Function to simplify units, e.g. m/ft -> dimensionless

[mdavezac]

@ajkeller34, unless I'm mistaken, the following functionality (simplify) does not yet exist. Happy to try and implement it if it doesn't:

notsosimple = 1u"m/ft*kg"
simple = simplify(notsosimple)
@assert units(simple) === units(1u"kg")

It should be possible to determine which unit to use of m or ft, the same way it is done for +, viapromote_type

[ajkeller34]

There's this:

julia> notsosimple = 1u"m/ft*kg"
1 ft^-1 kg m

julia> upreferred(ans)
1250//381 kg

This may not be what you want though, since the result would be in kg even if the mass unit in the first expression were a different unit. What this does is look at the dimensions of the quantity, and for each power of length, have that many powers of a meter, etc., where the dimension-to-unit mapping is given by the @preferunit macro in the defaults. So this basically is a "convert to base SI units" function given what is usually in deps/Defaults.jl.

If you have different functionality in mind, how would you simplify when you have units of mixed dimensions like joules where it's not just length or mass or time but a combination?

[mdavezac]

I guess I'm thinking of a function that would simplify units that have the exact same dimensions and leave those that partially overlap. More complex conversions would happen in #35 (for which upreferred would be one instance). I might trial an implementation so we have something concrete to discuss.