[racket-dev] for loops with interleaved escape continuations

On Jul 1, 2014, at 3:46 PM, Sam Tobin-Hochstadt <samth at cs.indiana.edu> wrote:

> I disagree strongly that this is un-rackety. Consider the following loop:
> 
> (define v ....)
> (let loop ([i 100])
> (define e (vector-ref v i))
> (cond [(zero? i) null]
>           [(= 999 e) null]
>           [(even? e) (loop (add1 i))]
>           [else (cons e (loop add1 i))]))
> 
> I don't think that's un-Rackety.
> 
> Here's that loop with break/continue:
> 
> (for/list ([i (in-range 100 0 -1)])
>  (define e (vector-ref v i))
>  (cond [(= 999 e) (break)]
>           [(even? e) (continue)]
>           [else e]))

You don’t like the non-capturing alternative

(for/list ([i (in-range 100 0 -1)]
           #:continue continue
           #:break break)
 (define e (vector-ref v i))
 (cond [(= 999 e) (break)]
          [(even? e) (continue)]
          [else e]))

?

Sorry, I’m working on implementing hygiene for Rust right now, and I’m kind of knee-jerk in favor of non-capturing….

John

> 
> I don't think that's un-Rackety either.
> 
> Sam
> 
> On Tue, Jul 1, 2014 at 10:59 PM, Neil Van Dyke <neil at neilvandyke.org> wrote:
>> If adding break&continue features to your fancy iteration syntax, I propose
>> that any uses of these features in source code be somehow very prominent.
>> 
>> For example, perhaps there is a keyword that must be at the top of the fancy
>> iteration form, something like
>> "#:enable-continue-here-because-programmer-cannot-be-bothered-to-write-this-in-idiomatic-racket".
>> 
>> This prominence warns readers of the code to be on the lookout for surprise
>> control flow (like they would on the rare occasion that they saw a "call/ec"
>> around a loop).  It also hints to newbie programmers that they are
>> discouraged from simply transliterating syntax from other languages (rather
>> than learning better control flow and structuring techniques).
>> 
>> Neil V.
>> 
>> 
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