let (ANSI and GNU Common Lisp Document)

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#### 5.3.18 let, let* [Special Operator]

`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 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.

#### See Also::

progv

Next: progv, Previous: destructuring-bind, Up: Data and Control Flow Dictionary