let, let*

ToC

Allegro CL

ANSI Common Lisp

5 Data and Control Flow

5.3 Dictionary of Data and Control Flow

5.3.18 let, let*

Special Operator

Syntax:

let ({var | (var [init-form ])}^*) {declaration}* {form}*

{result}*

let* ({var | (var [init-form ])}^*) {declaration}* {form}* {result}*

Arguments and Values:

var - a symbol.

init-form - a form.

declaration - a declare expression; not evaluated.

form - a form.

results - the values returned by the forms.

Description:

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.

Examples:

 (setq a 'top)   TOP
 (defun -function () a)   DUMMY-FUNCTION
 (let ((a 'inside) (b a))
    (format nil "~S ~S ~S" a b (-function)))  "INSIDE TOP TOP" 
 (let* ((a 'inside) (b a))
    (format nil "~S ~S ~S" a b (-function)))  "INSIDE INSIDE TOP" 
 (let ((a 'inside) (b a))
    (declare (special a))
    (format nil "~S ~S ~S" a b (-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.

See Also:

progv

Allegro CL Implementation Details:

None.