Add proverb exercise (#297)

[Adrien-LUDWIG]

I had some hesitation for the formatting of example.rkt. What is the best option?

• Current:

(define (recite lst)
  (cond
    [(empty? lst) '()]
    [else (define (verse index)
            (cond
              [(= index (sub1 (length lst))) (list (format "And all for the want of a ~a." (list-ref lst 0)))]
              [else (append
                     (list (format "For want of a ~a the ~a was lost."(list-ref lst index)(list-ref lst (add1 index))))
                     (verse (add1 index)))]))
          (verse 0)]))

• Define the inner function before cond to reduce indentation, but verse is defined even if lst is empty:

(define (recite lst)
  (define (verse index)
    (cond
      [(= index (sub1 (length lst))) (list (format "And all for the want of a ~a." (list-ref lst 0)))]
      [else (append
             (list (format "For want of a ~a the ~a was lost." (list-ref lst index) (list-ref lst (add1 index))))
             (verse (add1 index)))]))
  (cond
    [(empty? lst) '()]
    [else (verse 0)]))

• With a default argument, but we check if lst is empty for each value of index and this exposes weird usage possibilities:

(define (recite lst [index 0])
  (cond
    [(empty? lst) '()]
    [(= index (sub1 (length lst))) (list (format "And all for the want of a ~a." (list-ref lst 0)))]
    [else (append
           (list (format "For want of a ~a the ~a was lost." (list-ref lst index) (list-ref lst (add1 index))))
           (recite lst (add1 index)))]))

[github-actions]

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[Adrien-LUDWIG]

@BNAndras
Just to make sure you see the PR. 😉

[BNAndras]

Thanks again. I'll review tonight.

[BNAndras]

#2 is idiomatic since it signals a helper function is being used and it's defined before possibly being used. It's also an error to not have an expression after each internal define form so having the defines at the beginning makes that error a bit less likely to happen.

We could also forgo the helper function possibly by using a for/list form to manage the current index. The for/list form builds a list from the result from each iteration which is what we want.

[github-actions]

Hello. Thanks for opening a PR on Exercism. We are currently in a phase of our journey where we have paused community contributions to allow us to take a breather and redesign our community model. You can learn more in this blog post. As such, all issues and PRs in this repository are being automatically closed.

That doesn't mean we're not interested in your ideas, or that if you're stuck on something we don't want to help. The best place to discuss things is with our community on the Exercism Community Forum. You can use this link to copy this into a new topic there.

---

Note: If this PR has been pre-approved, please link back to this PR on the forum thread and a maintainer or staff member will reopen it.

[BNAndras]

There we go. The pause community contributions CI was misbehaving earlier.

[Adrien-LUDWIG]

Sorry for the force push, I mixed up proverb.rkt and example.rkt.

Indeed, for/list is very useful. You don't even need for an index.
What do you think of the result?

[BNAndras]

Looks good to me. Let me know if you're not getting Hacktoberfest credit for the PRs.