[plt-scheme] How to do something to all element of a list but the last

From: Robby Findler (robby at cs.uchicago.edu)
Date: Fri Aug 8 00:47:20 EDT 2008

I'm not sure, but that definition might require O(n^2) time. (Try
timing it and seeing ...)

This is how I'd write it but I'm old fashioned when it comes to little
loops like these. (And I didn't run the code. here's hoping no typos.)

#lang scheme
  [foo (-> (cons/c any/c (listof any/c))
           (-> any/c any)
           (-> any/c void?)

(define (foo args fun last-fun)
  (let loop ([fst (car args)]
             [rst (cdr args)])
      [(empty? rst) (last-fun fst)]
      [else (fun fst) (loop (car rst) (cdr rst))])))


2008/8/7 Grant Rettke <grettke at acm.org>:
> Hi folks,
> While reviewing some code I wrote a while back, I found 'do' used in
> many places. On further review, I found that this use of 'do' doesn't
> qualify as "there was no better way" but more like "when I wrote it I
> didn't know any other way".
> The usage is like that of 'for-each', but for the fact that different
> functionality is executed depending on whether or not the element
> being processed is the last element of the list, or not.
> Is there a "best way" to do something like this?
> I haven't thought too much into this because I am too tired, but, I
> did come up with this for the fun of it. I will think about it more
> tomorrow.
> #lang scheme
> (define (foo args fun last-fun)
>  (match args
>    [(list arg args ..1)
>       (fun arg)
>       (foo args fun last-fun)]
>    [(list arg)
>       (last-fun arg)]))
> (foo '(a b c d e)
>       (λ (x) (printf "Got: ~a~n" x))
>       (λ (x) (printf "Last One: ~a~n" x)))
> Best wishes,
> Grant
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