Good article. Suggestion: When you post an article like this to hacker news it would help to include the programming language in the title, like "Getting rid of NULL (in PHP)". I wasn't aware that this is a PHP-related post until I've read the first bunch of paragraphs until you actually mention "PHP".
I'm not sure that's necessary, it's not a PHP specific post. The author is just using PHP to demonstrate his point. Most, if not all object oriented languages should be able to replicate this functionality.
Null is simply a marker for non-value. Meaning the variable os initialized (ie not undefined) but the value hasn't been explicitly set.
To cheery-pick an example from the article.
> When your function usually returns an integer, return 0 instead of null.
0 carries a meaning that may be directly relevant to the context of where it's used.
Using that as a default state will eventually cause unexpected side effects
Look no further than the kerfuffle caused by c using true==1 & false==0 to see how this approach may cause unexpected bugs.
A more specific use case that wasn't mentioned is bool. I ran into this issue in one of the libraries I wrote. It contained inline hooks into a parser where user-defined code could be injected or returned false if the hook is unset.
Unfortunately, eventually I had a user that was using the hook that needed to return the value false. The only way to fix it with out breaking further edge cases would be to use null.
Null doesn't exist because of environments where the dev can easily reason about how the code will be used. It's used for code that will be used by others where it's not easy to reason about all of the possible use cases.
With that said, I think languages should have built-in null checking support of some form. Requiring if (val == null) checks everywhere makes its usage seem like an anti-pattern.
Null has a clearly defined and useful purpose. Even if most devs never have to explicitly set a value to set a value to null themselves.
