[plt-scheme] map-sequence using make-do-sequence and define-sequence-syntax

I've merged these corrections into the docs.

Thanks,
Matthew

At Sun, 16 Mar 2008 18:22:14 -0400, Doug Orleans wrote:
> It seems like the docs for make-do-sequence are out of date.  Here's
> what they currently say:
> 
>   The thunk results define the generated elements as follows:
> 
>     * The first result is a next-pos procedure that takes the current
>       position and returns the next position.
>     * The second result is a pos->element procedure that takes the
>       current position and returns the value(s) for the current
>       element. It is called only once per position.
>     * The third result is the initial position.
>     * The fourth result takes the current element value(s) and returns
>       a true result if the sequence includes the value, and false if
>       the sequence should end instead of including the value.
>     * The fifth result is like the fourth result, but it determines a
>       sequence end after the current element is already included in
>       the sequence.
>     * The sixth result is like the fourth result, but it takes both
>       the current position and the current element value(s).
> 
> I think it should say something like this (please correct me if I'm
> wrong):
> 
>   The thunk results define the generated elements as follows:
> 
>     * The first result is a pos->element procedure that takes the
>       current position and returns the value(s) for the current
>       element. It is called only once per position.
>     * The second result is a next-pos procedure that takes the current
>       position and returns the next position.
>     * The third result is the initial position.
>     * The fourth result takes the current position and returns a true
>       result if the sequence includes the value(s) for the current
>       position, and false if the sequence should end instead of
>       including the value(s).
>     * The fifth result is like the fourth result, but it takes the
>       current element value(s) instead of the current position.
>     * The sixth result is like the fourth result, but it takes both
>       the current position and the current element values(s) and
>       determines a sequence end after the current element is already
>       included in the sequence.
> 
> Here's my implementation of map-sequence (is there a better name for
> this? maybe in-mapped?):
> 
>   ;; Return a sequence in which each element is the result of fun
>   ;; applied to the corresponding element of seq.
>   (define (map-sequence fun seq)
>     (make-do-sequence
>      (lambda ()
>        (let-values (((more? next) (sequence-generate seq)))
> 	 (values
> 	  (lambda (pos) (fun (next)))
> 	  (lambda (pos) pos)
> 	  (void)
> 	  (lambda (pos) (more?))
> 	  (lambda (val) #t)
> 	  (lambda (pos val) #t))))))
> 
>   > (for/list ((n (map-sequence (lambda (x) (* x x)) (in-range 5)))) n)
>   (0 1 4 9 16)
> 
> I want to make a version with define-sequence-syntax, but I'm still
> digesting those docs.  (The doc for :do-in is missing post-guard, by
> the way.)
> 
> --dougorleans at gmail.com
> _________________________________________________
>   For list-related administrative tasks:
>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme