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@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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