[ghc-steering-committee] Proposal: Accept proposal 37, Hex Float literals

[Christopher Allen]

This is a well written proposal. Hex float literals as proposed are
fine and I am in favor of it. Hex notation for floats has pedagogical
benefits as well.

The deviation in the proposal is from the C standard, not the IEEE
standard and essentially brings the proposal into conformance with the
IEEE spec anyway. I'd suggest that we concern ourselves more
exclusively with IEEE compliance than mimicking the C spec.

This is CLC territory but for backwards compatibility we might suggest
that a newtype be offered for re-obtaining the old Read instance for
Double as a temporary compatibility affordance. (2-3 releases?)

Beyond that, this should be good to go.

On Tue, Feb 28, 2017 at 11:58 AM, Joachim Breitner
<mail at joachim-breitner.de> wrote:
> Hi,
>
> I think that's fine, and the benefits outweigh the downsides.
>
> This is treading on CLC space a bit, though. Should we coordinate with them?
>
> Joachim
>
>
>
> Am 28. Februar 2017 09:30:16 PST schrieb Iavor Diatchki <iavor.diatchki at gmail.com>:
>>One tricky bit that occurred to me: we can't really change the behavior
>>of
>>the Read instance depending on the extensions enabled.
>>
>>Thus, we would either have to always parse hex floats, even when the
>>extension is not on, or never parse them.
>>
>>I am leaning towards parsing them always, even without the extension.
>>What
>>do others think?
>>
>>-Iavor
>>
>>
>>
>>On Tue, Feb 28, 2017 at 5:36 AM, Simon Marlow <marlowsd at gmail.com>
>>wrote:
>>
>>> No objections from me.  Looks useful and not too costly in terms of
>>extra
>>> complexity.
>>>
>>> On 27 February 2017 at 21:34, Iavor Diatchki
>><iavor.diatchki at gmail.com>
>>> wrote:
>>>
>>>> Hello,
>>>>
>>>> I was assigned to be the shepherd for the Hex Float proposal  (
>>>> https://github.com/ghc-proposals/ghc-proposals/pull/37), so I would
>>like
>>>> to propose that we accept it for implementation in GHC.
>>>>
>>>> This is a small change, which can be summarized as follows:
>>>>     - allow floating point numbers to be written using hex digits.
>>>>
>>>> The format is exactly the same as decimal floating point numbers,
>>except
>>>> for:
>>>>     - the literals start with 0x
>>>>     - the digits are in hex
>>>>     - the exponent symbol is `p` or `P`, instead of `e` or `E`
>>>>     - the exponent is in base 2, rather than base 10
>>>>
>>>> This notation has become popular among people working with floating
>>point
>>>> numbers, as the numbers you write can be represented exactly, which
>>is not
>>>> the case for base 10 numbers.
>>>>
>>>> The following points were discussed:
>>>>    - the exact format to use, compared to what's allowed by other
>>>> languages: we decided to just follow Haskell's decimal float
>>notation, for
>>>> least surprise
>>>>    - should overflow (which becomes `Inf`) result in a warning?   We
>>>> decided that this is an orthogonal issue, also relevant to decimal
>>floating
>>>> point and made a GHC ticket (#13232)
>>>>     - there is an odd interaction between floating point (both
>>decimal
>>>> and hex) and -XNegativeLiterals, related to negative 0, see ticket
>>#13211
>>>>      - changing the Read instances for Float and Double to recognize
>>hex
>>>> floats could break some programs, although that does not seem all
>>that
>>>> likely
>>>>      - there is a question of how many extra pretty printing
>>functions to
>>>> add to `Numeric`: the current thinking is that maybe just one
>>`showHFloat`
>>>> is sufficient;  the alternative is to add 5, mirroring the
>>>> `show[E,F,G]Float` functions for decimals.
>>>>
>>>> I also had a stab at implementing the basic notation here:
>>>> https://phabricator.haskell.org/D3066
>>>> I haven't done the changes to the libraries yet.
>>>>
>>>> Please let me know if you have any objections or suggestions on what
>>>> might needs to be changed.
>>>>
>>>> Cheers,
>>>> -Iavor
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
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>>>> ghc-steering-committee at haskell.org
>>>>
>>https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>>>>
>>>>
>>>
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-- 
Chris Allen
Currently working on http://haskellbook.com