Floats, the true ieee next generation Re: Add Ord Laws to next Haskell Report

[David Feuer]

No, no, no. Int and Word are *rings*, which let's us apply a ton of
mathematical reasoning to their arithmetic. Trapping overflow would throw
all that completely out the window. If you want to trap overflow, please
use different types!

On Fri, Feb 8, 2019, 2:07 PM Lennart Augustsson <lennart at augustsson.net
wrote:

> I would *hate* to lose quiet NaNs.  They can be very useful.  But I’d be
> fine having them as a separate type.
>
> And while we’re at it, why not make Int overflow and underflow cause a
> trap as well?  With a different type if you want to wrap.
>
>
> On Fri, Feb 8, 2019 at 08:34 Carter Schonwald <carter.schonwald at gmail.com>
> wrote:
>
>> Thanks for eloquently summarizing , better than I would , what I thought
>> I had laid out.
>>
>> Ieee floating point has fantastic hardware support .  May as well be the
>> first real language to actually use it correctly. :)
>>
>> On Fri, Feb 8, 2019 at 5:21 AM Merijn Verstraaten <merijn at inconsistent.nl>
>> wrote:
>>
>>>
>>>
>>> > On 8 Feb 2019, at 10:57, Sven Panne <svenpanne at gmail.com> wrote:
>>> >
>>> > Am Do., 7. Feb. 2019 um 23:31 Uhr schrieb Merijn Verstraaten <
>>> merijn at inconsistent.nl>:
>>> > Our goal is to make "compare NaN n" impossible to happen. [...]
>>> >
>>> > Well, what is supposed to happen then when you *do* see a NaN, e.g.
>>> one produced from a foreign call? You *will* see NaNs in Haskell if you
>>> interact with other languages, most of them take a far less religious
>>> approach to floating points calculations.
>>>
>>> This is not true. As Carter pointed out we can setup the CPU to trap
>>> NaNs *even in foreign calls*. So, in theory we CAN rule this out safely.
>>> Doing this we can simply convert the trap into an exception at the FFI
>>> boundary.
>>>
>>> Now, there are cases were this is problematic, so as said before we will
>>> probably need to allow people to optionally switch on 'value NaNs', because
>>> the foreign code isn't exception safe or for other reasons, but this is
>>> manageable. Via, for example having an annotation on foreign imports
>>> whether you want to trap or not.
>>>
>>> In the scenario where someone switches to value NaNs, we are *still* not
>>> worse off than we are now. The things you suggest already happen *now*, so
>>> the only thing we're advocating is making it possible to have more sane
>>> behaviour in the future.
>>>
>>> Any IEEE-754 compliant implementation of Double that doesn't use
>>> trapping NaN can, by definition, never ever be a sane implementation of
>>> Ord. As IEEE-754 *requires* "NaN /= NaN", so equality symmetry doesn't
>>> apply to NaNs and there is *no* safe way to sort/order data containing NaNs.
>>>
>>> I've run into several nasty issues of trying to sort lists containing
>>> NaNs (not just Haskell, also Python and C) and it's *not* just the NaNs
>>> that are affected, entire subsequences end up getting sorted wrong based on
>>> the comparison with NaN and you end up with completely garbled and unsorted
>>> data.
>>>
>>> In other words, there are only two ways to get sane behaviour from
>>> Double with regards to ordering:
>>>
>>> 1. Trapping NaN represenation
>>> 2. Deviate from IEEE-754 semantics
>>>
>>> To me, option 2 is out of the question, it's the one consistent thing
>>> across language we have when it comes to floating point. I understand that
>>> *always* using trap representation isn't feasible, but allowing people to
>>> optionally switch to value NaNs leaves us no worse off than we are *right
>>> now*, and per above, there is literally no way to improve the situation wrt
>>> value NaNs without sacrificing IEEE-754 compliance.
>>>
>>> Cheers,
>>> Merijn
>>> _______________________________________________
>>> Libraries mailing list
>>> Libraries at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>>
>> _______________________________________________
>> Libraries mailing list
>> Libraries at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20190208/dfa3eca4/attachment.html>