; Examples to illustrate closures.
(define (book-sales book) (cadr book))
; Return a list of all books with at least THRESHOLD copies sold.
(define (best-selling-books2 threshold) 
  ; let creates the lexical environment
  (let (( book-list '(((Gone With the Wind) 1000) ((Freakonomics) 1500) ((Cryptonomicon) 500))))
    (filter ; higher-order function, similar to map but returns selected items
     (lambda (book) ; here's the anonymous function
       (>= (book-sales book) threshold)) 
     ; book-list is a "free variable" within the anonymous function, accesses the lexical
     ; environment created by let 
     book-list)))
 

; If the anonymous function accesses a global variable, it will point to the global 
; variable location.  Changes to the global variable WILL be seen in the function.
(define the-book-list '(((Gone With the Wind) 1000) ((Freakonomics) 1500) ((Cryptonomicon) 500)))
(define (best-selling-books threshold) 
  (filter
   (lambda (book) 
     (>= (book-sales book) threshold)) 
   the-book-list)) 

; TRY:  (set! the-book-list (append '(((Blown To Bits) 400)) the-book-list)) 

 
; Higher-order function examples 
(define (print-result operation) (display (operation 2 1)) (newline))(print-result +) 

(define (map-result operation theList) (map operation theList))

(define (exceedsLimit x) (if (> x 100) #t #f))

(define complement (lambda (f) (lambda (x) (not (f x)))))

; closure example from Schani web page
(define cl 
  (lambda (x)
    (lambda (y)
      (+ x y))))

; examples using cl(cl 3)
(apply (cl 3) '(4))
(map (cl 3) '(4 5))


(define (testClosure y)
  (let ((z 1))
    (let ((f (lambda (y) (* y z)))) 
; definition environment
      (let ((z 2))
        (display "calling f with z value: ")
        (display z)
        (display #\space)
        (f y))))) ; activation environment

; Currying examples from: http://www.engr.uconn.edu/~jeffm/Papers/curry.html
(define (mult y) (lambda (x) (* y x)))
(define (double x) ((mult 2) x))

(define (curry fun . args)
  (lambda x
    (apply fun (append args x))))

(define triple (curry * 3))

(define lister1 (curry list 'args))
(define lister2 (curry lister1 'one))
(define lister3 (curry lister2 'two))