I am doing an exercise from EDX course based on H2DP book and am having trouble
implementing a `find` function using the abstract function fold-elt. The
function should find the element and return its data.
-----
#lang htdp/isl
(define-struct elt (name data subs))
;; Element is (make-elt String Integer ListOfElement)
;; interp. An element in the file system, with name, and EITHER data or subs.
;; If data is 0, then subs is considered to be list of sub elements.
;; If data is not 0, then subs is ignored.
;; ListOfElement is one of:
;; - empty
;; - (cons Element ListOfElement)
;; interp. A list of file system Elements
(define F1 (make-elt "F1" 1 empty))
(define F2 (make-elt "F2" 2 empty))
(define F3 (make-elt "F3" 3 empty))
(define D4 (make-elt "D4" 0 (list F1 F2)))
(define D5 (make-elt "D5" 0 (list F3)))
(define D6 (make-elt "D6" 0 (list D4 D5)))
; D6
; / \
; / \
; D4 D5
; / \ |
; / \ |
; F1 F2 F3
;; The abstract fold/reduce function for Element.
(define (fold-elt c1 c2 b e)
(local [(define (fn-for-element e) ; -> X
(c1 (elt-name e) ;String
(elt-data e) ;Integer
(fn-for-loe (elt-subs e))))
(define (fn-for-loe loe) ; -> Y
(cond [(empty? loe) b]
[else
(c2 (fn-for-element (first loe))
(fn-for-loe (rest loe)))]))]
(fn-for-element e)))
;; String Element -> Integer or false
;; Search the given tree for an element with the given name,
;; produce data if found; false otherwise
(check-expect (find "F3" F1) #f)
(check-expect (find "F3" F3) 3)
(check-expect (find "D4" D4) 0)
(check-expect (find "D6" D6) 0)
(check-expect (find "F3" D4) #f)
(check-expect (find "F1" D4) 1)
(check-expect (find "F2" D4) 2)
(check-expect (find "F1" D6) 1)
(check-expect (find "F3" D6) 3)
(define (find n elt)
(local [(define (c1 name data loe)
(if (string=? n name) data loe))]
(fold-elt c1 ??? #f elt)))
-----
So, what should I do in the place marked with "???"?
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