Why are missing values not ignored by default?

[stevengj]

@lmiq

Seems to me from the discussion that the best would be to have a package that deals with another type, such as unsafe_missing, that implemented all the functions desired, so that someone can just replace all the missings in their data with that one and move forward with their preferred choice.

It would make it easier to do something like that if we added a new supertype AbstractMissing >: Missing to the Base package. (This can be done in 1.x — adding a new supertype is not considered a breaking change IIRC.)

That way, a lot of packages like DataFrames.jl could more easily support alternative missing types, e.g. an ImputedMissing type with some kind of semantics of automatic imputation, without having to specifically depend on new packages.