Proposal: Add log1p and expm1 to GHC.Float.Floating

[Casey McCann]

I expect the largest audience involved here are the group that doesn't
know or care about these functions, but definitely wants their code to
work.

As such, I'm opposed to anything that would break code that doesn't
need the benefits of these functions. Two specific scenarios come to
mind:

- Floating instances written for DSLs or AST-like types (the only
common example of Floating instances not already mentioned) needing
implementations of functions their author may not have heard of in
order to compile without warnings. This could be mitigated by good
documentation and providing "defaultFoo" functions suitable for
implementations that don't or can't do anything useful with these
functions anyway.

- Programmers who don't actually need the extra precision using these
functions anyway due to having a vague sense that they're "better".
Yes, in my experience this sort of thing is a common mentality among
programmers. Silently introducing runtime exceptions in this scenario
seems completely unacceptable to my mind and I'm strongly opposed to
any proposal involving that.

As far as I can see, together these rule out any proposal that would
directly add functions to an existing class unless default
implementations no worse than the status quo (in some sense) are
provided.

For default implementations, I would prefer the idea suggested earlier
of a (probably slower) algorithm that does preserve precision rather
than a naive version, if that's feasible. This lives up to the claimed
benefit of higher precision, and as a general rule I feel that any
pitfalls left for the unwary should at worst provide the correct
result more slowly.

Also, of all the people who might be impacted by this, I suspect that
the group who really do need both speed and precision for number
crunching are the most likely to know what they're doing and be aware
of potential pitfalls.

- C.



On Wed, Apr 23, 2014 at 11:55 PM, Edward Kmett <ekmett at gmail.com> wrote:
> Let's try taking a step back here.
>
> There are clearly two different audiences in mind here and that the
> parameters for debate here are too narrow for us to achieve consensus.
>
> Maybe we can try changing the problem a bit and see if we can get there by
> another avenue.
>
> Your audience would wants a claim that these functions do everything in
> their power to preserve accuracy.
>
> My audience wants to be able to opportunistically grab accuracy without
> leaking it into the type and destroying the usability of their libraries for
> the broadest set of users.
>
> I essence here it is your audience is the one seeking an extra
> guarantee/law.
>
> Extra guarantees are the sort of thing one often denotes through a class.
>
> However, putting them in a separate class destroys the utility of this
> proposal for me.
>
> As a straw-man / olive-branch / half-baked idea:
>
> Could we get you what you want by simply making an extra class to indicate
> the claim that the guarantee holds, and get what I want by placing these
> methods in the existing Floating with the defaults?
>
> I rather don't like that solution, but I'm just trying to broaden the scope
> of debate, and at least expose where the fault lines lie in the design
> space, and find a way for us to stop speaking past each other.
>
> -Edward
>
>
> On Wed, Apr 23, 2014 at 11:38 PM, Edward Kmett <ekmett at gmail.com> wrote:
>>
>> On Wed, Apr 23, 2014 at 8:16 PM, John Lato <jwlato at gmail.com> wrote:
>>>
>>> Ah.  Indeed, that was not what I thought you meant.  But the user may not
>>> be compiling DodgyFloat; it may be provided via apt/rpm or similar.
>>
>>
>> That is a fair point.
>>
>>>
>>> Could you clarify one other thing?  Do you think that \x -> log (1+x)
>>> behaves the same as log1p?
>>
>>
>> I believe that \x -> log (1 + x) is a passable approximation of log1p in
>> the absence of a better alternative and that I'd rather have the user get
>> something no worse than they get today if they refactored their code to take
>> advantage of the extra capability we are exposing, than just wind up in a
>> situation where they have to choose between trying to use it because the
>> types say they should be able to call it and getting unexpected bottoms they
>> can't protect against, so that the new functionality can't be used in a way
>> that can be detected at compile time.
>>
>> At this point we're just going around in circles.
>>
>> Under your version of things we put them into a class in a way that
>> everyone has to pay for it, and nobody including me gets to have enough
>> faith that it won't crash when invoked to actually call it.
>>
>> -Edward
>>
>>>
>>> On Wed, Apr 23, 2014 at 5:08 PM, Edward Kmett <ekmett at gmail.com> wrote:
>>>>
>>>> I think you may have interpreted me as saying something I didn't try to
>>>> say.
>>>>
>>>> To clarify, what I was indicating was that during the compilation of
>>>> your 'DodgyFloat' supplying package a bunch of warnings about unimplemented
>>>> methods would scroll by.
>>>>
>>>> -Edward
>>>>
>>>>
>>>> On Wed, Apr 23, 2014 at 8:06 PM, Edward Kmett <ekmett at gmail.com> wrote:
>>>>>
>>>>> This does work.
>>>>>
>>>>> MINIMAL is checked based on the definitions supplied locally in the
>>>>> instance, not based on the total definitions that contribute to the
>>>>> instance.
>>>>>
>>>>> Otherwise we couldn't have the very poster-chid example of this from
>>>>> the documentation for MINIMAL
>>>>>
>>>>> class Eq a where
>>>>>     (==) :: a -> a -> Bool
>>>>>     (/=) :: a -> a -> Bool
>>>>>     x == y = not (x /= y)
>>>>>     x /= y = not (x == y)
>>>>>     {-# MINIMAL (==) | (/=) #-}
>>>>>
>>>>>
>>>>>
>>>>> On Wed, Apr 23, 2014 at 7:57 PM, John Lato <jwlato at gmail.com> wrote:
>>>>>>
>>>>>> There's one part of this alternative proposal I don't understand:
>>>>>>
>>>>>> On Mon, Apr 21, 2014 at 5:04 AM, Edward Kmett <ekmett at gmail.com>
>>>>>> wrote:
>>>>>>>
>>>>>>>
>>>>>>> * If you can compile sans warnings you have nothing to fear. If you
>>>>>>> do get warnings, you can know precisely what types will have degraded back
>>>>>>> to the old precision at compile time, not runtime.
>>>>>>
>>>>>>
>>>>>> I don't understand the mechanism by which this happens (maybe I'm
>>>>>> misunderstanding the MINIMAL pragma?).  If a module has e.g.
>>>>>>
>>>>>> > import DodgyFloat (DodgyFloat) -- defined in a 3rd-party package,
>>>>>> > doesn't implement log1p etc.
>>>>>> >
>>>>>> > x = log1p 1e-10 :: DodgyFloat
>>>>>>
>>>>>> I don't understand why this would generate a warning (i.e. I don't
>>>>>> believe it will generate a warning).  So the user is in the same situation
>>>>>> as with the original proposal.
>>>>>>
>>>>>> John L.
>>>>>>
>>>>>>>
>>>>>>> On Mon, Apr 21, 2014 at 5:24 AM, Aleksey Khudyakov
>>>>>>> <alexey.skladnoy at gmail.com> wrote:
>>>>>>>>
>>>>>>>> On 21 April 2014 09:38, John Lato <jwlato at gmail.com> wrote:
>>>>>>>> > I was just wondering, why not simply numerically robust algorithms
>>>>>>>> > as
>>>>>>>> > defaults for these functions?  No crashes, no errors, no loss of
>>>>>>>> > precision,
>>>>>>>> > everything would just work.  They aren't particularly complicated,
>>>>>>>> > so the
>>>>>>>> > performance should even be reasonable.
>>>>>>>> >
>>>>>>>> I think it's best option. log1p and exp1m come with guarantees
>>>>>>>> about precision. log(1+p) default makes it impossible to depend in
>>>>>>>> such
>>>>>>>> guarantees. They will silenly give wrong answer
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
>
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