Re: [Chicken-users] Named let*

[Michele La Monaca]

On Wed, May 29, 2013 at 11:22 PM, Michele La Monaca
<address@hidden> wrote:
> On Wed, May 29, 2013 at 11:10 PM, Michele La Monaca
> <address@hidden> wrote:
>> On Wed, May 29, 2013 at 10:20 PM, Patrick Li <address@hidden> wrote:
>>> Hi Michele,
>>>
>>> I realized after posting my version of named-let*, that you actually
>>> *cannot* use it to accomplish all of what you want. For that you do need
>>> loop to be a syntactic extension, as mentioned by Jorg.
>>>
>>> For instance, my named-let* macro would not simplify the example you posted
>>> earlier:
>>>
>>> (let loop ((i (some-function)) (ch (string-ref buf (some-function))))
>>>   (do-something)
>>>   (if (some-condition-is-true)
>>>     (loop (+ i 1)
>>>           (string-ref buf (+ i 1)))))
>>>
>>> The key issue underlying this is, when you call (loop), would you like to
>>> call it with one or two arguments?
>>>
>>
>> Two. Your macro seems good to me. For example, let's say I want to
>> print a string starting from a random position:
>>
>> (define buf "foobar")
>>
>> (named-let* loop ((i (random (string-length buf))) (ch (string-ref buf i)))
>>   (display ch)
>>     (if (< (+ i 1) (string-length buf))
>>       (loop (+ i 1) (string-ref buf (+ i 1)))))
>>
>>
>> or even better:
>>
>> (named-let* loop ((len (string-length buf)) (i (random len)) (ch
>> (string-ref buf i)))
>>   (display ch)
>>     (if (< (+ i 1) len)
>>       (loop len (+ i 1) (string-ref buf (+ i 1)))))
>>
>>
>> Michele
>
> Or even even better (or worse depending on your tastes):
>
> (named-let* loop ((len (string-length buf)) (i (random len)) (j (+ 1
> i)) (ch (string-ref buf i)))
>   (display ch)
>     (if (< j len)
>       (loop len j (+ j 1) (string-ref buf j))))
>

I think this one can't be beaten:

(named-let* loop ((len (string-length buf))
                  (i (+ 1 (random len)))
                  (ch (string-ref buf (- i 1))))
  (display ch)
  (if (< i len)
    (loop len (+ i 1) (string-ref buf i))))

Ciao,
Michele