Racket recursive variable?

Question

Fig. 65 in "How to Design Programs" is as follows:

; Nelon -> Number
; determines the smallest number on l
(define (inf l)
  (cond
    [(empty? (rest l)) (first l)]
    [else
     (local ((define smallest-in-rest (inf (rest l))))
       (cond
         [(< (first l) smallest-in-rest) (first l)]
         [else smallest-in-rest]))]))

Can somebody explain how variable smallest-in-rest works. I get recursion in a function but a variable has me confused

C. K. Young · Accepted Answer · 2016-03-04 12:37:14Z

1

It's just a shorthand (longhand ;-)) for the following:

(let ((smallest-in-rest (inf (rest l))))
  (cond
    [(< (first l) smallest-in-rest) (first l)]
    [else smallest-in-rest]))

The let should make it clear that we're just storing the result of the (inf (rest l)) so that it only has to be written once in the code, rather than once for each branch of the cond.

answered Mar 4, 2016 at 12:37

C. K. Young

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3 Comments

Sylwester Over a year ago

Wonder why racket goes to such lengths to teach students something completely different than their main language. It's like teaching JavaScript by first teaching them Python..

stchang Over a year ago

The HtDP curriculum is not about learning Racket (or any language). As the first line of the book says, any idiot can pick up any language in a few days. HtDP teaches how to program, a significantly harder undertaking. It uses a series of streamlined beginner languages to do so, which allows students to focus on what's important, and not on idiosyncrasies of syntax (think public static void main).

Alex Knauth Over a year ago

As someone who learned local first, it made more sense to me over let, because we could reuse the same syntax for definitions that we had already learned. We spent less time learning about syntax that way.

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