[racket-dev] Short-circuiting comprehensions

I agree that #:while and #:until are easily confused with #:when and
#:unless.  I slightly prefer #:stop- to #:break- as a prefix here, it seems
a more natural word.  I like the idea of allowing these clauses at the end
of the body to give a notion of stopping after the current iteration.  I
had been wondering how to do that, and hadn't come up with anything nearly
so simple.

On Fri, Sep 14, 2012 at 11:40 AM, Matthew Flatt <mflatt at cs.utah.edu> wrote:

> I think this is a good idea. The technique to implement it is embedded
> in `for/vector' (to handle a vector length), and I can generalize that
> and move it into `for...'.
>
> Also, I think the names `#:while' and `#:until' are too close to
> `#:when' and `#:unless'. I suggest `#:break-when' and `#:break-unless'.
> Compare:
>
>  > (for*/list ([j 2] [i 10] #:when (i . < . 5)) i)
>  '(0 1 2 3 4 0 1 2 3 4)
>  > (for*/list ([j 2] [i 10] #:break-unless (i . < . 5)) i)
>  '(0 1 2 3 4)
>
> I imagine that `#:break-when' and `#:break-unless' are allowed among
> the clauses much like `#:when' and `#:unless', but also allowed at the
> end of the body. Is that what you had in mind?
>
> At Fri, 14 Sep 2012 10:09:52 -0400, Carl Eastlund wrote:
> > I would like the for/... comprehension macros to have #:while and #:until
> > clauses similar to the #:when and #:unless clauses.  I often find I want
> to
> > short-circuit the sequence at some point, but there is no elegant way to
> do
> > it.  I could probably write sequence-while and sequence-until, but I
> don't
> > want to move this condition into the sequence any more than I want to
> write
> > sequence-filter instead of #:when or #:unless.
> >
> > Has this been brought up before?  I can't recall.  Does anyone else run
> > into the same issue?
> >
> > Carl Eastlund
> > _________________________
> >   Racket Developers list:
> >   http://lists.racket-lang.org/dev
>
>
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