Immutable symbolic unit

[t-bltg]

I have a code computing an estimation of the glomerular filtration rate where the results for the eDFG are adjusted for body surface area (see https://en.wikipedia.org/wiki/Glomerular_filtration_rate#Normal_ranges) resulting in the custom unit mL/min/1.73m².

julia> using DynamicQuantities
julia> eDFG = 60us"mL/min/1.73m^2"
34.68208092485549 m⁻² min⁻¹ mL  # ← current output (not what I want)
60.0 m⁻² min⁻¹ mL 1.73⁻¹  # ← I want the result in m⁻² min⁻¹ mL 1.73⁻¹

How can I achieve that ?

[t-bltg]

I have tried :

julia> using DynamicQuantities
julia> @register_unit DFG us"mL/min/1.73m^2"
julia> 60us"DFG"
60.0 DFG  # ← output (not what I want)
60.0 m⁻² min⁻¹ mL 1.73⁻¹  # ← I want something alike

[MilesCranmer]

I think the best workaround is something like this:

julia> using DynamicQuantities

julia> @register_unit BSA173 1.73u"m^2"

julia> 60us"mL/min/BSA173"
60.0 min⁻¹ mL BSA173⁻¹

julia> 60u"mL/min/1.73m^2" |> us"mL/min/BSA173"
59.999999999999986 min⁻¹ mL BSA173⁻¹

Then you can have a string postprocessing step to map BSA173 => 1.73.

[MilesCranmer]

Or, for really custom stuff, you can also define your own custom dimensions type entirely, similar to the radians example in the docs:

julia> using DynamicQuantities: DynamicQuantities as DQ, Quantity, AbstractDimensions

julia> struct GFRDimensions{R} <: AbstractDimensions{R}
           volume::R
           time::R
           bsa_ref::R
       end

julia> function DQ.dimension_name(::GFRDimensions, k::Symbol)
           names = (volume = "mL", time = "min", bsa_ref = "(1.73m²)")
           get(names, k, string(k))
       end

julia> const GFR = Quantity(1.0, GFRDimensions(volume=1, time=-1, bsa_ref=-1))
1.0 mL min⁻¹ (1.73m²)⁻¹

julia> 60GFR
60.0 mL min⁻¹ (1.73m²)⁻¹

julia> 60GFR^2 - 30GFR^2
30.0 mL² min⁻² (1.73m²)⁻²

But you would have to define your own converter to SI units.