Support for different kinds of missing values in Julia

Note : This package is currently experimental. It relies on overwriting some fundamental Julia Base methods until they are changed to use ismissing(x) instead of hardcoding x === missing.

TypedMissings.jl provides the TypedMissing type which is similar to Missing but allows representing multiple kinds of missing values. Values of type TypedMissing behave identically to missing, except that isequal(TypedMissing(kind), missing) is true if and only if kind == MissingKinds.NI. The default kind is MissingKinds.NI ("No information"), which is equivalent to missing.

TypedMissing values propagate their kind across operations which involve only missing values of the same kind or non-missing values. Operations mixing TypedMissing values of different kinds fall back to the MissingKinds.NI kind.

If provided, kind must be an instance of the MissingKind enum. Supported kinds are NI, lowercase letters from a to z, and "null flavour" values defined by FHIR HL7 v3/ISO 21090:

• NI: No information
• INV: Invalid
• OTH: Other
• NINF: Negative infinity
• PINF: Positive infinity
• UNC: Unencoded
• DER: Derived
• UNK: Unknown
• ASKU: Asked but unknown
• NAV: Temporarily not available
• NAVU: Not available
• QS: Sufficient quantity
• NASK: Not asked
• TRC: Trace
• MSK: Masked
• NA: Not applicable

julia> TypedMissing()
TypedMissing()

julia> TypedMissing(MissingKinds.NI)
TypedMissing()

julia> TypedMissing() + 1
TypedMissing()

julia> TypedMissing(MissingKinds.a) + 1
TypedMissing(MissingKinds.a)

julia> TypedMissing(MissingKinds.NASK) + 1
TypedMissing(MissingKinds.NASK)

julia> TypedMissing(MissingKinds.NASK) + TypedMissing(MissingKinds.INV)
TypedMissing()

julia> TypedMissing(MissingKinds.NASK) + missing
TypedMissing()

julia> TypedMissing(MissingKinds.NASK) + 1
TypedMissing(MissingKinds.NASK)

julia> isequal(TypedMissing(MissingKinds.NASK), TypedMissing(MissingKinds.NASK))
true

julia> isequal(TypedMissing(MissingKinds.NASK), TypedMissing(MissingKinds.INV))
false

julia> isequal(TypedMissing(MissingKinds.NASK), missing)
false

julia> isequal(TypedMissing(MissingKinds.NI), missing)
true