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

[Ian Kelly]

On Mon, Jul 18, 2016 at 9:55 AM, Marko Rauhamaa <marko at pacujo.net> wrote:
> 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:

Okay, so how is that wavelength defined?

If you needed to mark a meter stick, and all you had was the
definition of c and the second, how would you do it without measuring
anything?