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References: Standard streams (pp. 327-329)

Category: ADDITION

Edit history: Version 1 by Pierson 5-Jul-88, add redesign per Pitman

Version 2 by Moon, 10-May-89, remove controversial parts

Version 3 by Moon, 12-May-89, improve wording and examples

Problem description:

Programs cannot currently distinguish between interactive use and batch

(or background) use without using implementation-dependent extensions.

For example, there is currently no way to tell whether it is useful to

ask a question when an unexpected situation is encountered.

Note: earlier versions of this issue tried to solve another problem

as well. See discussion section.


Add the following function:


Returns T if the stream is interactive, otherwise NIL. Signals

an error of type TYPE-ERROR if the argument is not a stream.

The precise meaning of INTERACTIVE-STREAM-P is implementation

dependent, and may depend on the underlying operating system. However

the general intent is to distinguish between interactive and batch (or

background or command-file) operations.

Some examples of the things that might identify a stream as interactive


1. The stream is connected to a person (or equivalent) in

such a way that the program can prompt for information and

expect to receive different input depending on the prompt.

2. The program is expected to prompt for input and support

"normal input editing".

3. READ-CHAR might hang waiting for the user to type something

instead of quickly returning a character or EOF.

*TERMINAL-IO* might or might not be interactive.


(when (> measured limit)

(let ((error (round (* (- measured limit) 100)


(unless (if (interactive-stream-p *query-io*)

(yes-or-no-p "The frammis is out of tolerance by ~D%.~@

Is it safe to proceed? " error)

(< error 15)) ;15% is acceptable

(error "The frammis is out of tolerance by ~D%." error))))


INTERACTIVE-STREAM-P has been proposed several times and is clearly

needed by any program that alters its behavior depending on whether

it is interacting with a user or running in a "batch" mode.

Current practice:

Most implementations have this feature already, often under a

different name.

Cost to Implementors:

Implementations will have to support this new function. Correct support

will require some thought for each operating system supported.

Cost to Users:

None, this is an upward-compatible extension.

Cost of non-adoption:

Less featureful language.

Performance impact:



More featureful language.


More featureful language.


It's not possible to make a specific definition of "interactive" that

applies to all operating systems, we concluded from earlier discussion.

However, "everyone knows" appoximately what it means. Hence the idea

of describing it by example rather than defining it.

Note that some proposed features for telling whether two streams are

connected to the same source or sink of information have been removed

from this version of the proposal. These were the functions


and STREAM-DESTINATION-ID-LIST. These could be revived in another

proposal if desired, but Moon thought INTERACTIVE-STREAM-P was

important and didn't want it to be lost due to controversy over these

unrelated functions.

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