Re: SI Units Checked and Unchecked

["Dan'l Miller" <optikos@verizon.net>]

On Friday, August 10, 2018 at 1:45:47 AM UTC-5, Dmitry A. Kazakov wrote:
> On 2018-08-10 00:12, Dan'l Miller wrote:
> 
> > Actually, I am talking about compile-time checks within Ada source code for a gauge as would be utilized on the GUI control screen of a chemical plant or a nuclear reactor, not some toy string-processing app that is the analogue of GNU Units.
> 
> As I explained this design is technically not viable.

in today's Ada.

> Even taking an 
> insanely huge overhead of making all components generic with measurement 
> unit as a formal parameter, that would be less safe than a design based 
> on constraints. With generics you can only pass same actual parameter or 
> use a generic instance as a parameter, you cannot constraint generic 
> instances in a way the algebra of measurement units works, e.g. length 
> divided to time gives speed. So you will end up with generic instances 
> for each particular unit but no mechanism to combine instances (of 
> different generic types) in a unit-safe way. It will be in effect 
> *untyped*. (Which is does not really matter since internal mechanisms 
> like queuing of events, callbacks etc cannot be made generic anyway.)
> 
> Compile-time unit checks can only be used in some numeric problems, for 
> instance in the control loop. Everywhere else checks are run-time, which 
> in case of widgets does not mean any real safety loss because 
> practically all these checks are done upon initialization. When you 
> create a gauge you connect a signal from a slider widget to it. Doing so 
> the units of the gauge and of the slider's signal get checked. That 
> happens once before the system starts. If you use a GUI design tool then 
> this check could be performed even earlier before deployment. Chances of 
> getting a run-time check fail are zero.
> 
> >> The scalars are not unitless. The unit is a discriminant:
> >>
> >>      http://www.dmitry-kazakov.de/ada/units.htm
> > 
> > It looks like a string-processing library to support a toy units converter app,
> 
> No string processing involved. It is same as saying that Integer is 
> string processing because there exists Integer'Image.
> 
> -- 
> Regards,
> Dmitry A. Kazakov
> http://www.dmitry-kazakov.de