#lang racket
(require rackunit)

;; -----------------------------------------------------------------------
;; Racket functions are bound to names in the same way as any other value!
;; In this expression, + is a free variable:
;; 
;; (lambda (x y)
;;   (+ (* x x) (* y y)))))
;;
;; You can verify that + is free for yourself by evaluating:

(define (capture-+ +)         ; Receives a function as an argument,
                              ; ** binds it to + **.
    (lambda (x y)             ; Returns a new-two-argument function
      (+ (* x x) (* y y)))))  ; that uses the captured +.

(define sum-of-squares (capture-+ *))          ; pass in *

;; (check-equal? (sum-of-squares 3 4) 25)        ; 9 + 16 = 25, right?

;; -------------------------------------------------------------------------
;; Try these in the REPL.  Dr. Racket provides strong support for software
;; engineering by not letting us do this sort of mental gymnastics in our
;; programs.

;; (if (zero? 0) 1 foo)     ; foo cannot be reached

;; (and #f foo)             ; foo cannot be reached

;; (
;;   (lambda (x)            ; this function ignores its argument...
;;     (lambda (z) z))
;;   (lambda (x) foo)       ; ... so foo cannot be reached
;; )

;; See:
;;
;; (((lambda (x) (lambda (z) z))
;;   (lambda (x) foo))
;;  'x)