On Tue, 5 Apr 2022 at 04:19, Ethan Furman <et...@stoneleaf.us> wrote: > > On 4/3/22 22:39, Chris Angelico wrote: > > On Mon, 4 Apr 2022 at 14:22, Brian McCall wrote: > > >> Related to these questions, there is the question of what to do about > mixed systems? > > Should 2.54 in / 1 cm evaluate to 2.54 in/cm or should it evaluate to 1? > > > > I would say that 2.54in/1cm should be equal to 1. Units should > > completely cancel out. > > It seems like the point of this exercise is to *not* have units cancel out -- > at least, not unnecessarily. > > 2.54in / 1 cm should be 2.54in/cm, otherwise multiplying by 5 kelvin will be > one of those hard-to-find bugs.
Hmm, fair point, I guess. It gets tricky, though. For an example, let's look at fuel efficiency. Outside of the US, vehicle fuel economy is measured in liters per hundred kilometers. What unit category should this be considered to be? 5 L/100km in SI units is 5e-3 m³ / 1e5m. That's 5e-8 m². Or if you prefer, 50mm². Fuel economy is a unit of area. https://what-if.xkcd.com/11/ This DOES, as Munroe points out, have a geometric interpretation. I think it's reasonable to say that fuel economy cancels down to a unit of area, and it's *also* reasonable to say that it doesn't, and that it simply remains as volume-per-distance. Some unit cancellations really do result in pure scalars. The ratio of a circle's circumference to its diameter isn't a unit of m/m any more than the ratio of a circle's area to that of a circumscribed square is a unit of m²/m². They're both just numbers. On the other hand, a radian is a very real unit of distance/distance (based on its definition of arc length), and it's a unit of angle. I suspect that the rules of cancellation would be best handed off to libraries, and there will be different choices for different applications. Maybe at some time in the future, there'll be a proposal to lock it down and define it more by the language. It wouldn't be the first time - type hints are now the only officially supported form of annotations, but the precise meanings of those hints is still partly up to the library. > Of course, it's actually 2.54cm/in. > (I actually didn't even spot that part) ChrisA _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/T6GF6J53KOSO74USIQTFIE23H4G7LR4S/ Code of Conduct: http://python.org/psf/codeofconduct/
