[racket-dev] for loops with interleaved escape continuations

On Wed, Jul 2, 2014 at 12:52 AM, John Clements
<clements at brinckerhoff.org> wrote:
>
> 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 meant to write the hygenic version that you provide.

Sam

>
> 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.
>>>
>>>
>>> _________________________
>>>  Racket Developers list:
>>>  http://lists.racket-lang.org/dev
>>>
>> _________________________
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>>  http://lists.racket-lang.org/dev
>