added basic rrules, ProjectTo for Quantity

Author: SBuercklin

Project.toml

@@ -3,6 +3,7 @@ uuid = "1986cc42-f94f-5a68-af5c-568840ba703d"
 version = "1.9.2"
 
 [deps]
+ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
 ConstructionBase = "187b0558-2788-49d3-abe0-74a17ed4e7c9"
 Dates = "ade2ca70-3891-5945-98fb-dc099432e06a"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"

src/Unitful.jl

@@ -27,6 +27,8 @@ import Random
 
 import ConstructionBase: constructorof
 
+import ChainRulesCore: rrule, NoTangent, ProjectTo
+
 export logunit, unit, absoluteunit, dimension, uconvert, ustrip, upreferred
 export @dimension, @derived_dimension, @refunit, @unit, @affineunit, @u_str
 export Quantity, DimensionlessQuantity, NoUnits, NoDims
@@ -69,5 +71,6 @@ include("logarithm.jl")
 include("complex.jl")
 include("pkgdefaults.jl")
 include("dates.jl")
+include("chainrules.jl")
 
 end

src/chainrules.jl

@@ -0,0 +1,34 @@
+function rrule(UT::Type{Quantity{T,D,U}}, x::Number) where {T,D,U}
+    unitful_x = Quantity{T,D,U}(x)
+    projector_x = ProjectTo(x)
+    uq_pullback(Δx) = (NoTangent(), projector_x(Δx) * oneunit(UT))
+    return unitful_x, uq_pullback
+end
+
+function ProjectTo(x::Quantity)
+    project_val = ProjectTo(x.val) # Project the literal number
+    return ProjectTo{typeof(x)}(; project_val = project_val)
+end
+
+function (projector::ProjectTo{<:Quantity})(x::Number)
+    new_val = projector.project_val(ustrip(x))
+    return new_val*x
+end
+
+# Project Unitful Quantities onto numerical types by projecting the value and carrying units
+(project::ProjectTo{<:Real})(dx::Quantity) = project(ustrip(dx))*unit(dx)
+(project::ProjectTo{<:Complex})(dx::Quantity) = project(ustrip(dx))*unit(dx)
+
+function rrule(::typeof(*), x::Quantity, y::Units, z::Units...)
+    Ω = *(x, y, z...)
+    project_x = ProjectTo(x)
+    function times_pb(Δ)
+        δ = project_x(Δ)
+        units = (y, z...)
+        return (NoTangent(), *(δ, y, z...), ntuple(_ -> NoTangent(), length(units))...)
+    end
+    return Ω, times_pb
+end
+
+rrule(::typeof(/), x::Number, y::Units) = rrule(*, x, inv(y))
+rrule(::typeof(/), x::Units, y::Number) = rrule(*, x, inv(y))