Related issues: WITH-OPEN-FILE-STREAM-EXTENT
Edit history: v1, 13 Feb 1991, Sandra Loosemore
v2, 11 Mar 1991, Sandra Loosemore
It isn't clear whether it is permissible for the variable bound to
a stream by WITH-OPEN-FILE, WITH-OPEN-STREAM, WITH-INPUT-FROM-STRING,
or WITH-OUTPUT-TO-STRING may be explicitly assigned to within the body.
The expansion of these macros must include some code to close the stream
after executing the body forms. Can this code refer to the stream
using the user-supplied variable, or must it use another "hidden"
variable in case the user-supplied variable has been assigned to in
the body forms?
Clarify that the consequences of altering the values of the variables
bound to streams by WITH-OPEN-STREAM, WITH-INPUT-FROM-STREAM,
WITH-OUTPUT-TO-STREAM, and WITH-OPEN-FILE (by using SETQ, for example)
are undefined. A Common Lisp compiler may choose to issue a warning
if such a variable appears in a SETQ.
This is consistent with what CLtL says about altering the value of
the counter variable in a DOTIMES form.
Lucid CL, Allegro CL, the Chestnut Lisp-to-C translator, and Symbolics
Common Lisp all implement WITH-OPEN-FILE in such a way that the
user-supplied variable is referenced to close the stream.
Cost to Implementors:
None, since this permits implementations to continue to use the
Cost to Users:
Probably none. Any user programs that depend on it working the other
way are already clearly not portable.
Cost of non-adoption:
The language specification will be vague. Users may have portability
Users will know what to expect and will avoid a portability pitfall.
Doesn't look too bad.
Kent Pitman notes:
It probably is worth mentioning that the most common abuse of this
situation I've seen is that sometimes users will create broadcast
streams, echo streams, etc., or even implementation-dependent
encapsulations of the indicated stream to the opened file and then set
it back to the indicated variable. (Whether the later call to CLOSE
is a good idea to do to the compound stream which results is something
that probably varies from implementation to implementation. This
proposal certainly makes it more clear that users who do this are
living on the edge.)
Anyway, I just wanted to emphasize that the only scenario was not
something bizarre like someone doing
(WITH-OPEN-FILE (TEMP ...)
(PRINT X STREAM)
(SETQ X (+ 7 2)) ...)