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Special OperatorLET, LET*

Syntax:

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

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

Arguments and Values:

var---asymbol.

init-form---aform.

declaration---adeclare expression; not evaluated.

form---aform.

results---thevalues returned by theforms.

Description:

let andlet* create new variablebindings and execute a series offorms that use thesebindings.let performs thebindings in parallel andlet* 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 expressionsinit-form-1,init-form-2, and so on, in that order, saving the resulting values. Then all of the variablesvarj are bound to the corresponding values; eachbinding is lexical unless there is aspecial declaration to the contrary. The expressionsformk are then evaluated in order; the values of all but the last are discarded (that is, the body of alet is animplicit progn).

let* is similar tolet, but thebindings of variables are performed sequentially rather than in parallel. The expression for theinit-form of avar can refer tovars previously bound in thelet*.

The form

 (let* ((var1 init-form-1)        (var2 init-form-2)        ...        (varm init-form-m))   declaration1   declaration2   ...   declarationp   form1   form2   ...   formn)

first evaluates the expressioninit-form-1, then binds the variablevar1 to that value; then it evaluatesinit-form-2 and bindsvar2, and so on. The expressionsformj are then evaluated in order; the values of all but the last are discarded (that is, the body oflet* is an implicitprogn).

For bothlet andlet*, if there is not aninit-form associated with avar,var is initialized tonil.

The special formlet has the property that thescope of the name binding does not include any initial value form. Forlet*, a variable'sscope 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; althoughx is indeed set before it is used, and is set to a value of the declared typeinteger, neverthelessx initially takes on the valuenil in violation of the type declaration.

Affected By: None.

Exceptional Situations: None.

See Also:

progv

Notes: None.

The followingX3J13 cleanup issues,not part of the specification, apply to this section:

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