Issue: FORMAT-COLON-UPARROW-SCOPE (version 3)

[Masinter.pa@Xerox.COM]

This apparently did not get mailed before. 

This issue is too simple to present two proposals. I've tried to represent
TEST-FOR-REMAINING-SUBLISTS as the single proposal and eliminate
TEST-FOR-REMAINING-ARGUMENTS. 


!
Issue: FORMAT-COLON-UPARROW-SCOPE

References:    CLtL p. 406 and also p. 403

Category:      CLARIFICATION

Edit history:  version 1: Guy Steele, 30 November 1987
	       version 2: Guy Steele, 18 January 1988
	       version 3: Masinter,  5 February 1988

Problem description:

Implementations currently differ on the question of what is tested by the FORMAT
command "~:â??".  Some implementations test to see whether any arguments remain in
the sublist for the current iteration step; others test to see whether any
sublists remain.  The text on page 406 is not clear on this point.

Proposal (FORMAT-COLON-UPARROW-SCOPE:TEST-FOR-REMAINING-SUBLISTS):
~:â?? may be used only if the command it would terminate is ~:{ or ~:@{. The
entire iteration process is terminated if and only if the sublist that is
supplying the arguments for the current iteration step is the last sublist (in
the case of ~:{) or the last FORMAT argument (~:@{). Note that ~:â?? is *not*
equivalent to ~:#â??; the latter terminates the entire iteration if and only if no
arguments remain for the current iteration step.

Test Cases/Examples:

(format nil "~:{~@?~:â??...~}" '(("a") ("b")))

Under this proposal, this yields "a...b", rather than "a".

Rationale:

This proposal is desirable because otherwise there is no way to test whether any
sublists remain. The text on page 406 may be construed to hint at this proposal
indirectly.  To quote Nick Gall:

   If one thinks about the intent of the parenthetical

      (because in the standard case it tests for remaining arguments of
      the current step only)

   one should agree that "a...b" will be returned.  In referring to ~â?? as
   the "standard case", which tests the arguments remaining in the
   current argument sublist, this parenthetical implies that there is
   an `other case', which tests `something else.'  The only `other case'
   discussed is ~:â??, which therefore must test `something else.'  I claim
   that the parentheical makes no sense if we interpret ~:â?? as testing
   the same condition as ~â??.  If they both test the same condition, why
   have the parenthetical explanation?

   If ~:â?? doesn't test the same condition as ~â??, then what does it test?
   I claim that the only test that makes sense is for ~:â?? to test the
   only thing that affects the "entire iteration process:" the number of
   sublists.  When there are no more sublists, "the entire iteration
   process" is terminated.

Current practice:

Some implementations already have the proposed behavior, include Symbolics
Common Lisp and TI Lisp.

Many other implementations currently have a different interpretation: the test
case returns "a", since ~:â?? in those implementations test for the remaining
arguments rather than remaining sublists. These currently include  Kyoto Common
Lisp, Allegro Common Lisp, GCLISP, Xerox Common Lisp, Spice Lisp, and VAXLISP.

Cost to Implementors:

Many implementations will have to make a small change, probably a one-liner.


Cost to Users:

It is unlikely that much user code depends on the behavior of testing for
remaining arguments, but it is possible.  The author of this writeup (Steele)
judges it somewhat more likely that user code might depend on the behavior of
testing for remaining sublists..


Cost of non-adoption:

Users would have to be warned not to use ~:â?? in code that is meant to be
portable.


Benefits:

Elimination of yet one more ambiguity.

The proposed semantics allows greater semantic power (there are more things one
can test).

Esthetics:

``Absolutely none.  We're talking about FORMAT here.'' -- Guy L. Steele Jr.

Discussion:

Guy Steele very strongly prefers the interpretation
FORMAT-COLON-UPARROW-SCOPE:TEST-FOR-REMAINING-SUBLISTS.

David Moon, Kent Pitman, Pavel Curtis, Dan Pierson, Rob Poor, Scott Fahlman and
Nick Gall
favor FORMAT-COLON-UPARROW-SCOPE:TEST-FOR-REMAINING-SUBLISTS.

Kevin Layer and Rich Robbins have spoken in favor f an alternative proposal, to
test for the remaining arguments.

Historical note: Steele first implemented this "feature", in Zetalisp, and so
the code in Symbolics Common Lisp is likely a direct descendant of the original
code.  This might cause some to give weight to Steele's opinion. There are two
arguments against such credence.  First, there is no reason why the original
code should be regarded as part of the specification of Common Lisp any more
than any other implementation; plainly, Steele botched the specification when he
wrote the book.  Second, a professor of literature (I believe) once told Isaac
Asimov concerning a short story of his (I quote from memory): "Tell me, Dr.
Asimov, just because you wrote the story, what makes you think you know what it
means?"