CLHS: Issue PEEK-CHAR-READ-CHAR-ECHO Writeup

Issue PEEK-CHAR-READ-CHAR-ECHO Writeup

Status: Passed, Jan 89 X3J13

Issue: PEEK-CHAR-READ-CHAR-ECHO

References: READ-CHAR (p379), UNREAD-CHAR (p379), PEEK-CHAR (p379),

MAKE-ECHO-STREAM (p330), Streams (p327-328),

READ-PRESERVING-WHITESPACE (p376),

READ-DELIMITED-LIST (p377)

Category: CLARIFICATION/CHANGE

Edit history: 06-Mar-87, Version 1 by Pitman

23-Jun-88, Version 2 by Pitman (remove interactive stuff)

8-Oct-88, Version 3 by Pitman & Masinter

Problem Description:

The interaction between PEEK-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 '---)

(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.

Proposal (PEEK-CHAR-READ-CHAR-ECHO:FIRST-READ-CHAR):

Ammend the description of 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.

Rationale:

Although this is not known to be in use in any particular system,

nothing prevents its use. It proposes a more rational interpretation

of the echoing behavior of 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.

Current Practice:

A wide variety of behaviors are in use.

Cost to Implementors:

Unknown.

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.

Cost to Users:

Any change is effectively upward compatible since the previous

behavior is so ill-specified.

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.

Cost of Non-Adoption:

The streams returned by MAKE-ECHO-STREAM would continue to be

significantly hard to use portably.

Benefits:

A number of applications involving of parsers, batch script

interpreters, and such would be possible to implement

straightforwardly and portably.

Aesthetics:

Discussion:

Pitman supports 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.