Add uconvert and chemistry example

README.md

@@ -207,6 +207,12 @@ julia> expand_units(x^2)
 8.987551787368176e16 m² s⁻⁴
 ```
 
+You can also convert a quantity in regular base SI units to symbolic units with `uconvert`:
+```julia
+julia> uconvert(us"nm", 5e-9u"m") # can also write 5e-9u"m" |> uconvert(us"nm")
+5.0 nm
+```
+
 ### Arrays
 
 For working with an array of quantities that have the same dimensions,

docs/make.jl

@@ -39,6 +39,7 @@ makedocs(;
     ),
     pages=[
         "Home" => "index.md",
+        "Examples" => "examples.md",
         "Utilities" => "api.md",
         "Units" => "units.md",
         "Constants" => "constants.md",

docs/src/examples.md

@@ -0,0 +1,35 @@
+# Toy Examples with Code
+
+## 1. Solving a Chemistry Homework Problem
+
+On your chemistry homework, you are faced with the following problem on the photoelectric effect[^1]:
+
+[^1]: Attribution: [MIT OCW](https://ocw.mit.edu/courses/5-111sc-principles-of-chemical-science-fall-2014/resources/mit5_111f14_lec04soln/)
+
+> In a photoelectric effect experiment, electrons are ejected from a titanium surface (work function ``\Phi = 4.33\mathrm{eV}``) following irradition with UV light.
+> The energy of the incident UV light is ``7.2 \cdot 10^{-19} \mathrm{J}`` per photon. Calculate the wavelength of the ejected electrons, in nanometers.
+
+Let's solve this problem with `DynamicQuantities.jl`!
+```jldoctest examples
+julia> using DynamicQuantities
+
+julia> using DynamicQuantities.Constants: h, m_e
+
+julia> Φ = 4.33u"Constants.eV" # work function
+6.93742482522e-19 m² kg s⁻²
+
+julia> E = 7.2e-19u"J" # incident energy
+7.2e-19 m² kg s⁻²
+
+julia> p = sqrt(2 * m_e * (E - Φ)) # momentum of ejected electrons
+2.1871890716439906e-25 m kg s⁻¹
+
+julia> λ = h / p # wavelength of ejected electrons
+3.029491247878056e-9 m
+
+julia> uconvert(us"nm", λ) # return answer in nanometers
+3.0294912478780556 nm
+```
+Since units are automatically propagated, we can verify the dimension of our answer and all intermediates.
+Also, using `DynamicQuantities.Constants`, we were able to obtain the (dimensionful!) values of all necessary constants without typing them ourselves.
+

docs/src/symbolic_units.md

@@ -19,3 +19,9 @@ To convert a quantity to its regular base SI units, use `expand_units`:
 ```@docs
 expand_units
 ```
+
+To convert a quantity in regular base SI units to corresponding symbolic units, use `uconvert`:
+
+```@docs
+uconvert
+```

src/DynamicQuantities.jl

@@ -5,7 +5,7 @@ export AbstractQuantity, AbstractDimensions
 export Quantity, Dimensions, SymbolicDimensions, QuantityArray, DimensionError
 export ustrip, dimension
 export ulength, umass, utime, ucurrent, utemperature, uluminosity, uamount
-export uparse, @u_str, sym_uparse, @us_str, expand_units
+export uparse, @u_str, sym_uparse, @us_str, expand_units, uconvert
 
 include("fixed_rational.jl")
 include("types.jl")

src/symbolic_dimensions.jl

@@ -98,13 +98,37 @@ end
 
 Expand the symbolic units in a quantity to their base SI form.
 In other words, this converts a `Quantity` with `SymbolicDimensions`
-to one with `Dimensions`.
+to one with `Dimensions`. The opposite of this function is `uconvert`,
+for converting to specific symbolic units, or `convert(Quantity{<:Any,<:SymbolicDimensions}, q)`,
+for assuming SI units as the output symbols.
 """
 function expand_units(q::Q) where {T,R,D<:SymbolicDimensions{R},Q<:AbstractQuantity{T,D}}
     return convert(constructor_of(Q){T,Dimensions{R}}, q)
 end
 expand_units(q::QuantityArray) = expand_units.(q)
 
+"""
+    uconvert(qout::AbstractQuantity{<:Any, <:SymbolicDimensions}, q::AbstractQuantity{<:Any, <:Dimensions})
+
+Convert a quantity `q` with base SI units to the symbolic units of `qout`, for `q` and `qout` with compatible units.
+Mathematically, the result has value `q / expand_units(qout)` and units `dimension(qout)`. 
+"""
+function uconvert(qout::AbstractQuantity{<:Any, <:SymbolicDimensions}, q::AbstractQuantity{<:Any, <:Dimensions})
+    @assert isone(ustrip(qout)) "You passed a quantity with a non-unit value to uconvert."
+    qout_expanded = expand_units(qout)
+    dimension(q) == dimension(qout_expanded) || throw(DimensionError(q, qout_expanded))
+    new_val = ustrip(q) / ustrip(qout_expanded)
+    new_dim = dimension(qout)
+    return new_quantity(typeof(q), new_val, new_dim)
+end
+
+"""
+    uconvert(qout::AbstractQuantity{<:Any, <:SymbolicDimensions})
+
+Create a function that converts an input quantity `q` with base SI units to the symbolic units of `qout`, i.e 
+a function equivalent to `q -> uconvert(qout, q)`.
+"""
+uconvert(qout::AbstractQuantity{<:Any, <:SymbolicDimensions}) = Base.Fix1(uconvert, qout)
 
 Base.copy(d::SymbolicDimensions) = SymbolicDimensions(copy(getfield(d, :nzdims)), copy(getfield(d, :nzvals)))
 function Base.:(==)(l::SymbolicDimensions, r::SymbolicDimensions)

test/unittests.jl

@@ -591,6 +591,30 @@ end
         @test_throws "rad is not available as a symbol" sym5.rad
 end
 
+@testset "uconvert" begin
+    @test uconvert(us"nm", 5e-9u"m") ≈ (5e-9u"m" |> uconvert(us"nm")) ≈ 5us"nm"
+    @test_throws DimensionError uconvert(us"nm * J", 5e-9u"m")
+
+    q = 1.5u"Constants.M_sun"
+    qs = uconvert(us"Constants.M_sun", 5.0 * q)
+    @test qs ≈ 7.5us"Constants.M_sun"
+    @test dimension(qs)[:kg] == 0
+    @test dimension(qs)[:g] == 0
+    @test dimension(qs)[:M_sun] == 1
+    @test expand_units(qs) ≈ 5.0 * q
+
+    # Refuses to convert to non-unit quantities:
+    @test_throws AssertionError uconvert(1.2us"m", 1.0u"m")
+    VERSION >= v"1.8" &&
+        @test_throws "You passed a quantity" uconvert(1.2us"m", 1.0u"m")
+
+    # Different types require converting both arguments:
+    q = convert(Quantity{Float16}, 1.5u"g")
+    qs = uconvert(convert(Quantity{Float16}, us"g"), 5 * q)
+    @test typeof(qs) <: Quantity{Float16,<:SymbolicDimensions{<:Any}}
+    @test qs ≈ 7.5us"g"
+end
+
 @testset "Test ambiguities" begin
     R = DEFAULT_DIM_BASE_TYPE
     x = convert(R, 10)