alt Hacker NewsThis seems really silly to me. Why does get_owner need to differentiate between the document exists and maybe has an owner or the document doesn’t exist and thus has no owner. In both cases getting no owner should suffice? Why are you using get_owner to try and figure out if the document exists? Can you give an example where this distinction actually matters?
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> In both cases getting no owner should suffice?
Depends on what exactly you plan on doing with the information, I'd imagine. I don't think it's necessarily a given that "no document" and "no owner" should be handled identically (e.g., searching for the updated document to try again vs. returning an operation-not-allowed error).
> Why are you using get_owner to try and figure out if the document exists?
Potential TOCTOU issues, perhaps? As I mentioned the document could have vanished out from under you so detecting this situation could let handle things appropriately (though to be fair that argument could probably extend to future operations as well).
Maybe a versioned filesystem would have made a bit more sense, where None would indicate a new version is available? Another example I've seen mentioned is trying to get a key-value pair from a JSON where the key may be missing.
But in any case, I did say it was a bit of a contrived example, so cut me a bit of slack :P It was just the first thing that came to mind.
The examples sibling comments give are probably more practical.
Maybe a different example works better for your mental model:
A user can set custom settings, and those settings may or may not have a value assigned to them. For example, one setting is optimization level, another option is a debug flag.
You want to have a unified interface to get information about settings, queried through some intermediate system. (lets just say it is limited to querying specific settings, rather than getting the entire list, due to legacy constraints in this case) Because for some settings you just care about them existing and for others you care about the actual value, you have to differentiate between a setting that contains something, a setting that contains nothing, and one that doesnt exist.
It's analogous to how we use undefined and null in JS.
Undefined = No value known. Think 0 rows found, the 1st row's column is undefined.
Null = Value known. It is void of value. The 1st row is populated, and this column has no value.
You run into this issue any time you do a LEFT JOIN in sql and the field you care about is nullable.
The right side value could be null because it was missing, or because it was literally null. You resolve by checking the join key, and if that is itself nullable, you’re forced to introduce a sentinel