Strategies for dealing with missings

The following types support methods to deal with presence of missing values in their inputs.

MissingStrategies.MissingStrategy — Type

MissingStrategy()

Suptertype of Abstract and Singleton Types that signify how a method should deal with missing values

└PassMissing: Singleton: return missing, if a missing value is encountered
└HandleMissingStrategy: Abstract type: take missing values consciously into account
- └SkipMissing: ignore missing values
- └ExactMissing: unbiased processing

See @handlemissings_typed on how to extend functions by methods handling these strategies.

source

Usually, methods should return missing if there are missings present in the input. This PassMissing() strategy ensures, that missing are not accidently bypassed unconsciously. Hence, PassMissing() is a good default value for the strategy.

If a caller knows that there might by missing values present, he can consciously request to handle missings by supplying one of subtypes of HandleMissingStrategy. For an explicit dealing with missings that may cause some cost in computation use ExactMissing. For ignoring missing values use SkipMissing.

Design choices

The design strategies alternatively could be passed by keyword arguments, e.g. skipmissing::Bool=true or by an enum type. Modelling the strategies by a type system allows the dispatch system and the compiler to work on them.

This comes with several advantages:

Avoid method matching ambiguities across existing methods that cannot handle missing values and their extensions by the same name and argument types aside from the MissingStrategy.
Generate efficient code, because if branches can be pruned at compile time
Extensible to further strategies