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let  ({var | (var [init-form])}*) {declaration}* {form}* ⇒  {result}*
let*  ({var | (var [init-form])}*) {declaration}* {form}* ⇒  {result}*
var—a symbol.
init-form—a form.
declaration—a declare expression; not evaluated.
form—a form.
results—the values returned by the forms.
let and let* create new variable bindings and execute a series of forms that use these bindings. let performs the bindings in parallel and let* does them sequentially.
The form
 (let ((var1 init-form-1)
       (var2 init-form-2)
       ...
       (varm init-form-m))
   declaration1
   declaration2
   ...
   declarationp
   form1
   form2
   ...
   formn)
first evaluates the expressions init-form-1, init-form-2, and so on,
in that order, saving the resulting values. Then all of the variables varj are bound to the corresponding values; each binding is lexical unless there is a special declaration to the contrary. The expressions formk are then evaluated in order; the values of all but the last are discarded (that is, the body of a let is an implicit progn).
let* is similar to let, but the bindings of variables are performed sequentially rather than in parallel. The expression for the init-form of a var can refer to vars previously bound in the let*.
The form
 (let* ((var1 init-form-1)
        (var2 init-form-2)
        ...
        (varm init-form-m))
   declaration1
   declaration2
   ...
   declarationp
   form1
   form2
   ...
   formn)
first evaluates the expression init-form-1, then binds the variable var1 to that value; then it evaluates init-form-2 and binds
var2, and so on. The expressions formj are then evaluated in order; the values of all but the last are discarded (that is, the body of let* is an implicit progn).
For both let and let*, if there is not an init-form associated with a var, var is initialized to nil.
The special form let has the property that the scope of the name binding does not include any initial value form. For let*, a variable’s scope also includes the remaining initial value forms for subsequent variable bindings.
 (setq a 'top) ⇒  TOP
 (defun dummy-function () a) ⇒  DUMMY-FUNCTION
 (let ((a 'inside) (b a))
    (format nil "~S ~S ~S" a b (dummy-function))) ⇒  "INSIDE TOP TOP" 
 (let* ((a 'inside) (b a))
    (format nil "~S ~S ~S" a b (dummy-function))) ⇒  "INSIDE INSIDE TOP" 
 (let ((a 'inside) (b a))
    (declare (special a))
    (format nil "~S ~S ~S" a b (dummy-function))) ⇒  "INSIDE TOP INSIDE"
The code
(let (x) (declare (integer x)) (setq x (gcd y z)) ...)
is incorrect; although x is indeed set before it is used, and is set to a value of the declared type integer, nevertheless x initially takes on the value nil in violation of the type declaration.
Next: progv, Previous: destructuring-bind, Up: Data and Control Flow Dictionary