[Python-ideas] Units in type hints

[Steven D'Aprano]

On Thu, May 14, 2015 at 02:03:39PM +0300, Koos Zevenhoven wrote:
> Hi all,
> 
> How about extending the type annotations for int, float and complex to 
> optionally include also a unit?

I really, really like the idea of having unit-aware calculations.

But this is not the way to do it. See below:


> For instance,
> 
>     def sleep(duration : Float['s']):
>         ...
> 
> Now the type checker could catch the error of trying to pass the sleep 
> duration in milliseconds, Float['ms']. 

But that's not an error. Calling sleep(weight_in_kilograms) is an error. 
But calling sleep(milliseconds(1000)) should be the same as calling 
sleep(seconds(1)). If the user has to do the conversion themselves, 
that's a source of error:

sleep(time_in_milliseconds / 1000)  # convert to seconds

If you think that's too obvious an error for anyone to make, (1) you're 
wrong, I've made that error, yes even that simple, and (2) you should 
try it with more complex sets of units. How many pound-foot per minute 
squared in a newton?

Having the language support unit calculations is not just to catch the 
wrong dimensions (passing a weight where a time is needed), but to 
manage unit conversions automatically without the user being responsible 
for getting the conversion right. A type checker is the wrong tool for 
the job.

If you want to see what a good unit-aware language should be capable of,  
check out:

- Frink: https://futureboy.us/frinkdocs/

- the HP-28 and HP-48 series of calculators;

- the Unix/Linux "units" utility.

There are also some existing Python libraries which do unit 
calculations. You should look into them.



-- 
Steve