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.toro...@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.ors...@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
____________________
Racket Users list:
http://lists.racket-lang.org/users
____________________
Racket Users list:
http://lists.racket-lang.org/users
____________________
Racket Users list:
http://lists.racket-lang.org/users
____________________
Racket Users list:
http://lists.racket-lang.org/users
