• 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 var_j are bound to the corresponding values; each binding is lexical unless there is a special declaration to the contrary. The expressions form_k 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 var_j 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.

(defparameter *a* 'top) 

→

*A* 



(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 example below 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.

(let (x) 
  (declare (integer x)) 
  (setf x (gcd y z)) ...)

None.

None.

Special Operator PROGV

None.

\issue{DECLS-AND-DOC} \issue{VARIABLE-LIST-ASYMMETRY:SYMMETRIZE} \issue{KMP-COMMENTS-ON-SANDRA-COMMENTS:X3J13-MAR-92} \issue{KMP-COMMENTS-ON-SANDRA-COMMENTS:X3J13-MAR-92}