I'm working through Exercise 21.2.3 of HtDP on my own and was wondering if this is idiomatic usage of the various functions. This is what I have so far:
(define-struct ir (name price))
(define list-of-toys (list
(make-ir 'doll 10)
(make-ir 'robot 15)
(make-ir 'ty 21)
(make-ir 'cube 9)))
;; helper function
(define (price< p toy)
(cond
[(< (ir-price toy) p) toy]
[else empty]))
(define (eliminate-exp ua lot)
(cond
[(empty? lot) empty]
[else
(filter ir? (map price< (build-list (length lot)
(local ((define (f x) ua)) f)) lot))]))
To my novice eyes, that seems pretty ugly because I have to define a local function to get build-list to work, since map requires two lists of equal length. Can this be improved for readability? Thank you.
You could implement eliminate-exp with filter alone:
(define (eliminate-exp ua lot)
(define (price< toy) (< (ir-price toy) ua))
(filter price< lot))