(f(x, y, z) for x in xs, y in ys, z in zs if p(x, y, z))

using BenchmarkTools

function sumcart_manual(A::AbstractMatrix)
    s = 0.0
    @inbounds for j = 1:size(A,2), i = 1:size(A,1)
        s += A[i,j]
    end
    s
end

function sumcart_iter(A)
    s = 0.0
    @inbounds for I in CartesianIndices(size(A))
        s += A[I]
    end
    s
end

function sumcart_foldl(A)
    foldl(CartesianIndices(size(A)); init=0.0) do s, I
        @inbounds s + A[I]
    end
end

A = rand(10^4, 10^4);
@btime sumcart_manual($A);  # 126.509 ms (0 allocations: 0 bytes)
@btime sumcart_iter($A);    # 145.124 ms (0 allocations: 0 bytes)
@btime sumcart_foldl($A);   # 125.753 ms (0 allocations: 0 bytes)

using BenchmarkTools
suite = BenchmarkGroup()
suite["sum(x == y for x in xs, y in ys)"] = @benchmarkable sum(
    x == y for x in (1, 2, 3, 4, 5, 6, 7, 8), y in $(rand(1:50, 10^3))
)
suite["sum(x * y for (x, y) in zip(A, transpose(B)))"] = @benchmarkable sum(
    x * y for (x, y) in zip($(rand(100, 100)), transpose($(rand(100, 100))))
)
suite["copyto!(A, transpose(B))"] = @benchmarkable begin
    A = $(zeros(100, 100))
    B = transpose($(rand(100, 100)))
    foreach(eachindex(A, B)) do I
        @inbounds A[I] = B[I]
    end
end