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Comment by stux | Hacker News Reader

This is interesting! A field being nullable because it's legitimately optional in the domain model is one thing, but for new fields which shouldn't be nullable in the domain model, unless you can pick a reasonable identity value, you need a concept of absence that's different from null. Luckily the intersection of "non-nullable fields" and "fields with no reasonable identity value" and "fields which didnt exist in v1 of the domain model" is normally pretty small, but it's painful when it happens.

This reminds me of frozen/nonfrozen enums in Swift. You can do exhaustive case analysis on frozen enums, but case analysis on nonfrozen enums requires adding an `@unknown default` case.

https://docs.swift.org/swift-book/documentation/the-swift-pr...

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> you need a concept of absence that's different from null

Could you give an example? Null _is_ absence, the way I see it

What they are saying is that the field is always present in the domain model but we don't have the information to backfill it. For example, say you have a customers table. Originally, it just stored their name and internal ID. But now you are adding in their government ID as well. Except that you already have thousands of customers and you don't have their government ID. So you either make the column nullable and slowly backfill it over time. Or you find some default value which isn't null but the code understands it to still be empty. And again, you slowly backfill over time.

> So you either make the column nullable and slowly backfill it over time. Or you find some default value which isn't null but the code understands it to still be empty. And again, you slowly backfill over time.

What would be the justification for using some other “default value” in this case? That’s just null with extra steps. Null _is_ the default value

There’s nothing gross or unholy about null values. Very smart people many years ago envisioned them as part of relational databases specifically for the use cases like these.

I want to represent that a field should never get new null values, it should be treated as non-nullable for the purpose of writing; however there are historical records which were made before the introduction of the field, so it should be treated as nullable for the purpose of reading.

> I want to represent that a field should never get new null values, it should be treated as non-nullable for the purpose of writing

That's a job of DB triggers — arbitrary restrictions that are automatically applied when you create or update records. You can restrict new null values, you can restrict numeric values to be odd or prime, you can make sure that only a single field can be null out of two, but not two at the same time — all the things that you can't even describe in a type system.

If null is an actual value, you need another value to represent "undefined".

Null is also considered a billion dollar mistake by very smart people.

If you are referring to Tony Hoares talk, it is not nulls which is the billion dollar mistake. The mistake is type systems which does not distinguish between nullable and non-nullable.

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