abstraction.rkt

;; A function to remove all the 2's from a list
(define (remove-2 lst)
  (if (null? lst)
      lst
      (let ([ item (car lst) ])
        (if (= 2 item)
            (remove-2 (cdr lst))
            (cons item (remove-2 (cdr lst)))))))
            
;; A function to remove all the 7's from a list
(define (remove-7 lst)
  (if (null? lst)
      lst
      (let ([ item (car lst) ])
        (if (= 7 item)
            (remove-7 (cdr lst))
            (cons item (remove-7 (cdr lst)))))))
            
;; Abstract the argument to allow removing any number
(define (remove-n n lst)
  (if (null? lst)
      lst
      (let ([ item (car lst) ])
        (if (= n item)
            (remove-n n (cdr lst))
            (cons item (remove-n n (cdr lst)))))))
            
;; Abstract over the comparison to allow removing any object equal to the argument
;; such as strings, lists, etc.
(define (remove obj is-same? lst)
  (if (null? lst)
      lst
      (let ([ item (car lst) ])
        (if (is-same? obj item)
            (remove obj is-same? (cdr lst))
            (cons item (remove obj is-same? (cdr lst)))))))
            
;; Abstract over the comparison to allow comparisons other than equality
;; such as is-odd?, contains-only-primes?, is-non-empty-string?, etc.
(define (remove pred? lst)
  (if (null? lst)
      lst
      (let ([ item (car lst) ])
        (if (pred? item)
            (remove pred? (cdr lst))
            (cons item (remove pred? (cdr lst)))))))
            
;; Recognize and use built-in higher order functions
(define (remove pred? lst)
  (filter (compose not pred?) lst))
  
;; Allow reusing specific instances
(define remove-2 (curry remove is-2?))
(define remove-foo (curry remove (curry string=? "foo")))