let, let
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)
let, let∗
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)
Data and Control
(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:
progvExpanded Reference: let, let
TODO: Please contribute to this page by adding explanations and examples
(let, let )