[plt-scheme] How to fold it

The general approach I follow in these kind of cases is an idea I got
from the Design Recipe:

1- The value you produce in the empty case is the empty case of foldr.
2- The function you pass to foldr (let's call it f) is the else part of
your condition, wrapped inside a lambda with two parameters.
3- All the (first a-list)'s in your code is the first parameter of f.
4- The result of the recursion is the second parameter of f.

Do the substitutions and your foldr'd program is ready.

I think it is possible to convert any program that is a simple
structural recursion on one value to a foldr using this methodology. I
guess somebody already proved it somewhere.

Jordan Johnson der ki:
> This feels clunky to me, but how about:
>
> (define (slide ls)
>   (reverse (foldl (lambda (x ls)
>                     ;; e.g.: 2 '((0 1) (0)) ==> ((0 1 2) (0 1) (0))
>                     (if (null? ls)
>                       (list (list x))
>                       (cons (append (car ls) (list x))
>                             ls)))
>                   '()
>                   ls)))
>
> I also tried it as an unfold, which was cute by comparison, at the cost 
> of some reversing:
>
> (require srfi/1)
>
> (define (slide2 ls)
>   (unfold-right null?
>                 reverse
>                 rest
>                 (reverse ls)))
>
> Cheers,
> jmj
>
> On May 15, 2009, at 8:04 PM, wooks wrote:
>
>> ;;slide :: [*] -> [[*]]
>> ;;Given a list like '(0 1 2 3 4) produces '((0) (0 1) (0 1 2) (0 1 2
>> 3) (01 2 3 4))
>> (define slide
>>   (lambda (a-list)
>>     (cond
>>       [(empty? a-list) empty]
>>       [else (cons (list (first a-list))
>>                   (map (lambda (x) (cons (first a-list) x))
>>                        (slide (rest a-list))))])))
>>
>> This code is in the spirit of HTDP 12.4.2. The map prefixes the
>> inductive case with the first of the list and
>>
>>         cons (list (first a-list))
>>
>> was what was required to get from what I had to what I wanted.
>>
>> It looks like something that should be foldable. How to get there?
>>
>> _________________________________________________
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>
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-- 
R. Emre Başar
İstanbul Bilgi University
Department of Computer Science
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