Issue CLOSE-CONSTRUCTED-STREAM Writeup

Status:	DRAFT

Forum:	Cleanup

Issue:         CLOSE-CONSTRUCTED-STREAM


References:    Close (CLtL p. 332), WITH-OPEN-STREAM 

	(CLtL p 330), make-synonym-stream, make-broadcast-stream,

	make-concatenated-stream, make-two-way-stream,

	make-echo-stream, make-string-input-stream,

	make-string-output-stream, with-input-from-string,

	with-output-from-string


Related issues: CLOSED-STREAM-OPERATIONS


Category:      Clarification/Change


Edit history:  Masinter,  6-Jan-89, Version 1

               Masinter, 12-Jan-89, Version 2


Problem description:


First some terminology:


A "composite" stream is one created with MAKE-SYNONYM-STREAM, 

MAKE-BROADCAST-STREAM, MAKE-CONCATENATED-STREAM, MAKE-TWO-WAY-STREAM, 

MAKE-ECHO-STREAM. 


The "constituents" of a composite stream are the streams it points to:

the SYMBOL-VALUE of the symbol given to MAKE-SYNONYM-STREAM

the streams given to MAKE-BROADCAST-STREAM or MAKE-CONCATENATED-STREAM

the input-stream and output-stream given to MAKE-TWO-WAY-STREAM or MAKE-ECHO-STREAM.


A "constructed" stream is either a composite stream or one created with 

MAKE-STRING-INPUT-STREAM, MAKE-STRING-OUTPUT-STREAM, WITH-INPUT-FROM-STRING,

 WITH-OUTPUT-FROM-STRING.


The function "CLOSE" is currently described in 21.3, which starts "This

section contains discussion of only those operations that are common to all

streams."  This would seem to imply that they apply to constructed streams. 


The definition of CLOSE "The argument must be a stream. No further input/output

 operations may be performed on it. ... "  It further states "... and it is 

permissible to close an already closed stream."


However, the behavior on the constructed streams is not well defined, and

implementations (apparently) differ.


Proposal (CLOSE-CONSTRUCTED-STREAM:IS-ERROR): 


It "is an error" to call CLOSE on a constructed stream. CLOSE might signal an

error, or the result of CLOSE could cause the constituent streams to be closed,

etc, but the effect is implementation-dependent.


Proposal: (CLOSE-CONSTRUCTED-STREAM:SIGNALS-ERROR)


Calling CLOSE on a constructed stream signals an error.


Proposal (CLOSE-CONSTRUCTED-STREAM:ARGUMENT-STREAM-ONLY):


The effect of CLOSE on a constructed stream is to close the "argument" stream

only. No i/o operations are permitted after the call to CLOSE on the stream

given to CLOSE; There is no effect on the constituents of composite streams.


For streams created with MAKE-STRING-OUTPUT-STREAM, the result of

GET-OUTPUT-STREAM-STRING is unspecified after CLOS. For composite streams,

the call to CLOSE has no effect on any of the constituent streams.


The "links" to the constituents may be broken; if the proposal in STREAM-ACCESS 

passes, the results of the accessor functions there are unspecified after the

call to CLOSE.)


Proposal: (CLOSE-CONSTRUCTED-STREAM:CLOSE-CONSTITUENTS)


CLOSE first closes its argument; it then operates recursively on the constituents

of composite streams.


Examples:


>>not written; sorry<<


Rationale:


Specifying seems better than not saying what happens, even if it is

"implementation-dependent".


Current practice:


Implementations seem to vary widely.


Cost to Implementors:


Varying, depending on the current level of conformance. Making the changes

themselves is probably trivial (to the "close" method for each kind of

constructed stream type) but it is possible that system code might depend

on the behavior.


Cost to Users:


Likely small; we've not found any good uses for CLOSE on composite streams.


Cost of non-adoption:


Continued minor ambiguity


Performance impact:


near zero


Benefits:


The language would be more well specified.


Esthetics:


Well-specified languages are usually easier to deal with.


Discussion:


Signalling an error is reasonable if no Common Lisp program ever needs to

call CLOSE on a composite stream. We could not come up with a legitimate

case where you wouldn't instead close the underlying stream if that's what

you wanted.


Allowing the result to be implementation dependent is the "status quo". 


ARGUMENT-STREAM-ONLY is probably the "safest" in that it makes it

harder to accidentally close a stream that wasn't intended. It

seems counterintuitive and yet it probably wouldn't be harmful

if it were well-defined that this was what it did.


CLOSE-CONSTITUENTS could be an incompatible change for some

implementations. It makes more sense for things like broadcast streams

(which are usually non-interactive) than it does for echo streams

(which are sometimes interactive).