Re: [Sbcl-devel] rebinding in LET and lambda lists (was [patch]...)

[<bdo...@la...>]

On Fri, Jan 05, 2007 at 12:20:42PM -0600, William Harold Newman wrote:
>   * Whether or not &KEY arguments allow binding the same variable
>     name more than once, it is probably an error to use duplicate
>     keywords.

I think by the wording of the spec using duplicate keyword names in a
lambda list (not duplicate variable names - that's just as uncertain as
anything else in this discussion) is valid.

For instance,

(defun foo (&key ((:x x)) ((:x y)))
  (print (list x y)))

3.4.1.4 states:

| The keyword parameter specifiers are, like all parameter specifiers,
| effectively processed from left to right. For each keyword parameter
| specifier, if there is an argument pair whose name matches that
| specifier's name (that is, the names are eq), then the parameter
| variable for that specifier is bound to the second item (the value) of
| that argument pair. If more than one such argument pair matches, the
| leftmost argument pair is used.

I take that to mean that these results are unambiguously correct:

(foo :x 4) => (4 4)
(foo :x 4 :x 'no) => (4 4)

Of course, it may still be less confusing to disallow duplicate keyword
names, as at least one implementations fails this in compiled code
(Allegro 7.0):

CL-USER(5): (foo :x 4)
(4 NIL)

-bcd