References: Close (CLtL p. 332), WITH-OPEN-STREAM
(CLtL p 330), make-synonym-stream, make-broadcast-stream,
Related issues: CLOSED-STREAM-OPERATIONS
Edit history: Masinter, 6-Jan-89, Version 1
Masinter, 12-Jan-89, Version 2
First some terminology:
A "composite" stream is one created with MAKE-SYNONYM-STREAM,
MAKE-BROADCAST-STREAM, MAKE-CONCATENATED-STREAM, MAKE-TWO-WAY-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,
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.
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.
Calling CLOSE on a constructed stream signals an error.
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.)
CLOSE first closes its argument; it then operates recursively on the constituents
of composite streams.
>>not written; sorry<<
Specifying seems better than not saying what happens, even if it is
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
The language would be more well specified.
Well-specified languages are usually easier to deal with.
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
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).