Re: [Caml-list] string_of_float (0.1 +. 0.2)

[Gabriel Scherer <gabriel.scherer@gmail.com>]

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Nathanaëlle Courant, with help from Julien Lepiller and myself, wrote a
rather complete interpreter/evaluator for OCaml parsetrees (in OCaml) as
part of the Camlboot project

  https://arxiv.org/abs/2202.09231
  https://github.com/Ekdohibs/camlboot
  https://github.com/Ekdohibs/camlboot/tree/master/interpreter

We take as input OCaml parsetrees, but we evaluate into a type of "value"
that we defined ourselves, and stores a "float" for floating-point numbers:

https://github.com/Ekdohibs/camlboot/blob/2692b14a4e685387194556e511fe23057d25c6c3/interpreter/data.ml#L46-L66

type value =
...
| Float of float
...

Kenichi, I don´t understand what your own constraints, but in general I
have the impression that "float" is better than "string" to represent
double values used for computation. "string" was meant to accurately
represent the source value and avoid any serialization/portability issue,
but those constraints are rather for data exchange.

On Wed, Jun 15, 2022 at 4:00 AM Kenichi Asai <asai@is.ocha.ac.jp> wrote:

> On OCaml 4.12.0 on M1 mac, I got:
>
> # 0.1 +. 0.2;;
> - : float = 0.300000000000000044
> # string_of_float (0.1 +. 0.2);;
> - : string = "0.3"
>
> Why don't I obtain "0.300000000000000044" here?
>
> Here is some background.  I am writing an OCaml interpreter that
> mimics most part of the original OCaml interpreter.  In the OCaml
> interpreter, 0.1 and 0.2 are represented as
>
> Pexp_constant (Pconst_float ("0.1", None))
> Pexp_constant (Pconst_float ("0.2", None))
>
> When I add these two numbers, I would have to execute
>
> let a = float_of_string "0.1"
> let b = float_of_string "0.2"
> let c = a +. b
> let d = string_of_float c
>
> and then return
>
> Pexp_constant (Pconst_float (d, None))
>
> At this point, however, since d is "0.3" instead of
> "0.300000000000000044" (even though c is 0.300000000000000044), I
> cannot return 0.300000000000000044 as a result.  How can I mimic the
> OCaml behavior?
>
> Sincerely,
>
> --
> Kenichi Asai
>

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