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5.3.57 prog, prog* [Macro]

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.

resultsnil 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))

returns the car of the value of 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

This can be accomplished more perspicuously as follows:

      (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
See Also::

block , let , tagbody , go , return , 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)))