Issue: APPEND-DOTTED (Version 4)

[Masinter.pa@Xerox.COM]

At X3J13, Bekerly (sp?) expressed some concern that APPEND might in some
circumstances return a non-list. I quoted more extensively from CLtL so
that it would be more obvious that this was the only reasonable
clarification that was compatible with the current description.

!


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

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.

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).

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:

If there are any implementations which have microcoded APPEND or NCONC
incompatibly, the small change may nevertheless be 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.