Just Showing Off

May 19, 2017

As I have mentioned on several previous occasions, I frequently receive email from students asking for homework help, which I routinely ignore. Less frequently, I receive email from someone who tells me that Scheme is a horrible programming language, they don’t understand it, it is unreadable, there are too many parentheses, blah, blah, blah, and wouldn’t it be better if I wrote my blog in C#. (I don’t know what’s wrong with C# people, but I get more of them than any other language zealots.) Usually I ignore them, too, but the other day I engaged one of those correspondents who singled out macros as a particular wart on the face of Scheme. So I wrote to him and gave him this macro, which I used to calculate fibonacci numbers; the whole story is on the next page:

(define-syntax define-memoized
  (syntax-rules ()
    ((define-memoized (f arg ...) body ...)
      (define f
        (let ((cache (list)))
          (lambda (arg ...)
            (cond ((assoc `(,arg ...) cache) => cdr)
            (else (let ((val (begin body ...)))
                    (set! cache (cons (cons `(,arg ...) val) cache))
                    val)))))))))

When I showed him how to speed up the calculation of fibonacci numbers by memoizing sub-computations, he grudgingly agreed there might be something there, but it wouldn’t translate to C# (I didn’t disagree with that comment).

Your task is to write a program that shows off some special feature of your favorite programming language; tell the story how it makes your language better than any others, and give a real-life example. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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