Type container equivalence

[aplavin]

@anon56330260

If you worry about type stability, then never use Union type!

Maybe this is a bit too extreme, but only a bit.
Whenever you put these unions into containers, type stability can easily be lost. Here’s a small example using only Base types — floats, missings, tuples, and arrays:

Is there any reason to use NaN instead of missing?

Yes, there are lots of performance (type stability) reasons to use NaN! The overhead with missing can be more than an order of magnitude even in this simple example: julia> x_nan = [([1., 2., 3.],), ([1., 2., 3., NaN],)] julia> x_missing = [([1., 2., 3.],), ([1., 2., 3., missing],)] julia> @btime map(x -> sum.(x), $x_nan) 35.578 ns (1 allocation: 80 bytes) julia> @btime map(x -> sum.(x), $x_missing) 378.745 ns (8 allocations: 240 bytes)

Despite the simplicity of this example, the type-stable (float-only) code is 10x faster than float+missing.