References: CLtL, pages 331, 386
Edit history: Version 1, 25-Mar-88 JonL
Version 2, 29-Mar-88 JonL (fix typos; comments by Daniels)
Version 3, 23-May-88 JonL (fix nits raised by Masinter)
Version 4, 23-May-88 JonL (change issue name -- only 1 proposal)
Version 5, 7-Jun-88 Masinter (more nits)
CLtL p386 says that FORMATting to a fill-pointer'd string should add
characters "as if by use of VECTOR-PUSH-EXTEND"; but CLtL p331 says that
WITH-OUTPUT-TO-STRING will work "as if using VECTOR-PUSH-EXTEND if the
string is adjustable, and otherwise as if using VECTOR-PUSH". It's very
unlikely that the original authors of these parts intended differing
semantics for these two cases. Furthermore, the semantics for
WITH-OUTPUT-TO-STRING permit the inconspicuous loss of characters
written to the string, since VECTOR-PUSH will just "drop on the floor"
any characters that would go beyond the end.
Change the documentation of WITH-OUTPUT-TO-STRING to be like that under
FORMAT. That is, replace the first sentence of the next-to-last paragraph
on CLtL p331 by:
"If *string* is specified, it must be a string with a fill pointer;
the output is incrementally appended to the string (as if by use of
(let ((str (make-array 4 :element-type 'string-char :fill-pointer 0)))
(with-output-to-string (s str) (princ "You Luz, Bunkie!" s))
CLtL behaviour will return "You "; proposed behaviour will signal an error.
It's unlikely that the mention of VECTOR-PUSH in CLtL p331 was intended
to suggest that characters could be quietly "dropped on the floor". In
any case, there is no practical or theoretical reason to make FORMAT and
WITH-OUTPUT-TO-STRING act differently on non-adjustable strings.
VaxLisp 2.2 and Lucid 3.0 implement the proposal; Lucid 2.1 and earlier
versions implement CLtL. For WITH-OUTPUT-TO-STRING, Xerox Common Lisp
implements CLtL. Symbolics Genera 7.2 implements the proposal.
Cost to Implementors:
Cost to Users:
Less special-casing in the semantics of "printing" to strings.
More conformity with naive expectations about printing to strings.
Implementations may want to actually call VECTOR-PUSH, rather than
VECTOR-PUSH-EXTEND, on non-adjustable string in order to test the
result -- nil means an overflow of the total length of the string;
thus they may signal an error more directly related to the problem,
rather than permitting VECTOR-PUSH-EXTEND to complain about a non-
adjustable array. But either way, the semantics is still that of
VECTOR-PUSH-EXTEND: when you get to the end of the string, adjustable
strings are extended, and non-adjustable strings cause error signals.
It's perfectly acceptable to use VECTOR-PUSH-EXTEND with a non-adjustable
array. It's the error-signalling property of VECTOR-PUSH-EXTEND, as opposed
to the "dropping on the floor" of VECTOR-PUSH, that motivated this proposal.