[racket] Inconsistency of `in-range` and numerical issues

Should we consider making in-range signal an error for floats? I guess
backwards compatibility says "no" but maybe in a future racket? With a note
in the error message that points people in the right direction?

Robby

On Friday, February 27, 2015, Konrad Hinsen <konrad.hinsen at fastmail.net>
wrote:

> On 26/02/2015 15:13, Neil Toronto wrote:
>
>  The second alternative is a little harder to get right because of
>> fencepost errors [1]. Fortunately, Racket has a library function for it.
>> Unfortunately, it's buried in `plot/utils`. Here it is in action:
>>
>>  > (require (only-in plot/utils linear-seq))
>>
>>  > (linear-seq 0.0 1.0 4)
>> '(0.0 0.3333333333333333 0.6666666666666666 1.0)
>>
>
> That's the best solution in my opinion.
>
>  I should really move this function into `math/base`.
>>
>
> Yes!
>
>  If you must use a flonum step size, do something like this:
>>
>>    (define (flonum-range start end step)
>>      (define n (exact-ceiling (/ (- end start) step)))
>>      (for/list ([i  (in-range 0 n)])
>>        (+ start (* i step))))
>>
>> To get points with 0.5 ulp error each, which is the best you can do, add
>> (require math/flonum) to your program and change the loop body to (flfma
>> step (fl i) start).
>>
>> Arguably, `in-range` should do something like the above when given
>> floating-point step lengths. I don't know how feasible that is, though.
>>
>
> It would obviously break backward compatibility. And it would complicate
> the rather simple mental model one can have of in-range.
>
> More fundamentally, I am not sure it would be much of an advantage. Think
> of the two kinds of users:
>
> 1) Those who are not aware of the problem. They will still fall into the
> trap, only a bit later, and the cause of the problem will be even more
> hidden. It's a bit like languages that do decimal rounding of float output.
>
> 2) Those who are aware of the problem. They would use linear-seq, but
> might prefer the modified in-range in specific situations for compactness.
>
> I suspect group 1) is the bigger one, so the most important measure is to
> promote linear-seq instead of in-range for floats.
>
> Konrad.
>
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