2

#lang racket

(define/contract (filter f xs)
    (parametric->/c [a] (-> (-> a boolean?) (listof a) (listof a)))
    (if (null? xs)
        null
        (if (f (car xs))
            (cons (car xs) (filter f (cdr xs)))
            (filter f (cdr xs)))))

(filter odd? '(1 2 3 4 5 6 7 8 9))

(define (contains a xs)
    (member a xs))

(define (count a xs)
    (if (null? xs)
        0
        (if (equal? a (car xs))
            (+ 1 (count a (cdr xs)))
            (count a (cdr xs)))))

(define (all-inside xs f)
    (lambda (y)
        (define (iter xs)
            (if (null? xs)
                #t
                (if (and (f (car xs)) (not (contains (car xs) y)))
                    #f
                    (iter (cdr xs)))))
        (define (iter2 xs ys);;zmienic na contains
            (if (null? xs)
                #t
                (if (not (= (count (car xs) ys) (count (car xs) xs)))
                    #f
                    (iter2 (cdr xs) ys))))
        (and (iter xs) (iter2 y xs))))

(define/contract (filter2 f xs)
    (->i ([f procedure?]
          [x (listof any/c)])
          [result (f x) (and/c list?
                                (all-inside x f))])
    (if (null? xs)
        null
        (if (f (car xs))
            (cons (car xs) (filter2 f (cdr xs)))
            (filter2 f (cdr xs)))))

(filter2 odd? '(1 2 3 4 5 6 7))