[racket] Style. for/fold for/list for/lists

Replying to the entire list is hard to remember...

This doesn't have the full fanciness of the for--- series, but perhaps this
is a useful starting point along your suggested API?

(define (make-collector init-val mappend)
  (let ([v init-val])
    (λ args
      (case (length args)
        [(0) v]
        [(1) (set! v (mappend (car args) v))]
        [else (error "for/collect: blah blah blah")]))))

(define-syntax collector
  (syntax-rules (list sum)
    [(_ sum) (make-collector 0 +)]
    [(_ list) (make-collector '() cons)]
    ;etc.
    ))

(define-syntax-rule (for/collect ([acc type] ...) ([i lst] ...) body ...)
  (let ([acc (collector type)] ...)
    (let loop ([i lst] ...)
      (cond [(or (null? i) ...) (values (acc) ...)]
            [else (let ([i (car i)] ...)
                    body ...)
                  (loop (cdr i) ...)]))))

It does use set!, as you feared, but since it's entirely local to the
collector implementation I don't think it counts as breaking purity.


On Wed, Jan 22, 2014 at 6:49 PM, Sean Kanaley <skanaley at gmail.com> wrote:

> This doesn't have the full fanciness of the for--- series, but perhaps
> this is a useful starting point along your suggested API?
>
> (define (make-collector init-val mappend)
>   (let ([v init-val])
>     (λ args
>       (case (length args)
>         [(0) v]
>         [(1) (set! v (mappend (car args) v))]
>         [else (error "for/collect: blah blah blah")]))))
>
> (define-syntax collector
>   (syntax-rules (list sum)
>     [(_ sum) (make-collector 0 +)]
>     [(_ list) (make-collector '() cons)]
>     ;etc.
>     ))
>
> (define-syntax-rule (for/collect ([acc type] ...) ([i lst] ...) body ...)
>   (let ([acc (collector type)] ...)
>     (let loop ([i lst] ...)
>       (cond [(or (null? i) ...) (values (acc) ...)]
>             [else (let ([i (car i)] ...)
>                     body ...)
>                   (loop (cdr i) ...)]))))
>
> It does use set!, as you feared, but since it's entirely local to the
> collector implementation I don't think it counts as breaking purity.
>
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