Why can't we merge Missing and Nothing?

Internals & Design
15

Let me illustrate what bothers me about having to distinguish “has a value, but it is unknown” and “has no value” in programming:

  1. You may not know. If there is no information about siblings, you do not know whether to assign NameOfSister to missing (“has a value, but it is unknown”) or a nothing (“has no value”).

  2. No standard for optional arguments because the types are loaded with meaning:
    # Input, if you have a coupon
    buy(..., coupon::Union{String,Nothing}=nothing)
    # Input, if you know the DNA of your mom.
    healthanalysis(..., mom::Union{DNA,Missing}=missing)

  3. Production of missing types into codes that may not be programmed to handle it (kudos to @kristoffer.carlsson). Would you dare to define the generally intractable isconvex(function) or the generally undecidable ishalting(callable) as a Union{Bool,Missing} even though they are definitely of the “has a value, but it might be unknown” type? I never want that because the three-way-logic would hide bugs in my code.

What is more appropriate in my opinion is to distinguish “value not available (if it exists, it is unknown)” and “definitely has a value, but it is unknown”. The former would be the standard goto type for all items in the enumeration above, whereas the second would be a specialization you opt into when needed. In practice, this is how I see Nothing and Missing already being used today.