I have a Racket code:
#lang racket
(provide (all-defined-out))
(struct weather (name perspective temperature humidity wind class)
#:transparent)
;;Exercise 8
;;Todo implement
#;(define (read-examples file-name)
(file->lines file-name))
;;todo uncomment
#;(define examples (read-examples "examples.txt"))
;;Exercise 9
(define (add-example examples example)
(cons example examples))
;;Exercise 10
(define (attribute which examples)
(define command (eval (string->symbol(string-append "weather-"
(symbol->string which)))))
(if (null? examples) null
(cons (command (car examples)) (attribute which (cdr examples)))))
Also test file:
#lang racket
(require rackunit "xalt2.rkt")
(define examples (list (weather "Day1" 'cloudy -5 60 'yes '-)
(weather "Day2" 'cloudy 0 30 'no '+)
(weather "Day3" 'cloudy 10 45 'no '+)
(weather "Day4" 'cloudy 20 60 'yes '-)
(weather "Day5" 'cloudy 30 80 'no '+)))
;;tests for Exercise 9
(define argument-for-add-example (weather "Day77" 'cloudy 39 87 'no '+))
(define expected-result (cons argument-for-add-example examples))
(check-equal? (add-example examples argument-for-add-example) expected-result "addind to list is not working?")
;;tests for Exercise 10
(check-equal? (attribute 'wind examples) null "")
it gives me strange error about struct functions not being defined, but i have required the file with the struct defined and used in the test file... I do use Racket 6.8
The error occurs because you are using eval in the wrong namespace.
Recommendation #1: Don't use eval.
Consider writing a helper function that converts attribute symbols into accessors:
;; get-attribute-function : Symbol -> (Weather -> Any)
(define (get-attribute-function attr)
(case attr
[(name) weather-name]
....))
Or you could make a hash table that mapped symbols to functions instead.
Recommendation #2: If you must, must use eval, you need to make sure you're giving it the right namespace. Read the docs on reflection and dynamic evaluation, especially the parts about namespaces.