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

[Levent Erkok]

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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