float_to_list vs io:format ~w

[Viktor Söderqvist]

Thanks Thomas! That's a very useful answer.

I know I'd seen this topic somewhere but I couldn't find it. It was 
probably your PR I'd seen.

Cheers,
Viktor

PS. @zxq9 Yes, IEEE floats are a mess. Posits[1] is an interesting 
alternative but I think it'll take five decades before they replace IEEE 
floats, if ever.

[1]: https://www.youtube.com/watch?v=aP0Y1uAA-2Y


On 2021-06-28 11:33, Thomas Depierre wrote:
> Short answer: because the BIF use the libc sprintf, which does not 
> support the format offered by io:format
> Slightly more complex answer: it does not yet, but it is coming, PR at 
> https://github.com/erlang/otp/pull/4719 
> <https://github.com/erlang/otp/pull/4719>
> 
> Long answer:
> The io:format format is known as "shortest round-trip conversion". It 
> has the advantage of always giving the shortest string while keeping 
> perfect precision, making it both better for external data exchange 
> (less bytes used) and for human readability.
> It has historically been a problem. We had no fast algorithm to generate 
> that format, even less an algorithm that does not need arbitrarily large 
> integers. So it is not implemented in the libcs anywhere, as it was 
> really complex and expensive. It is not in the C spec. We only have %g 
> which is... its own mess.
> 
> This changed a few years ago thanks to Ryu, an algorithm presented by 
> Ulf Adams in 2018. https://github.com/ulfjack/ryu 
> <https://github.com/ulfjack/ryu>
> I have been working for 9 months bringing Ryu to OTP, which is slowly 
> happening. Hopefully before OTP 25.
> 
> Getting it in the libc will probably take a few decades, seeing how much 
> dev cycle they get for the amount of legacy they deal with.
> 
> Hope that answer the question ? Always available if someone want to dig 
> deeper.
> 
> Thomas Depierre
> Schedule a meeting with me 
> <https://calendar.x.ai/di4na?utm_medium=email&utm_source=user-signature>
> 
> 
> On Mon, 28 Jun 2021 at 11:23, Viktor Söderqvist <viktor@REDACTED 
> <mailto:viktor@REDACTED>> wrote:
> 
>     Hi!
> 
>     Apparently, float_to_binary/1 and float_to_list/1 return a different
>     textual representation than io:format("~w", [Float]) and the shell.
> 
>           Eshell V11.1.8  (abort with ^G)
>           1> float_to_list(10.2).
>           "1.01999999999999992895e+01"
>           2> 1.01999999999999992895e+01.
>           10.2
>           3> io:format("~w~n", [10.2]).
>           10.2
>           ok
>           4> 1.01999999999999992895e+01 =:= 10.2.
>           true
> 
>     Apparently, both are correct textual representations of the same
>     floating point value. Why don't they use the same algorithm?
> 
>     Viktor
>