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let* 290
I
vforms. Thus the bindings happen in parallel; all the vforms are evaluated
before any of the vars are bound. Finally, the bforms (the body) are
evaluated sequentially, the old values of the variables are restored, and the
result of the last bform is returned.
You can omit the vform from a let clause, in which case it is as if the
vform were nil: the variable is bound to nil. Furthermore, you can replace
the entire clause (the list of the variable and form) with just the variable,
which also means that the variable gets bound to nil. I t is customary to
write just a variable, rather than a clause, to indicate that the value to
which the variable is bound does not matter, because the variable is
setq'ed before its first use. Example:
(let «a (+ 3 3»
(b 'foo)
(c)
d)
... )
Within the body, a is bound to 6, b is bound to foo, c is bound to nil, and
d is bound to nil.
See the section "Special Forms for Binding Variables" in Symbolics Common
Lisp: Language Concepts.
let* ((var value) ... ) body... Special Form
The same as let, except that the binding is sequential. Each var is bound
to the value of its vform before the next vform is evaluated. This is useful
when the computation of a vform depends on the value of a variable bound
in an earlier vform. Example:
(let* «a (+ 1 2»
(b (+ a a»)
... )
Within the body, a is bound to 3 and b is bound to 6.
See the section "Special Forms for Binding Variables" in Symbolics Common
Lisp: Language Concepts.
let-and-make-dynamic-closure vars &body body Function
When using dynamic closures, it is very common to bind a set of variables
with initial values, and then make a closure over those variables. Furthermore,
the variables must be declared as "special".
let-and-make-dynamic-closure is a special form that does all of this. It is
best described by example: