take 1

src/Unitful.jl

@@ -69,5 +69,6 @@ include("logarithm.jl")
 include("complex.jl")
 include("pkgdefaults.jl")
 include("dates.jl")
+include("linearalgebra.jl")
 
 end

src/linearalgebra.jl

@@ -0,0 +1,42 @@
+using LinearAlgebra
+
+# This function is re-defined during testing, to check we hit the fast path:
+linearalgebra_count() = nothing
+
+function LinearAlgebra.mul!(C::StridedVecOrMat{<:AbstractQuantity{T}}, 
+        A::StridedMatrix{<:AbstractQuantity{T}}, 
+        B::StridedVecOrMat{<:AbstractQuantity{T}},
+        alpha::Bool, beta::Bool) where {T<:Base.HWNumber}
+    # This is exactly how A * B creates C = similar(B, T, ...)
+    eltype(C) == Base.promote_op(LinearAlgebra.matprod, eltype(A), eltype(B)) || error("bad eltypes")
+    C0 = ustrip(C)
+    A0 = ustrip(A)
+    B0 = ustrip(B)
+    mul!(C0, A0, B0)
+    linearalgebra_count()
+    return C
+end
+
+function LinearAlgebra.mul!(C::StridedVecOrMat{<:AbstractQuantity{T}}, 
+        A::LinearAlgebra.AdjOrTransAbsMat{<:AbstractQuantity{T}, <:StridedMatrix}, 
+        B::StridedVecOrMat{<:AbstractQuantity{T}},
+        alpha::Bool, beta::Bool) where {T<:Base.HWNumber}
+
+    eltype(C) == Base.promote_op(LinearAlgebra.matprod, eltype(A), eltype(B)) || error("bad eltypes")
+    C0 = ustrip(C)
+    A0 = A isa Adjoint ? adjoint(ustrip(parent(A))) : transpose(ustrip(parent(A)))
+    B0 = ustrip(B)
+    mul!(C0, A0, B0)
+    linearalgebra_count()
+    return C
+end
+
+function LinearAlgebra.dot(A::StridedArray{<:AbstractQuantity{T}}, 
+                           B::StridedArray{<:AbstractQuantity{T}}) where {T<:Base.HWNumber}
+    A0 = ustrip(A)
+    B0 = ustrip(B)
+    C0 = dot(A0, B0)
+    linearalgebra_count()
+    C = C0 * oneunit(eltype(A)) * oneunit(eltype(B))  # surely there is an official way
+    return C
+end

src/utils.jl

@@ -75,7 +75,7 @@ julia> a[1] = 3u"m"; b
  2
 ```
 """
-@inline ustrip(A::Array{Q}) where {Q <: Quantity} = reinterpret(numtype(Q), A)
+@inline ustrip(A::StridedArray{Q}) where {Q <: Quantity} = reinterpret(numtype(Q), A)
 
 @deprecate(ustrip(A::AbstractArray{T}) where {T<:Number}, ustrip.(A))
 

test/runtests.jl

@@ -94,6 +94,40 @@ end
     @test ConstructionBase.constructorof(typeof(1.0m))(2) === 2m
 end
 
+@testset "LinearAlgebra functions" begin
+    CNT = Ref(0)
+    Unitful.linearalgebra_count() = (CNT[] += 1; nothing)
+    @testset "> Matrix multiplication: *" begin
+        M = rand(3,3) .* u"m"
+        M_ = view(M,:,1:3)
+        v = rand(3) .* u"V"
+        v_ = view(v, 1:3)
+
+        CNT[] = 0
+
+        @test unit(first(M * M)) == u"m*m"
+        @test M * M == M_ * M == M * M_ == M_ * M_
+
+        @test unit(first(M * v)) == u"m*V"
+        @test M * v == M_ * v == M * v_ == M_ * v_
+
+        @test CNT[] == 10
+
+        @test unit(first(v' * M)) == u"m*V"
+        @test v' * M == v_' * M == v_' * M == v_' * M_
+
+        @test CNT[] == 15
+
+        @test unit(v' * v) == u"V*V"
+        @test v' * v == v_' * v == v_' * v == v_' * v_
+
+        @test CNT[] == 20
+    end
+    @testset "> Matrix multiplication: mul!" begin
+
+    end
+end
+
 @testset "Types" begin
     @test Base.complex(Quantity{Float64,NoDims,NoUnits}) ==
         Quantity{Complex{Float64},NoDims,NoUnits}