Define AbstractQuantity

[giordano]

Start addressing #186 by defining an AbstractQuantity type. I've marked this PR as work-in-progress because I'm not sure I've used the new abstract type wherever it's needed, at least tests pass locally.

[codecov]

Codecov Report

Merging #204 into master will increase coverage by 1.01%.
The diff coverage is 90.69%.

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
+ Coverage   78.94%   79.95%   +1.01%     
==========================================
  Files          14       14              
  Lines         964      983      +19     
==========================================
+ Hits          761      786      +25     
+ Misses        203      197       -6

Impacted Files	Coverage Δ
src/types.jl	94.28% <ø> (+0.34%)	⬆️
src/quantities.jl	91.56% <90.69%> (ø)	⬆️
src/conversion.jl	79.62% <0%> (-4.01%)	⬇️
src/Unitful.jl	100% <0%> (ø)	⬆️
src/logarithm.jl	69.66% <0%> (+0.29%)	⬆️
src/user.jl	88.8% <0%> (+1.3%)	⬆️
src/units.jl	92.5% <0%> (+4.16%)	⬆️
src/utils.jl	88% <0%> (+8%)	⬆️
src/pkgdefaults.jl	21.73% <0%> (+19.03%)	⬆️

---

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[codecov]

Codecov Report

Merging #204 into master will increase coverage by 1.01%.
The diff coverage is 90.69%.

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
+ Coverage   78.94%   79.95%   +1.01%     
==========================================
  Files          14       14              
  Lines         964      983      +19     
==========================================
+ Hits          761      786      +25     
+ Misses        203      197       -6

Impacted Files	Coverage Δ
src/types.jl	94.28% <ø> (+0.34%)	⬆️
src/quantities.jl	91.56% <90.69%> (ø)	⬆️
src/conversion.jl	79.62% <0%> (-4.01%)	⬇️
src/Unitful.jl	100% <0%> (ø)	⬆️
src/logarithm.jl	69.66% <0%> (+0.29%)	⬆️
src/user.jl	88.8% <0%> (+1.3%)	⬆️
src/units.jl	92.5% <0%> (+4.16%)	⬆️
src/utils.jl	88% <0%> (+8%)	⬆️
src/pkgdefaults.jl	21.73% <0%> (+19.03%)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 050f893...cffd842. Read the comment docs.

[giordano]

I've played with this branch in PhysicalConstants.jl and it seems to be sufficient for my current needs: JuliaPhysics/PhysicalConstants.jl#2 (comment)

The only thing I would recommend doing is to define an API function (value?) to access the value of an AbstractQuantity, which for the concrete type Quantity could be value(x::Quantity) = x.val. In my tests for PhysicalConstants.jl I've implemented Base.getproperty(c::Constant, sym::Symbol). This function wouldn't need to be exported, just imported by packages implementing a subtype of AbstractQuantity.

[c42f]

The only thing I would recommend doing is to define an API function to access the value of an AbstractQuantity.

Just wondering - is this just ustrip, or does it mean something different?

[giordano]

Ustrip is defined as

@inline ustrip(x::Quantity) = ustrip(x.val)

So strictly speaking it doesn't simply access to the val field. However, I'm having difficulties to find a case where the result would be different (maybe a quantity of quantity, whatever it is?)

[c42f]

Yes I haven't read enough of the internals to know why it's recursive, but it seems like it might have the meaning you want.

[giordano]

Yeah, anyway I'm not too concerned about that, for me it's sufficient this PR is merged

[ajkeller34]

I'm late on the explanation but ustrip is recursive because of types like Quantity{<:Level}, i.e. (20u"dBm/Hz").val == 20u"dBm", but presumably you want ustrip(20u"dBm/Hz") == 20. There is some special sauce needed to make logarithmic quantities work right.