Bobby Tables would be proud.

The way I use it is 'undefined' is literally undefined (not set), but null means no value - explicitly.

Don't call it a monad, call it a structured data type or something, that's what it is! Calling it a monad is like saying that you're using a curve of constant normal intersection point. Why not just say it's a wheel?

Yes, it's mathematically true that you're having a smooth ride precisely because the normals have a constant intersection point, but it's also true to say that it's a wheel and it goes round and isn't bumpy and doesn't scrape, and people can get a handle on that.

So yeah, use a Result or Option or Maybe structured data type because it keeps explicit track of whether there's a value or not, and yeah, you can change or combine them and preserve the tracking, but there's no point calling it a monad unless you're trying to make people believe that avoiding the $1bn mistake of allowing/using null requires category theory. It doesn't, it's just a structured data type. It's simpler than an array! Stop calling it a monad.

Yeah, for this reason null shouldn't be part of any production code. If there's the possibility of having a null value, you need to check every variable or returned value to be safe.

These monads tell the consumer of your functions to do something (a check for emptiness or wait for it to be ready, or iterate it) to access the value inside. In a safe language, if the value is not wrapped by a monad, then you should expect to access it without issues.

I kind of get why people don't want to call them monads, since it sounds like a heavy term and more things to learn that are not strictly "necessary", but the earlier you learn about their importance, the earlier you can use any of their benefits in your codebase.

JavaScript wat

maybe it's rather a box of tissues

Full scale analytics of this issue on Wikipedia

Or

Wikipedia

If you have 0 before use, you have a negative value after use.

Ease up mate CSS isn’t easy

response indicating that there was no value assigned in the first place.

You can explicitly assign undefined to a variable though.

Another fun fact about JavaScript is that undefined never used to be a keyword. If you did var foo = undefined, foo would indeed have a value of undefined, but it was only because there was no variable called undefined in scope!

You could do var undefined = 42 then var foo = undefined would actually set foo to 42! window.undefined = 42 would break all sorts of things.

Thankfully this was fixed with ES5 in 2009, although it took a few years for browsers to make the change.

There's deeper subtlety here. There are declared variables, uninitialised variables and variables that will at some point be declared, uninitialised or not, that don't exist in the logic of the program flow yet. Compilers / interpreters may or may not make distinctions between these, especially if memory is (perhaps only considered to be) in short supply.

What specific languages do in these cases is wide and varied. Some automatically give all variables a safe, but identifiably undefined value the moment they're declared, which may or may not be equivalent to the language's interpretation of null. Or is equivalent in some senses but not others. Looking at you, Javascript.

In the toilet paper example, the holder might actually still exist but the middle bar might be missing for "identifiably undefined", and the blank wall is that variable you have literally no idea about.

Maybe in trying to access it you amaze yourself by finding that you've punched through the wall and managed to grab some toilet paper from a neighbouring bathroom cubicle.

Maybe you instead discover your hand goes through a mincing machine and the pain, oh god why the pain nothing will ever be the same, is that my hard drive churning? No way to tell. Now that's undefined behaviour.

Depends on what undefinedwe're talking about. JavaScript undefined is just a value for undefined variables. In C undefined behavior could be anything, ranging from reading in random garbage to time travel or summoning eldritch terrors.

The only language I know where null equals 0 is German.