Week 4

Week 4 #

Local Bindings #

Creating variables with a local scope, and bind them to the result of expressions

  • Scope: Visibility of variables

let #

Usage:

(let ([var1 expr1] ... [varn exprn]) body)

  • expr1, ..., exprn are evaluated in an undefined order, has the appearance of running in parallel

  • The scope of var1, ..., varn is body

  • Note: vari is not in the scope of [varj, exprj] when i != j, due to the parallel nature

Example:

(define double-triple (x) 
  (let ([double (* 2 x)]
        [triple (* 3 x)])
    (list double triple)))

let* #

  • Same usage as let

  • expr1, ..., exprn are evaluated sequentially left to right

  • The scope of vari is everything to the right of vari

Example:

(define double-triple (x) 
  (let* ([double (* 2 x)]
        [triple (+ x double)])
    (list double triple)))

letrec #

  • Same usage and order of evaluation as let

  • The scope of vari is the entire letrec

Example:

;; Tail Recursion - allows var to be called in expr
(define len-of-list (lst)
  (letrec ([length-tail (lambda (lst len) (if (empty? lst) 
                              len 
                              (length-tail (rest lst) (+ len 1))))])
    (length-tail lst 0)))

;; another recursive example
(letrec 
  ([my-even? (lambda (x) (if (eq? x 0) #t (my-odd? (- x 1))))]
  [my-odd? (lambda (x) (if (eq? x 0) #f (my-even? (- x 1))))])
  ; allows my-even? to be in scope when called in my-odd?
  ; and vice versa
  (if (my-even? 4) 1 0))