Issue: APPEND-DOTTED (Version 5)

[Masinter.pa@Xerox.COM]

Sigh, making a status list, I come across little edits that are hard to avoid
making. This version incorporates a few typos and a report on Current Practice.
Please don't reply unless you have some objections, there'll be an opportunity
to say "OK" to the whole group.

!


Issue:        APPEND-DOTTED
References:   APPEND (p268)
Category:     CHANGE/CLARIFICATION
Edit history: 27-Jul-87, Version 1 by Pitman
              29-Oct-87, Version 2 by Pitman (loose ends)
              14-Nov-87, Version 3 by Masinter
              23-Nov-87, Version 4 by Masinter
              14-Jan-88, Version 5 by Masinter

Problem Description:

The description of APPEND on p268 is not adequately clear on the issue of what
happens if an argument to APPEND is a dotted list. The only case explicitly
mentioned is the last argument, viz:

"The last argument [to APPEND] actually need not be a list but may be any LISP
object, which becomes the tail end of the constructed list. For example, (append
'(a b c) 'd) => (a b c . d)."

While this specifies the behavior of APPEND when the last argument is not a
list, the behavior when any of the other arguments are not lists is not
specified.

Proposal (APPEND-DOTTED:REPLACE):

Define that the cdr of the last cons in any but the last argument given to
APPEND or NCONC is discarded (whether NIL or not) when preparing the list to be
returned.

In the degenerate case where there is no last cons (i.e., the argument is NIL)
in any but the last list argument, clarify that the entire argument is
effectively ignored. Point out that in this situation, if the last argument is a
non-list, the result of APPEND or NCONC can be a non-list.

Remove any text which suggests that (APPEND x '()) and (COPY-LIST x) are the
same, since these two might legitimately differ in situations involving dotted
lists. As such, deciding which to use is not just a stylistic issue.

Examples:

(APPEND '(A B C . D) '())       => (A B C)	;Proposed
(NCONC (LIST* 'A 'B 'C 'D) '()) => (A B C)	;Proposed

Note that (COPY-LIST '(A B C . D)) would still return (A B C . D).

(APPEND '(A B . C) '() 3)       => (A B . 3)	;Proposed
(NCONC (LIST* 'A 'B 'C) '() 3)  => (A B . 3)	;Proposed

(APPEND '() 17)   => 17			;Proposed
(NCONC (LIST) 17) => 17			;Proposed

Rationale: 

This function is used a lot and its behavior should be well-defined across
implementations. This proposal upholds the apparent status quo in a number of
implementations.

Current Practice:

Symbolics Lisp, Vaxlisp, and Lucid Lisp appear to implement the proposed
interpretation (at least in the interpreter). Franz's Allegro Common Lisp
conforms to the proposed behavior except in the case of (NCONC (LIST) 17) => 17,
where it returns NIL instead of 17.

Kyoto Common Lisp signal an error when using APPEND or NCONC on a dotted list.
Xerox Common Lisp signals an error on APPEND and implements the proposed
interpretation on NCONC.

Cost to implementors:

Technically, the change should be relatively small for those implementations
which don't already implement it. However, implementations which have microcoded
APPEND or NCONC incompatibly may find the small change somewhat painful.

Some implementations may have optimized their APPEND or NCONC to expect only NIL
when SAFETY is 0. In this case, depending on implementation details, requiring
an ATOM check rather than a NULL check may slow things down.

Cost to users:

This change is upward compatible.

Benefits:

Since non-lists are allowed as a last argument and since APPEND and NCONC can
therefore produce dotted lists, some readers may have (incorrectly) assumed that
APPEND and NCONC can reliably deal in general with dotted lists, something that
doesn't appear to be guaranteed by a strict reading. The proposed extension
would happen to legitimize such assumptions.

Aesthetics:

Whether or not users will think this improves the aesthetics of the language
will depend largely on how they view the relation between lists and dotted
lists. Those who view dotted lists as a special kind of list may feel
differently than those who view lists as a special kind of dotted list.

Discussion:

The cleanup committee supports this proposal.