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

[Iavor Diatchki]

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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>>
>
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