_ use synonym
streams to solve some real problem.

- Alan

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

steve <sgonedes1977@gmail.com> writes:

Alan Bawden <alan@csail.mit.edu> writes:

> I no longer remember why we designed this whole synonym stream thing.

[ ... ]

bivalent streams are useful.

"In addition, SBCL supports various extensions of ANSI Common Lisp
streams:

*Bivalent Streams*
A type of stream that can read and write both ‘character’ and
‘(unsigned-byte 8)’ values.

I don't see any connection with synonym streams here.

> I'm not aware of any modern system where I/O has this feature.

POSIX man dup

This does not make a synonym in the sense that Common Lisp is using the
word. If it made a synonym (in the Common Lisp sense) then after:

old = open("old_file", O_RDONLY);
new = open("new_file", O_RDONLY);
fd = dup(old);
dup2(new, old);

it would be the case that reading from FD would read from "new_file".
But if you try it, you will find that FD is still reading from "old_file".

> I would be interested to know of any place that _does_ use synonym

I use the following for type 1 font dissasembler. I do not have the code
right now, but bivalent streams make it much easier to
read-char/read-byte.

The code you included isn't using synonym streams.

I think you haven't fully internalized what a weird feature synonym
streams really are. Its easy to breeze through the documentation and
think that a synonym stream is just a way to duplicate a stream. But
it's more than that: The new stream follows the _current_ value of a
dynamic variable.

- Alan

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Sun, 14 Apr 2024 11:36:11 -0400, steve wrote:

*Bivalent Streams*
A type of stream that can read and write both ‘character’ and
‘(unsigned-byte 8)’ values.

How is that supposed to work, exactly? Is each stream item tagged in some
way to identify it as a “character” versus an “unsigned-byte 8”?

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)