For the longest time, I was told Null was bad: “Null the billion-dollar mistake”

But… doesn’t database tables have nullable columns? And when there is no value, I put NULL in it? If using NULL is a billion-dollar mistake there, then what do we use instead?

Cue programmers of various unpopular languages to sweep in, “come! there’s no Null here, maybe”

type Maybe a = Nothing | Just a

message1 = Just "Hello, world!"
message2 = Nothing

So… Null is a mistake but Nothing is alright? Yes. No.

You see Null itself is not the problem: When something has no value, it is Null [1]. That is correct. It ISN’T better if we ignored that correctness and use a 0 instead, or "" empty string, or 0001-01-01 00:00:00 +0000 UTC!

ASIDE: Go programmers, life is not a binary choice between “uninitialized C variable causes core dump” vs “implicit zero values then”! [2] Zero values are not our friend: When we unmarshal a json string without error and some numbers are 0, how do we know if the values were missing (not good) or the decoder saw 0 in the json string and decoded it (good) ? We don’t know [3]. Null errors can still be located and fixed, but we can’t locate zero value data corruption like this. And if you argue “Oh but there’s no difference between an absent value and 0” 👀 you should know that’s your Stockholm syndrome talking. [4]

Null becomes a problem ONLY IF your language lets you use it like it isn’t

maybeUser = findUser users 42 // a nullable reference to User value                // your compiler is happy; runtime not so much

This convenience of referrencing the name attribute on a possibly null user — THAT is the billion-dollar mistake: the null reference. Not Null itself.

Back to the example of Nothing being the same as Null (they are!). The difference is that, in some languages, we can’t use a maybeUser value like a User

maybeUser = findUser users 42
^^^^^^^^^---- compiler error!

The only thing we can do with a Maybe value is to deal with each specific code path

case maybeUser of
    Just u ->
        "Winner is " ++ # guaranteed safe access

    Nothing ->
        "Nobody won"

Inconvenient, but the guarantees are bliss.

So, boys and girls, don’t avoid Null itself. Don’t make your table column NOT NULL because of the reputation. Don’t use an inaccurate zero value in place of the correct one. If a column is indeed nullable, be brave and make it nullable.

Add a Null check linter? Change your programming language? Anything palatable to you, but let the Nulls be Nulls.

[1] Or Nothing, nil, et al.

[2] The simplest correct solution to “uninitialized C variable causes core dump” is to require initialization. A little bit of typing goes a long way.

[3] Unless you look into the json string again. 🤦‍♂️

[4] After all these sacrifices of correctness for implicit zero values to protect against core dump, Go programmers still have to grapple with the nil zero value and all the same nasties as Null… and worse: why is my nil error value not equal to nil?. 🤣