PEEK-CHAR-READ-CHAR-ECHOPEEK-CHAR, READ-CHAR and streams made by MAKE-ECHO-STREAM is not made adequately clear about how many times a particular character may be echoed and at what time such echo is permissible.
For example, given: (WITH-INPUT-FROM-STRING (STRING-STREAM "A") (LET ((*STANDARD-INPUT* (MAKE-ECHO-STREAM STRING-STREAM *STANDARD-OUTPUT*))) (LET ((CHAR NIL)) (PEEK-CHAR) (PRIN1 '---) (PEEK-CHAR) (PRIN1 '---) (SETQ CHAR (READ-CHAR)) (PRIN1 '---) (UNREAD-CHAR CHAR) (PRIN1 '---) (READ-CHAR)))) what is seen on the terminal? There are at least these possibilities:
[1] PEEK-CHAR is implemented by READ-CHAR/UNREAD-CHAR. The first time a char is seen by READ-CHAR it's echoed, UNREAD-CHAR does not echo, re-fetching the char by READ-CHAR doesn't echo. A------------
[2] Characters are echoed whenever seen by PEEK-CHAR or READ-CHAR. Characters are not unechoed by UNREAD-CHAR. A---A---A---A---
[3] Characters are not echoed by PEEK-CHAR but are echoed by READ-CHAR. No `unecho' action is done by UNREAD-CHAR. ------A------A
[4] PEEK-CHAR is implemented by READ-CHAR/UNREAD-CHAR. READ-CHAR echoes but UNREAD-CHAR does not `unecho'. A---A---A------A
[5] PEEK-CHAR is implemented by READ-CHAR/UNREAD-CHAR. READ-CHAR echoes but UNREAD-CHAR unechoes (a magic Erase character must be presupposed for display terminals, a file stream that can randomly access during output and/or back up must be presupposed for files, paper terminals just lose): A<Erase>---A<Erase>---A---<Erase>---A
[6] PEEK-CHAR is implemented by peeking and does not echo. The first time a char is seen by READ-CHAR it's echoed, UNREAD-CHAR does not echo, re-fetching the char by READ-CHAR doesn't echo: ------A------
This list is not believed to be exhaustive. It is only to illustrate of the variety of possible ways in which the current specification can be implemented without technically being in conflict with the written word of CLtL. Obviously not all of these interpretations are considered useful by all people, but usefulness has not been determined to be criterial in satisfying the specification.
The description of streams (p327-328) is also [probably deliberately] fuzzy on this issue as it relates to operating systems on which echoing is done by the operating system. That is, some systems are line-at-a-time and all READ-CHAR and PEEK-CHAR operations happen after issues of echo have long since been resolved by a system call that reads and echoes input a line at a time. Other systems are character-at-a-time and these issues hit home in a different way. It will probably be necessary to continue leaving things slightly unspecified in order to accomodate the native style of the variety of operating systems now trying to support Common Lisp, but we should be more up front about the game we are playing. (For example, code which must port between character-at-a-time and line-at-a-time systems must be more careful about whether it does newline-preceded or newline-terminated output than many CL programmers might realize given the current wording.) Additionally, though, we should be on the lookout for less ambitious goals involving only partial compatibility to improve the situation wherever we can find a way to.
Abstract functions READ-PRESERVING-WHITESPACE and READ-DELIMITED-LIST are implicitly affected by any decisions made on this issue since their descriptions involve the use of UNREAD-CHAR and PEEK-CHAR, respectively.
READ-CHAR to say that when the stream is an echo stream (a stream created by MAKE-ECHO-STREAM), the character will be echoed on the stream the first time those characters are seen. (Characters which are not echoed by READ-CHAR are those which were put there by UNREAD-CHAR and hence are assumed to have been echoed already by a previous call to READ-CHAR.)
Ammend the description of UNREAD-CHAR to say that when the stream is an echo stream (a stream created by MAKE-ECHO-STREAM), no attempt will be made to undo any echoing of the character which might already have been done on the stream. However, characters placed on the stream by UNREAD-CHAR will be marked in such as way as to inhibit later re-echo by READ-CHAR.
Ammend the description of PEEK-CHAR to say that when the stream is an echo stream (a stream created by MAKE-ECHO-STREAM), characters which are only peeked at are not echoed. Note however that in the case that the PEEK-TYPE argument is not NIL, the characters which are passed by PEEK-CHAR are treated as if by READ-CHAR, and so are echoed unless they have been marked otherwise by READ-CHAR.
Ammend the description of abstract input functions READ-PRESERVING-WHITESPACE and READ-DELIMITED-LIST to acknowledge that they are implicitly affected by these new echoing rules of READ-CHAR, UNREAD-CHAR, and PEEK-CHAR.
Note: This is consistent with behavior [6] in the problem description.
Clarify that the echo behavior of interactive streams such as *TERMINAL-IO* continues to be implementation dependent.
UNREAD-CHAR which might make it possible for programmers concerned about echo behavior not to have to shy away from UNREAD-CHAR. (It would probably also improve the behavior of READ-PRESERVING-WHITESPACE with regard to echoing, since its description mentions using UNREAD-CHAR.)
Correct echoing behavior is important to programs which do batch processing, parsing, etc. Allowing multiple or premature echoing is clearly unsatisfactory.
The code to implement the proposed change itself is probably fairly localized.
In some operating systems, there may be echoing constraints which are overlooked here.
In some cases, there may be second order effects in the system itself which would also require a somewhat less predictable amount of work to fix.
Most users probably naively expect (perhaps even without realizing it explicitly) that echoing will take care of itself. That is, they probably expect that echoing will occur at the time of the READ-CHAR and probably do not give a lot of thought to the effect of PEEK-CHAR. As such, FIRST-READ-CHAR probably best supports most of their naive intuitions.
MAKE-ECHO-STREAM would continue to be significantly hard to use portably.PEEK-CHAR-READ-CHAR-ECHO:FIRST-READ-CHAR because he feels it is more practically coherent. However, he says he has only mental exercises and no actual personal experience upon which to base that belief.
Version 1 of this proposal treated interactive streams on par with echo streams, but while people agreed that this issue is a severe portability problem, some considered that the treatment of interactive streams got involved in operating system issues that were beyond the scope of the standard, so that part of the text was removed.