Proposal: modify `Read` instances for `Float` and `Double`

[Carter Schonwald]

As a member of the prime committee I would support adding hex floats to the
next standard.  I'm not current on the related Unicode topics mind you :)
On Sat, Mar 4, 2017 at 2:10 AM Edward Kmett <ekmett at gmail.com> wrote:

> This does bring up portability concerns and would cause further divergence
> of Read from the language standard. If not handled carefully, this drags us
> in an ever more implementation-defined rather than specification-defined
> direction.
>
> As a data point for this discussion, a similar proposal to extend the Read
> syntax to add support BinaryLiterals was rejected over portability and
> silent behavioral change concerns.
>
> https://ghc.haskell.org/trac/ghc/ticket/10092
>
> Whatever we do here, we may well want to be consistent with how we treat
> both of these proposals.
>
> If we do choose to accept this, we may well need to back and re-tackle
> #10092.
>
> Currently, we do have at least one chink in the armor, in that Read is
> currently more liberal in what it will accept Unicode-wise than what the
> language specification states as a result of
>
> https://ghc.haskell.org/trac/ghc/ticket/10444
>
> I do think that whatever we do here, it should involve a conscious
> decision to either stick to the current report, or diverge from the current
> report and then to revise this part of the report.
>
> If we can get the Haskell Prime folks to fix the language report to
> include them in the next language standard (if by default, even better!)
> then I'm fully +1. I'm also fully on board with both these and binary
> literals going into the language standard.
>
> If we're doing this entirely on our own in the spirit of "being liberal in
> what you accept and conservative in what you output" then I'm personally
> far more dubious of the merits of that approach in practice, and will wait
> to weigh in from a CLC perspective until more feedback is in place.
>
> -Edward
>
> On Fri, Mar 3, 2017 at 5:41 PM, Carter Schonwald <
> carter.schonwald at gmail.com> wrote:
>
> Plus one from me
>
> Also this is actually more ieee compliant than the c standards spec
> because we don't need suffixes on literals :)
>
> On Tue, Feb 28, 2017 at 8:20 PM Iavor Diatchki <iavor.diatchki at gmail.com>
> wrote:
>
> To me, the notation makes sense if you think of the binary representation
> of the number: each hex digit is 4 bits, and the base 2 exponent allows you
> to move the decimal point by one bit.  I would guess that the exponent is
> written in base 10, because that's easier for most people to understand,
> and its bit-pattern representation is not all that important.
>
>
>
> On Tue, Feb 28, 2017 at 5:07 PM, Levent Erkok <erkokl at gmail.com> wrote:
>
> Henning:
>
> Indeed, the proposal follows the description in p57-58 of
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf, which dates
> back to 2007. (Some Haskell related deviations do exist, like dropping the
> final suffix, since Haskell doesn't need it; and requiring digits both
> before and after the dot.)
>
> I think of the format as precisely representing the value "mantissa x
> 2^exp"; where the mantissa is written in hexadecimal, and the exponent is
> left as a regular decimal integer. The discrepancy is rather weird, but I
> guess it made more sense when the standard was drafted. More importantly,
> all the other languages (C, Java, Python:
> http://www.exploringbinary.com/hexadecimal-floating-point-constants/)
> follow this convention as well; so it would be unfortunate if Haskell
> diverged.
>
> For the change in semantics for "reads:" That is indeed unfortunate since
> we lose backwards compatibility. But it's a very minor one and I would be
> curious if anyone depended on the existing semantics for any legitimate
> reason. I personally do not see any issues with it.
>
> -Levent.
>
> On Tue, Feb 28, 2017 at 11:20 AM, Henning Thielemann <
> lemming at henning-thielemann.de> wrote:
>
>
> On Tue, 28 Feb 2017, Iavor Diatchki wrote:
>
> This may affect existing programs---although it doesn't seem very likely.
> Here is an example:
>
> current behavior:
>
> reads "0x10p10" = [(16.0,"p10")]
>
> new behavior:
>
> reads "0x10p10" = [(16384,"")]
>
>
>
> "p" refers to a power of two and the exponent is written in decimal for a
> hexadecimal mantissa. Looks pretty confusing to me but it seems that the
> standard was made somewhen before this proposal.
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