Syntax:

prog ({var | (var [init-form])}*) declaration* {tag | statement}*

=> result*

prog* ({var | (var [init-form])}*) declaration* {tag | statement}*

=> result*

Arguments and Values:

var---variable name.

init-form---a form.

declaration---a declare expression; not evaluated.

tag---a go tag; not evaluated.

statement---a compound form; evaluated as described below.

results---nil if a normal return occurs, or else, if an explicit return occurs, the values that were transferred.

Description:

Three distinct operations are performed by prog and prog*: they bind local variables, they permit use of the return statement, and they permit use of the go statement. A typical prog looks like this:

(prog (var1 var2 (var3 init-form-3) var4 (var5 init-form-5)) declaration* statement1 tag1 statement2 statement3 statement4 tag2 statement5 ... )

For prog, init-forms are evaluated first, in the order in which they are supplied. The vars are then bound to the corresponding values in parallel. If no init-form is supplied for a given var, that var is bound to nil.

The body of prog is executed as if it were a tagbody form; the go statement can be used to transfer control to a tag. Tags label statements.

prog implicitly establishes a block named nil around the entire prog form, so that return can be used at any time to exit from the prog form.

The difference between prog* and prog is that in prog* the binding and initialization of the vars is done sequentially, so that the init-form for each one can use the values of previous ones.

Examples:

(prog* ((y z) (x (car y))) (return x))

z

(setq a 1) => 1 (prog ((a 2) (b a)) (return (if (= a b) '= '/=))) => /= (prog* ((a 2) (b a)) (return (if (= a b) '= '/=))) => = (prog () 'no-return-value) => NIL

(defun king-of-confusion (w) "Take a cons of two lists and make a list of conses. Think of this function as being like a zipper." (prog (x y z) ;Initialize x, y, z to NIL (setq y (car w) z (cdr w)) loop (cond ((null y) (return x)) ((null z) (go err))) rejoin (setq x (cons (cons (car y) (car z)) x)) (setq y (cdr y) z (cdr z)) (go loop) err (cerror "Will self-pair extraneous items" "Mismatch - gleep! ~S" y) (setq z y) (go rejoin))) => KING-OF-CONFUSION

(defun prince-of-clarity (w) "Take a cons of two lists and make a list of conses. Think of this function as being like a zipper." (do ((y (car w) (cdr y)) (z (cdr w) (cdr z)) (x ' () (cons (cons (car y) (car z)) x))) ((null y) x) (when (null z) (cerror "Will self-pair extraneous items" "Mismatch - gleep! ~S" y) (setq z y)))) => PRINCE-OF-CLARITY

Affected By: None.

Exceptional Situations: None.

See Also:

block, let, tagbody, go, return, Section 3.1 (Evaluation)

Notes:

prog can be explained in terms of block, let, and tagbody as follows:

(prog variable-list declaration . body) == (block nil (let variable-list declaration (tagbody . body)))