[Small-cl-src] with-carry

[Peter Lewerin]

#|
In a reply to me in c.l.l., Adam Warner suggested a control structure for the
cases when a computation is made in stages, with the result of each stage used
somewhere in the next stage.  The following is basically his code; I added a
SYMBOL-MACROLET wrapper to be able to name the carry-over variable.

See c.l.l. thread "Which style do you prefer?"
|#

(defmacro with-carry (c stages &body body)
   (symbol-macrolet ((_ c))
                    `(let* (,@(loop for item in stages collect (list _ item)))
                       , at body)))

;; (with-carry x ((+ 2 3) (* x 10)) (print x))
;; => 50
;; expands to: (LET* ((X (+ 2 3)) (X (* X 10))) (PRINT X))