[racket] Plot is voracious

I don't think there is a problem with adding that exception, except
that it seems pretty laborious. And probably someone (and I can't
think of anyone better than you to do this, Neil!) may find that there
are actually 2 or 3 exns that are the right set.

I think that if you wanted to take this on, Racketeers everywhere
would thank you.

Robby

On Wed, May 21, 2014 at 1:05 PM, Neil Toronto <neil.toronto at gmail.com> wrote:
> Based on experience using Plot, I believe allowing all errors is a terrible
> default.
>
> Jay's suggestion would make it easier to ignore only some errors, especially
> if it were a parameter. I could see doing
>
>   (plot-swallow-exception? exn:fail:contract:divide-by-zero?)
>
> To make it as backwards-compatible as possible while being precise, it would
> be nice if it could swallow only math domain errors by default. But again,
> there's no way to distinguish those from other errors.
>
> Is there a problem with adding `exn:fail:contract:math` and making
> `exn:fail:contract:divide-by-zero` a subtype?
>
> Neil ⊥
>
>
> On 05/21/2014 11:25 AM, Jay McCarthy wrote:
>>
>> Or
>>
>> #:catch-these-exns-plot! [catch-exn? void]
>>
>> Jay
>>
>> On Wed, May 21, 2014 at 11:22 AM, Robby Findler
>> <robby at eecs.northwestern.edu> wrote:
>>>
>>> How about adding yet another argument to plot #:errors-go-to-nan or
>>> something like that, such that plot's default behavior is to let the
>>> errors thru, but that, if you turn that flag on, then it just swallows
>>> all errors?
>>>
>>> Robby
>>>
>>> On Wed, May 21, 2014 at 12:18 PM, Neil Toronto <neil.toronto at gmail.com>
>>> wrote:
>>>>
>>>> Division by zero and other math domain errors, primarily. There are two
>>>> good
>>>> reasons to ignore these.
>>>>
>>>> 1. Plotting is for visualizing functions. Users shouldn't have to ensure
>>>> to
>>>> Plot that the function they're plotting is total when all they want is
>>>> to
>>>> *see* it, not use it for some critical calculation.
>>>>
>>>> More concretely, raising all exceptions means that this:
>>>>
>>>>    (plot3d (surface3d / -1 1 -1 1))
>>>>
>>>> has to be this instead:
>>>>
>>>>    (plot3d (surface3d
>>>>             (λ (x y) (with-handlers ([exn?  (λ (e) +nan.0)])
>>>>                        (/ x y)))
>>>>             -1 1 -1 1))
>>>>
>>>> or it has to use an explicit domain test (i.e. test (= y 0)). That's a
>>>> lot
>>>> of work just to visualize the division function.
>>>>
>>>> 2. Plotting necessarily approximates. Plot's behavior should be as
>>>> consistent as possible regardless of exactly how it approximates.
>>>>
>>>> Currently, plotting the division function in any subdomain or with any
>>>> sampling interval just works. Suppose Plot allowed errors to be raised.
>>>> Then
>>>> this would plot just fine:
>>>>
>>>>    (plot3d (surface3d / -2 1 -2 1))
>>>>
>>>> because sampling 41 points (the default) within [-2,1] skips over 0. But
>>>> this would raise an error:
>>>>
>>>>    (plot3d (list (surface3d / -2 1 -2 1)
>>>>                  (surface3d + -1 2 -1 2)))
>>>>
>>>> because the bounds are extended to [-2,2]x[-2,2] accommodate the `+`
>>>> surface
>>>> renderer, so 0 would be sampled.
>>>>
>>>> ----
>>>>
>>>> In a sense, what I really want is impossible. With any
>>>> exception-swallowing
>>>> rule I make, someone could write a function that raises an exception
>>>> that
>>>> Plot eats when it shouldn't, or re-raises when it shouldn't. Someone
>>>> could
>>>> try plotting a function that, for some reason, tries to compute (gamma
>>>> 0) if
>>>> a `vector-ref` goes out of bounds.
>>>>
>>>> But I think it could behave properly in the most common cases if it were
>>>> possible to tell the difference between the errors raised by (gamma 0)
>>>> and
>>>> (vector-ref vs (length vs)). Right now, there's no way to do that.
>>>>
>>>> Neil ⊥
>>>>
>>>>
>>>> On 05/21/2014 10:16 AM, Robby Findler wrote:
>>>>>
>>>>>
>>>>> I meant to suggest that plot should raise any error that a function it
>>>>> calls raises. Is there something wrong with doing that?
>>>>>
>>>>> Robby
>>>>>
>>>>> On Wed, May 21, 2014 at 11:09 AM, Neil Toronto <neil.toronto at gmail.com>
>>>>> wrote:
>>>>>>
>>>>>>
>>>>>> Referencing an identifier before its definition raises an
>>>>>> `exn:fail:contract:variable`. So that would put Plot in the weird
>>>>>> position
>>>>>> of having to distinguish different kinds of contract errors, but not
>>>>>> by
>>>>>> using subtypes or any other simple rule.
>>>>>>
>>>>>> Maybe we should have an `exn:fail:contract:math` or
>>>>>> `exn:fail:contract:domain` or `exn:fail:contract:argument` and make
>>>>>> `exn:fail:contract:divide-by-zero` a subtype of it. I'd gladly change
>>>>>> Plot
>>>>>> to ignore just those, and change the math library to raise more
>>>>>> specific
>>>>>> errors.
>>>>>>
>>>>>> (log 0) raises an `exn:fail:contract:divide-by-zero`, which is a
>>>>>> terminology
>>>>>> stretch that suggests we need a bit of refactoring anyway.
>>>>>>
>>>>>> Neil ⊥
>>>>>>
>>>>>>
>>>>>> On 05/21/2014 09:51 AM, Robby Findler wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> IMO, plot should raise that exception.
>>>>>>>
>>>>>>> Robby
>>>>>>>
>>>>>>> On Wed, May 21, 2014 at 10:37 AM, Laurent <laurent.orseau at gmail.com>
>>>>>>> wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> ... it eats everything! (particularly exceptions)
>>>>>>>> For example, it is perfectly happy with the following:
>>>>>>>>
>>>>>>>> % racket
>>>>>>>> Welcome to Racket v6.0.1.7.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> (require plot)
>>>>>>>>> (plot (function (lambda(x)(+ x n)))
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>      #:x-min 0 #:x-max 10 #:y-min 0 #:y-max 10)
>>>>>>>> (object:2d-plot-snip% ...)
>>>>>>>>
>>>>>>>> ... although n is clearly undefined. Presumably this is to avoid
>>>>>>>> breaking
>>>>>>>> on
>>>>>>>> math errors like `(/ 0)` ?
>>>>>>>>
>>>>>>>> Would it make sense to have an `exn:fail:arithmetic` exception
>>>>>>>> struct
>>>>>>>> so
>>>>>>>> that `function` would only catch those?
>>>>>>>>
>>>>>>>> Laurent
>>>>>>>>
>>>>>>>>
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>>
>>
>