Zach Beane <firstname.lastname@example.org> wrote:
| jayessay <email@example.com> writes:
| > Does anyone remember the historical (or other) reason for why _labels_
| > is called "labels"? It always seemed that the most likely name would
| > have been flet* (the above usage is particularly indicative of this in
| > comparison with let and let*). Just curious, as I've never seen this
| > "explained"...
| My turn to be picky. LABELS has an important difference from LET*: a
| LET* variable binding can refer only to preceding bindings, while
| LABELS can refer to any function binding, before or after, in the group.
Right. Actually, Scheme gets it right for a Lisp1, with LET/LET*
and LETREC instead of CL's LET/LET*/FLET and LABELS. But LETREC
and LABELS have different syntaxes and semantics. LETREC binds
arbitrary *values*, not functions [though in a Lisp1 a "value"
which is an evaluated LAMBDA expression "is" a function], while
LABELS binds *only* functions, like FLET.
CL *could* have used FLETREC instead of LABELS, I suppose, except
LABELS was already in MacLisp (I think) and predated Scheme.
Note that even with LABELS, CL really has no direct counterpart
for Scheme's LETREC [unless you write a CL macro that expands it
the same way most Schemes do, as a LET that binds all the vars to
NIL first then SETFs them later]. That is, the following works in
Scheme, but there is no standard operator in CL that you can replace
the LETREC with that will work:
> ((lambda (n)
(letrec ((f (lambda (x)
(if (<= x 1)
(* x (apply f (- x 1) '()))))))
(apply f n '())))
[Note: The funny APPLY forms, which work in both Scheme & CL,
are a workaround for FUNCALL per se not existing in Scheme.]
Rob Warnock <firstname.lastname@example.org>
627 26th Avenue <URL:http://rpw3.org/>
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