Dear Viet,

I am not sure this is an issue with OCaml (as you can verify using your
favourite C compiler).  Rather, I think IEEE 754 double-precision binary
floating-point numbers can only represent numbers between 10^{-308} and
10^308 with full decimal digits precision. Numbers smaller than that can
only be represented with reduced precision.

Best wishes,
Nicolas

On Tue, Aug 1, 2017 at 12:47 PM, Viet Le <vietlq85@gmail.com> wrote:

> Hi all,
>
> I'm writing a JSON parser in pure OCaml and have encountered an issue with
> small float values:
>
> 10 zeros in between
> # 1.00000000001e-312 ;;
> - : float = 1.00000000001e-312
>
> 11 zeros in between
> # 1.000000000001e-312 ;;
> - : float = 1.00000000000341e-312
>
> # 5e-324 ;;
> - : float = 4.94065645841e-324
>
>
> I haven't found precise limit, but as a rule of thumb (*not precise*),
> for a positive float value to keep its precision, it *should* not be
> smaller than 1.00000000001e-312. To use JSON as precise serializer, it
> would be necessary to preserve accuracy.
>
> I checked https://github.com/ocaml/Zarith and it supports only big int &
> quotients, not floating point.
>
> For values smaller than the limit above, should I just treat as 2 values:
> (normalized: float, exponent: float), so we will have:
>
> 5e-324 -> (5, -324)
>
> Comments and suggestions are appreciated. Thanks.
>
> --
> Kind regards,
> Viet
>