Issue WITH-OUTPUT-TO-STRING-APPEND-STYLE Writeup

Issue:         WITH-OUTPUT-TO-STRING-APPEND-STYLE


References:    CLtL, pages 331, 386


Category:      CLARIFICATION


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)


Problem description:


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.



Proposal (WITH-OUTPUT-TO-STRING-APPEND-STYLE:VECTOR-PUSH-EXTEND):


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

   VECTOR-PUSH-EXTEND)."



Test Case: 

    (let ((str (make-array 4 :element-type 'string-char :fill-pointer 0)))

      (with-output-to-string (s str) (princ "You Luz, Bunkie!" s))

      str)

CLtL behaviour will return "You "; proposed behaviour will signal an error.



Rationale:


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.



Current Practice:


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:


Very small.


Cost to Users:


Virtually none.



Benefits:


Less special-casing in the semantics of "printing" to strings.

More conformity with naive expectations about printing to strings.



Aesthetics:


Minor impact.


Discussion:


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.