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#lang racket
(provide (contract-out
[with-labels with-labels/c]
[foldr-map foldr-map/c]
[pair-from pair-from/c]))
(provide with-labels/c foldr-map/c pair-from/c)
(define with-labels/c (parametric->/c [a b] (-> (-> a b) (listof a) (listof (cons/c b (cons/c a null?))))))
(define (with-labels f xs)
(if (null? xs)
null
(cons (list (f (car xs)) (car xs)) (with-labels f (cdr xs)))))
(define foldr-map/c (parametric->/c [x a f] (-> (-> x a (cons/c f a)) a (listof x) (cons/c (listof f) a))))
(define (foldr-map f a xs)
(define (it a xs ys)
(if (null? xs)
(cons ys a)
(let [(p (f (car xs) a))]
(it (cdr p)
(cdr xs)
(cons (car p) ys)))))
(it a (reverse xs) null))
(define pair-from/c (parametric->/c [x fx gx] (-> (-> x fx) (-> x gx) (-> x (cons/c fx gx)))))
(define (pair-from f g)
(lambda (x) (cons (f x) (g x))))
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