;; The first three lines of this file were inserted by DrScheme. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "reader.ss" "plai" "lang")

;; Define is kind of an imperative kluge that we use at "top level":

(define g  9.81)
(define t  2)
(define v  0)
;; final velocity is:
(+ v (* t (* g g)))


;; We can write this in a purely functional style with LET:

(let ([g  9.81]
      [t   2]
      [v 0])
      (+ v (* t (* g g))))


;; We can also bind procedure values using LET, of course:
(let ([is-even?   (lambda (x) (equal? 0 (remainder x 2)))])
      (or (is-even? 7) (is-even? 3)))

;; Although we get into a problem with recursive procedures:
;(let ([is-even?   (lambda (x) (equal? 0 (remainder x 2)))]
;      [is-odd?    (lambda (x) (not (is-even? x)))])
;      (or (is-even? 7) (is-odd? 3)))
;
;(let ([factorial   (lambda (x) (if (<= x 1) 1 (* x (factorial (- x 1)))))])
;      (factorial 7))
 
; The solution is LETREC, which allows the values on the
; left to appear on the right:
(letrec ([factorial   (lambda (x) (if (<= x 1) 1 (* x (factorial (- x 1)))))])
      (factorial 7))