[racket-dev] Short-circuiting comprehensions

From: J. Ian Johnson (ianj at ccs.neu.edu)
Date: Fri Sep 14 12:17:07 EDT 2012

I've been using let/ec for this same functionality, and it's made me sad.
----- Original Message -----
From: "Carl Eastlund" <cce at ccs.neu.edu>
To: "Matthew Flatt" <mflatt at cs.utah.edu>
Cc: "PLT Developers" <dev at lists.racket-lang.org>
Sent: Friday, September 14, 2012 11:49:20 AM GMT -05:00 US/Canada Eastern
Subject: Re: [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'. 

> (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 
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