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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.
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