方案（DrRacket） - 將兩個高階函數與Lambda結合定義

Question

僅供參考我正在使用Scheme使用DrRacket進行編程。

我試圖代碼，消耗了城市名稱和航班的列表，並返回城市（串）的列表定義daily-to到航空公司有從給定的城市每日航班。

從我的理解這個定義需要同時使用filter和map，與lambda一起，因爲這是一個問題，關於higher order functions和lambda。雖然我似乎沒有正確地做到這一點。

希望我能得到我的daily-to定義的幫助，因爲它運行不正確。

這裏是我的程序：

(define-struct flight (from to frequency)) 
;; Example: (make-flight "Boston" "Chicago" 「daily」)) 
;; The frequency is one of 「daily」, 「weekday」 or 「weekend」. 

(define myflights 
    (list 
    (make-flight "Boston" "Chicago" "daily") 
    (make-flight "New York" "Providence" "weekend") 
    (make-flight "Paris" "Moscow" "weekday"))) 

;; Signature: service-between?: String String ListOfFlights -> Boolean 
;; Purpose: consumes two city names and a list of flights and determines 
;;   whether (true or false) there is a flight from the first city to the second. 
;; Tests: 
(check-expect (service-between? "Boston" "Chicago" myflights) true) 
(check-expect (service-between? "New York" "Providence" myflights) true) 
(check-expect (service-between? "Paris" "Moscow" myflights) true) 
(check-expect (service-between? "Boston" "Providence" myflights) false) 
;; Definition: service-between? 
(define (service-between? city1 city2 alof) 
    (ormap 
    (lambda (str) 
    (and (string=? city1 (flight-from str)) 
     (string=? city2 (flight-to str)))) alof)) 

;; Signature: daily-to: String ListOfFlights -> ListOfString 
;; Purpose: consumes a city name and a list of flights and returns a list 
;;   of cities (strings) to which the airline has a daily flight from the given city. 
;; Tests: 
(check-expect (daily-to "Boston" myflights) (list "Chicago")) 
(check-expect (daily-to "New York" myflights) empty) 
(check-expect (daily-to "Paris" myflights) empty) 
;; Definition: daily-to 
(define (daily-to city alof) 
    (filter (map 
      (lambda (str) 
      (cond 
       [(and (string=? city (flight-from str)) (string=? "daily" (flight-frequency str))) (flight-to str)] 
       [else empty])) alof))) 

預先感謝您！

Answer 1

這裏有一個daily-to的定義，這似乎是有道理的我的頭頂。它沒有經過測試。

(define (daily-to city alof) 
    (map flight-to 
     (filter (lambda (flight) 
       (and (string=? city (flight-from flight)) 
         (string=? "daily" (flight-frequency flight)))) 
       alof)))