昇順の整数のリストが複数あったとき、それらの共通部分を抜き出すやつ

intersection.scm

; https://twitter.com/golden_lucky/status/1352094822148915201

(use srfi-1)

(define (intersection lss)
  (let R ((as (car lss))
          (bss (drop-less-all (caar lss) (cdr lss)))
          (result '()))
    (if (or (null? bss) (null? as))
        (reverse result)
        (R (cdr as)
           (if (null? (cdr as))
               '()
               (drop-less-all (cadr as) bss))
           (if (every (^a (= (car as) (car a))) bss)
               (cons (car as) result)
               result)))))

(define (drop-less-all target lss)
  (define (drop-less ls)
    (drop-while (^a (< a target)) ls))
  (call/cc
    (^b
      (let R ((lss lss)
              (result '()))
        (if (null? lss)
            (reverse result)
            (let1 ds (drop-less (car lss))
              (if (null? ds)
                  (b '())
                  (R (cdr lss)
                     (cons ds result)))))))))

(define l1 '(1 2 3 4 5 6 8))
(define l2 '(1 4 8))
(define l3 '(1 2 8))

(intersection `(,l1 ,l2 ,l3))