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

[Carter Schonwald]

I’m not sure if they currently have full ring structure , but  I do agree
that trapping and non trapping int and word are useful.

Simple example where all the finite signed ints work wrong today :

There’s no proper additive inverse for minBound :: int

Likewise , what’s our current definition of negate on finite word types?

On Fri, Feb 8, 2019 at 2:12 PM David Feuer <david.feuer at gmail.com> wrote:

> 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
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