A Common Lisp macro that threads a value through a handful of forms, updating that value if the predicate returns true.

cond-<>.lisp

;; from On Lisp
(defun single (lst)
  (and (consp lst) (not (cdr lst))))

(defmacro cond-<> (data &body body)
  "A combination of this: https://clojuredocs.org/clojure.core/cond-%3E
and this: https://github.com/rplevy/swiss-arrows#a-generalization-of-the-arrow
returns last value of <>"
  (let ((<> (intern "<>")))
    `(let ((<> ,data))
        ,@(mapcar
            (lambda (l)
              `(if ,(car l)
                   (setf <>
                         ,(if (single (cdr l))
                              (cadr l)
                              `(progn ,@(cdr l))))))
            body)
      <>)))

;; Example
(cond-<> (list 1 2 3)
  ((member 4 <>) (mapcar (lambda (x) (* x x)) <>))
  ((is-full-moon-p) (do-crazy-database-transaction <> :its-a-full-moon) (reduce #'* <>))
  ((cust-want-min-max-p) (list (apply #'min <>) (apply #'max <>))))

;; Expands to:
(progn
  (setf <> (list 1 2 3))
  (if (member 4 <>)
      (setf <> (mapcar (lambda (x) (* x x)) <>)))
  (if (is-full-moon-p)
      (setf <>
            (progn (do-crazy-database-transaction <> :its-a-full-moon)
                   (reduce #'* <>))))
  (if (cust-want-min-max-p)
      (setf <> (list (apply #'min <>) (apply #'max <>))))
  <>)