String macro to simplify type declarations with units

General Usage
9

Thanks very, very much for all your suggestions. My questions are resolved. Let me summarize:

The Julia idiomatic way is

using Unitful

Quantity(a,b) = Quantity{a, dimension(b), typeof(b)} 

mutable struct Data4{Float <: AbstractFloat}
    length::Quantity(Float,u"m")
end

d4 = Data4{Float64}(2.0u"mm")
@show d4.length

Nicer to read, but hard to understand the implementation behind it with the (bad) assumption that a type Float is defined outside of the macro (so the macro only works under this assumption):

using Unitful 

macro Float_str(str)
    return :( Quantity{$(esc(:Float)), dimension(@u_str($str)), typeof(@u_str($str))} )
end

mutable struct Data3{Float <: AbstractFloat}
    length::Float"m"  
end

d3 = Data3{Float64}(2.0u"mm")
@show d3.length