comparing every element in a list to every other element

stepwise comparison

; common lisp version, totally works
; (defun build (list1 list2)
;   (if (null list1)
;       ()
;       (append (mapcar #'(lambda (bubble)
;                           (list (first list1) bubble))
;                       list2)
;               (build (rest list1) list2))))

; if you tell lisp: (build '(1 2) '(1 2))
; lisp will happily reply: ((1 1) (1 2) (2 1) (2 2))

; literal clojure translation
(defn build [list-1 list-2]
  (if (nil? list-1)
    ()
    (concat (map (fn [bubble]
                     (list (first list-1) bubble))
                 list-2)
            (build (rest list-1) list-2))))

; if you tell clojure: (build '(1 2) '(1 2))
; lisp will stack overflow error all up in your business

; if you're curious there's a Ruby version here:
; https://github.com/gilesbowkett/tweenr/blob/master/bit101_style.rb#L2

The Common Lisp code reimplements MAPCAN. Use it:

(defun cp (l1 l2)
  (mapcan (lambda (e2)
            (mapcar (lambda (e1) (list e1 e2)) l1))
          l2))