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

[Iavor Diatchki]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-steering-committee/attachments/20170227/f2795a61/attachment.html>