Proposal: Adding units to Mimi

[jrising]

I think units are really important in building and validating models. In the next few weeks, I propose that I will add a unit system to Mimi, which will be optional unless it's required. I see that the MetaVariable and MetaParameter types have a units field, but I don't think they're in current use and I propose some changes.

The most advanced version of this would do unit conversion, and understand how arithmetic changes units. I want to put in something simpler, with some planning to help it be replaced later without too much trouble.

Parameters and variables will take an optional argument, units, which will be of any type; the typical case is that it would be a string. The units would be stored in a field in the MetaVariable and MetaParameter types-- so I would just change these from UTF8String to Any. The Any type will allow conversion-aware units to be handled later, using the overloaded unitcheck and unitconvert functions.

If a component is connected through a parameter or variable that has units, it will be required that the other side also have units and that these units match. connectparameter would make this check.

Similarly, setparameter for united parameters would require that the value is of type UnitedObject, which is just a wrapper around both a value::Any and a units::Any, and that the units match.

To support future work, the checks between components will use a unitcheck(Any, Any) function. Eventually, I want to have a system that would automatically create a whole component in between two connected components, if that's what's necessary to connect two components.

When using setparameter, the UnitedObject would be passed to a unitconvert(UnitedObject, unit::Any) function, which returns the raw data (possibly in a converted form). In version 1, unitconvert will just call unitcheck and then return value.

[davidanthoff]

Yes, I agree, we should add this!

A few random points:

• Might https://github.com/ajkeller34/Unitful.jl help? I only looked over it briefly, it looks neat, but I'm not sure whether this would fit our scenarios and could be used here...
• I'll have to mull a bit over the UnitedObject. This plays into the general parameter handling story. I'm not happy how that is done in general right now, but haven't really figured out a good alternative. Would be good to discuss this broader question.
• I also have a broader redesign of how models are connected in mind that would be good to discuss. I do think it would mostly work with the whole unit idea.
• We might think about a generic mechanism to "insert" conversion functions between components, I think that might in general be very useful, and maybe whatever we come up with could also be used for a automatic unit conversion step...

[jrising]

The Unitful package is just the sort of thing I would want to use. I'm worried though about runtime costs. Also, bizarrely, the Unitful documentation says nothing about arrays. I think we should not wed Mimi to any particular unit system.

[delgadom]

@jrising @davidanthoff Unitful includes support for arrays (now referenced in the docs here http://ajkeller34.github.io/Unitful.jl/stable/manipulations/). They claim to have a very small performance penalty.

[lrennels]

Let's bump this towards top of discussion as it would be a well-contained, but valuable, contribution. We also see interest on the forum here: https://forum.mimiframework.org/t/mimi-and-unitful/182/2