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

POSIX man dup

> 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.

;;
;;; Macros

(defmacro with-open-file-stream (direction type options &body body)
`(with-open-file
(,@options :direction ,direction :element-type ,type)
,@body))

(defmacro with-binary-input-stream (options &body forms)
`(with-open-file-stream :input '(unsigned-byte 8) ,options ,@forms))

(defmacro with-ascii-input-stream (options &body forms)
`(with-open-file-stream :input 'character ,options ,@forms))

(defmacro with-ascii-output-stream (options &body forms)
`(with-open-file-stream :output 'character ,options ,@forms))

(defmacro with-binary-output-stream (options &body forms)
`(with-open-file-stream :output '(unsigned-byte 8) ,options ,@forms))

> streams to solve some real problem.

i do not have real problems.

> - Alan

steve

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

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”?

Lawrence D'Oliveiro <ldo@nz.invalid> writes:

> 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”?

I don't have any code at the moment - you could use 'base-char which
includes

(with-open-file (out "/home/steve/file.txt" :direction :output :element-type 'base-char :if-exists :overwrite)
(write-string "this" out)
(terpri out)
(write out :stream out))
#<SB-SYS:FD-STREAM for "file /home/steve/file.txt" {1002E2A943}>
CL-USER>

File :
this
#<SB-SYS:FD-STREAM for "file /home/steve/file.txt" {1002E2A943}>

hope this helps...