Burke_et_al-NIL_Reference_Manual_0286-1984

Scope. Extent. and Hinding 12 Nil. Manu411
distinguished from more "ordinary'· lexically scoped variablcs.
variable special is with defvar (p41ge 24). ror instance.
(defvar -leaves-)
(defun find-alI-leaves (tree)
(let (-leaves- nil»
(find-all-leaves-l tree)
-leav.8s-
) )
(defun find-all-leaves-l (tree)
(cond «atom tree)
; Empty sctofleaves
; o rO\lcl over the tree
; And return the leaves found
One way to globally declare a
(cond «not (memq tree -leaves-»
(setq -leaves- (cons tree -leaves-»»)
(t (find-all-1eaves-! (car tree»
(find-all-leaves-1 (cdr tree»»)
There are more esoteric (or SCIIEMIHike) ways in which the above could have been perfunned.
without the. usc of the special variable -Ieaves-. but the above is fairly straightforward. fairly
effident. ,md will also run (both k terpreled and -compiled) compatibly in MACUSP and !.lSI'
MACHINE I.ISP.
The above intuitive (or. if you prefer. hand-waving and vague) description can now be used
to more fonnally define the tenns of scope and extellt which are used to describe the accessibility
and lifetimes of things. of which variable bindings are one instance. The scope of something tells
where it may be validly referred to~ To say that something has lexical scope then means that it
iHay UC U)CU cJllywiaclc ·'i.cxluaiiy'" within Lhc conSlfUct which "creatcs" the objcct (e.g.. the
lambda-expression which binds a variable). Note that this docs not in itself imply that the
reference becomes invalid if that construct is exited. That dimension is the extelll of the object.
which tens the lime during which the object (e.g.. variable binding) is valid dynamic extent
means that the object (reference) is only vaHd during the execution of the construct. illdefillite
extellt means that there is no such limitation. Variable bindings in the NIL interpreter (which are
not special) have lexical scope and indefinite· extent lbismeans upward funarg capability.
indefinite scope means that there is no restriction on wbere a valid reference may occur from.
This is the case with special variables; the "free" references may be made from any piece of
code. The bindings of such variables. however, have only dynamic extent; they become invalid
(are "unbound") when the binding construct is exited. This combination of scope and extent,
which is quite common, is· referred to as dYllamic scope.
Now, for the pragmatics. The current NIL compiler actually only implements local scope
instead of lexical scope. Its capabilities lie only in dctennining when it is losing. In many cases,
this· docs nOl matter because the constructs being used are expanded out into other construc~
making the references local. This is what happens for mapcar, for instance: in the construct
(let «22 (computate»)
(mapc #'(lambda (x) (mumblify x 22» some-list})
the reference to zz within the lambda expression is a non-local (but lexical) reference. That
expression is recoded by the compiJer~ however. as an iteration without a separate function. in
which the reference become local.
MC:NILMAN:BIND 27
23-DEC-83