[Wishlist] Define an AbstractQuantity type

[giordano]

A few months ago I've created a new package to provide physical constants: https://github.com/JuliaPhysics/PhysicalConstants.jl This package uses Unitful.jl and defines a new type which is mostly based on my proposal at #81. In order to be able to automatically use a Constant object in mathematical operations, I think it would be great to have an AbstractQuantity type here, that can be subclassed by PhysicalConstants.jl (see JuliaPhysics/PhysicalConstants.jl#2). Quantity would be another concrete subtype of AbstractQuantity. I don't think the change would be very disruptive, it'd only be necessary to define the interface of AbstractQuantity and use it throughout this package (rather than accessing named fields).

[ajkeller34]

Should this issue stay open given that #204 was merged, and if so what remains to be done for your needs?

[giordano]

It'd be good to have a getter function for the val field just as a nicer interface than a.val. This function would be semantically different from ustrip, and probably won't be exported, I don't expect users to need it.

Apart from this, I think this ticket can be closed.