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

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 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 :

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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 wrote:

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No objections from me. Looks useful and not too costly in terms of extra complexity.

On 27 February 2017 at 21:34, Iavor Diatchki wrote:

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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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