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

[Oleg <oleg@okmij.org>]

Actually the similar problem of accurately conveying floats also
occurs in MetaOCaml/Code generation. After all, what you are doing is
a sort of reflection.

First of all, accurate (lossless) printing of floats is a research
area in itself. The latest result is

  http://www.cs.tufts.edu/~nr/cs257/archive/florian-loitsch/printf.pdf
  https://github.com/google/double-conversion

Perhaps some day it can be incorporated in OCaml, so that
string_of_float truly returns *the* printable representation of a float.

Hexademical printout (which is also supported in C) 
mentioned by Daniel Buenzli is another way -- provided we don't
actually have to look at the printed value, because we probably won't
understand it anyway.

In practice in our recent project, we settled on

let float : float -> float cde = fun x ->
  let str = if Float.is_integer x then string_of_float x else
            Printf.sprintf "%.17g" x

which seems to work well. At least, it solved the problems when the
results of our generated signal processing code differed slightly from
the results of the hand-written reference C code, due to slightly
different printed FP values in FP array initializers (filter
coefficients). We used to use string_of_float back then. With the
above float, the problem is solved.

I have to add that float above does not account for NaN, plus/minus
infinity and -0. So, the fully production code also has to add these
cases.