What exactly is "exact" (was Clean Singleton Docstrings)

[Marko Rauhamaa]

Marko Rauhamaa <marko at pacujo.net>:

> Ian Kelly <ian.g.kelly at gmail.com>:
>> Off-topic, c being a fundamental constant is actually in the latter
>> category. Its *exact* value is 299792458 m/s.
>>
>> The length of the meter, on the other hand, is defined as the distance
>> traveled by light in a vacuum in 1/299792458 seconds and is subject to
>> the precision of measurements.
>
> Since both c and the second are exact magnitudes, so is the meter.
>
>    The second [...] is quantitatively defined in terms of exactly
>    9,192,631,770 periods of a certain frequency of radiation from the
>    caesium atom: a so-called atomic clock.
>    <URL: https://en.wikipedia.org/wiki/Second>

Which would became immediately apparent if you programmed in Scheme. One
meter is equal to the wavelength of said magnitude times:

   9192631770/299792458
   ==> 656616555/21413747
   (exact? 9192631770/299792458)
   ==> #t


Marko