Unchained VS mp-units

Anything with a finite (200 is small even) number of units misses the algebraic structure of the problem mentioned in https://news.ycombinator.com/item?id=36988497 wherein multiplying|dividing two things gives you a (potentially) new unit which implies an open ended "space" of units.

To be concrete (hah!), in C++ a template meta-type with 12 signed integer parameters (6 numerators & 6 denominators for rational exponents of SI base units) might be one way to model it.

Unlike C++ template stuff, Nim macros (like Lisp macros) makes metaprogramming more like procedural programming - just against abstract syntax trees. I think that helps to shield some of this type complexity from users, but the documentation README https://github.com/SciNim/Unchained does better job than I can in an HN comment.

Of course, for unit system conversion, the number of dimensions (6 in SI, 3 in CGS/Gaussian) changes. So, for full generality you need compile-time (if you want static type integration/CT errors) linear algebra over a rational field (at least & conventionally) to project|inverse project. That might be theoretically possible in C++. I would think it very un-fun and unlikely to ever have been done. There's probably a Mathematica package, though.

I've recently written a units library for Nim [0]. It's still WIP, but it's already proven extremely useful for me as a physicist.

Thanks to Nim's strong type system and metaprogramming features, it allows for a fully compile time design, without any runtime overhead (in form of special unit objects or such things; everything is a `distinct float`).

In addition Nim's unicode support, the code even looks nice!

A more complex use case (I can link more if desired): [1]

[0]: https://github.com/SciNim/Unchained/

[1]: https://github.com/SciNim/Unchained/tree/master/examples

It's not the type system you're complaining about, though. It takes a bit of work but you can write types that include units information with no loss of performance.

I recommend using this as reference https://github.com/mpusz/units it probably solves the same or similar problem.

Yep. And there are some libraries to provide strong-type int, depending on what you need: type_safe or even units.

Indeed you cannot. Then again, you couldn't write a library like fmtlib in C in the first place. I mean why do you complain about C++ features which enable you to write libraries you otherwise couldn't? How would you expect to implement equivalent libraries to EVE or mp-units in C alone?

How does it compare with https://github.com/mpusz/units ?

I don't have a book to recommend, but I have a fantastic library: https://github.com/mpusz/units

Good C++ library for that topic is [0]. You can even go further and combine with something like [1] which is super helpful for kalman filters and other stuff where you have heterogeneous units in one vector.

[0] https://github.com/mpusz/units

This seems to be on a progress to standardisation for c++23/26: https://github.com/mpusz/units