;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-intermediate-reader.ss" "lang")((modname lect22) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ())))

;;; READING: Scott: 3.1, 3.3 (excluding 3.3.{4,3})
;;;   [that's about 15pp total]

(define x 7)
(let {[x 4]            
      [z (+ x 2)]}    ; this is the "outer" x, 7.
  (+ x z))            ; this is the "inner" x, 4.




(let* {[x 7]
       [x (+ 4 x)]
       [z (+ x 2 (let {[x 99]} (* x x)))]}
  (+ x z))


(let* {[x 4]
       [y 19]
       [w (+ y x  (let ([x 19]) (* x x)))]
       [z (+ x 2)]}
  (+ x z))


;; In a program-text, an identifier-occurrence can be
;; a "binding occurrence" or a "applied occurrence" (?).

;; For every binding occurence, we should define its *scope*:
;; what region of the program-text corresponds to that binding occurrence?
;; (So that for every applied-occurrence, we should be able to answer:
;;   "Where is the binding occurrence [if any]"?)
;;

;; We say "an identifier v is free within the expression e" if v occurs in e
;; but is not bound *within e*.
;; So x is free in
(let {[z 5]}
  (+ x z))
;; but z is not.
;; Note that x is *not* free inside the larger expression
(let {[x 9]}
  (let {[z 5]} 
    (+ x z)))
;; (For a variable to be free within an entire program is probably
;;  an error 'undeclared variable'.)

;;; "alpha substitution" or "alpha renaming" --
;;;   lift to the top level, and re-name to not conflict.
;;;   But only lift *free* occurrences in the expression.

;;; "beta reduction" -- re-write the body of a function, with params
;;;   replaced by args (but only replace *free* occurrences in the body).

;;;;;;;;;;;;;;;;;
;; anonymous functions:
;;
;; A use-case:  sort;  big-bang's stop-when;  decorator pattern.
;;
;;   Java:   
;;   php:    
;;   racket: 

(sort (cons "hi" (cons "whoa" (cons "ayiyi" empty)))
      string<=?)