Relation to proposed SI extensions
The concept of dimensions has some limitations (see BIPM (2019), Nature Publishing Group UK (2017), and Steven T Bramwell (2017), which have led to many proposed extensions to the SI system, including many proposals to include angles as a dimension. Several such proposals include W E Eder (1982), Paul Quincey (2021), B P Leonard (2021), and references therein.
Summary of proposals
The exact form of the proposals and their effects on the equations of physics are all different, but most (i) include angle as a base dimension, (ii) use a "fundamental" constant, typically equal to one radian, to modify the equations of physics, (iii) define solid angle as a derived dimension equal to angle squared, and (iv) take different approaches to which equations or quantity units to modify.
Some examples of how common equations would be modified in such systems, using $θ₀=1rad$:
Trigonometric functions:
 $cos(θ) → cos(θ/θ₀)$
 $cos(ωt+φ) → cos([ωt+φ]/θ₀)$
Angular frequency and frequency:
 $ω=2πf → ω=2πfθ₀$
Arc length $s$ and other kinematic equations:
 $s=rθ → s=rθ/θ₀$
For dynamic equations there's more variability between the different proposals. As an example, Paul Quincey (2021) would modify torque (and its unit) while leaving the units of work intact, as
 $T=(𝐫×𝐅) → T=(𝐫×𝐅)/θ₀$
 $W=Tθ$
DimensionfulAngles.jl's relation to these proposals
DimensionfulAngles.jl extends the number of base dimensions solely for convenience when working with unitful quantities on a computer. It does not propose or promote any official extension of the SI system such as those summarized above. However, there's a few things to point out:
1. Defining constant
The constant θ₀=1rad
is provided as a hypothetical "defining unit" or "fundamental constant", mostly for analogy/consistency with the other base dimensions in the SI and Unitful.jl.
2. Extension to Base
and standard library functions
Additionally, note that the extensions to functions in Base
ensure that function calls like
julia> cos(45ua"°")
0.7071067811865476
work without having to use the constant θ₀
to normalize the argument. Please report any function in Base
or in the standard library that should handle arguments of a dimension that includes angles but is not currently covered by this package. The goal of this package is to cover all such functions as well as those whose output should be of dimensions that includes angles.
3. Functions in other packages
Functions in other packages for which it would make sense to provide arguments with dimensions including angles will not immediately work. You will likely need to convert to radians and then strip the units before providing the quantity as an argument to that function. This can be done in several ways, including through the use of the constant θ₀
for normalization. Alternatively, you can expand those functions to accept dimensionful angles by defining new methods. If you believe such expansion should be included in DimensionfulAngles.jl (e.g. to cover a very popular package) please create an issue in the GitHub repository.
4. Solid angles
Solid angles are considered a derived dimension equal to angle squared (𝐀²). This is in agreement with the proposals discussed above.
References
 SIBrochure_2019

BIPM, Le Système international d'unités / The International System of Units (‘The SI Brochure’), Bureau international des poids et mesures, 2019.
 Bramwell_2017

Steven T Bramwell, The invention of dimension, Nature physics, 13(8), 820–820, 2017.
 Eder_1982

W E Eder, A Viewpoint on the Quantity ``Plane Angle'', Metrologia, 18(1), 1, 1982.
 Quincey_2021

Paul Quincey, Angles in the SI: a detailed proposal for solving the problem, Metrologia, 58(5), 053002, 2021.
 Nature_2017

Nature Publishing Group UK, SI units need reform to avoid confusion, Nature, 548(135), 2017.
 Leonard_2021

B P Leonard, Proposal for the dimensionally consistent treatment of angle and solid angle by the International System of Units (SI), Metrologia, 58(5), 052001, 2021.