Feature request: dimension helper functions?

[icweaver]

Hi Miles, just following up on my evil master plan for JuliaAstro, would you be open to a PR adding some convenience is<dimension> functions?

I'm not sure if this has been discussed somewhere already, but we're basically looking for a convenient way to dispatch on dimensions, e.g.,

julia> using DynamicQuantities

julia> x = 3u"m"
3.0 m

julia> v = 3u"m/s"
3.0 m s⁻¹

julia> function do_stuff(q::UnionAbstractQuantity)
           if islength(q)
               # <do length stuff>
           elseif isspeed(q)
               # <do speed stuff>
           # ...
           else
           end
       end

I see that this could be done by factoring out some of the dimension display logic here, to something like:

julia> function islength(q::UnionAbstractQuantity)
           d = dimension(q)
           dd = filter(k -> !iszero(d[k]), keys(d))
           return dd == (:length,) && d.length == 1
       end
islength (generic function with 1 method)

julia> function isspeed(q::UnionAbstractQuantity)
           d = dimension(q)
           dd = filter(k -> !iszero(d[k]), keys(d))
           return dd == (:length, :time) && d.length == 1 && d.time == -1
       end
isspeed (generic function with 1 method)

julia> islength(3u"m")
true

julia> isspeed(3u"m/s")
true

Alternatively, would this make sense as a trait that could be dispatched on at the type level, or would this run counter to the dynamic runtime design here?

[MilesCranmer]

Thanks for the idea, can you say more about what the use case is and why existing tools won't cut it