Why can't we merge Missing and Nothing?

Nay (if we are summarizing :wink:). I think you misunderstood something.

That said, it is more helpful to think about interfaces than philosophical categories. The bottom line is that by default, most functions propagate missing, while they error for nothing.

A succinct way to demonstrate the difference is

julia> length(methodswith(Missing))
137

julia> length(methodswith(Nothing))
12
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