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Null isn't that bad -- or rather, the concept of a missing value. Certain languages handle null better than others, but even then, it seems like the more costly mistake has been the accumulation of made-up data to satisfy non-null requirements.[0] More costly for non-programmers who have to deal with the programmers' lazy insistence that not knowing a value for some data in their system is forbidden, anyway.

In any case I think the modern fashion of trying to eliminate null from PLs won't matter much in the effort to replace C, whereas something like a mandatory GC is an instant no-go (though Java at least was very successful at sparing the world a lot of C++). OTOH a language that makes more kinds of formal verification possible (beyond just type theory proofs) might one day replace C and have null-analysis as a subfeature too.

[0] http://john.freml.in/billion-dollar-mistake

I think the author of that blog post fundamentally misunderstands the point: The damage of nullable pointers is not that they are nullable, but that compilers allow you to write code everywhere that assumes they’re not null (in fact, this is the only possible way to code, when the language cannot express the notion of a non-nullable reference!)

For example, most older languages with “the billion dollar mistake” have no complaint whatsoever when your write “object.method();” where it’s unknown at this scope whether “object” is null or not.

The fact that such code compiles is the billion dollar mistake; not the fact that the pointer is nullable.

I don’t care if you want to write nullable references everywhere, or whatever else you prefer or your application demands. That’s fine, so long as:

1. Non-nullable reference types must exist.

2. Nullable references types must exist as statically distinct from #1.

3. The compiler must not let you write code that assumes a nullable reference is not null, unless you check via a control flow statement first.

Now to take a step back, the principle behind this certainly applies beyond just nullability (if that was the point you were trying to make): Generally, dynamic, untyped invalidation states are dangerous/bad, while statically typed invalidation states are ideal. And yes, this does include bad states internal to a non-null reference, just as much as to a null reference.

Sum types are the key to being able to statically declare what range of values a function may return (or accept), and ensure at compile time that these different cases are all accounted for. If you aren’t aware of how elegantly sum types solve this, you should look into it — and I suspect it will be quickly clear why nullable references are useless, outdated, and harmful.

But at the very least, we’ve solved the pain of null dereference — and virtually without compromise. So, it’s irresponsible or ignorant IMO to create a new language that doesn’t include this solution in its core.

It doesn't seem like you are familiar with how option types get rid of null. You don't have to make up data to satisfy things not being null. You set them none, and the language either forces, or encourages usually, you to always check if the option is none or some.

I use Option in Java quite a bit because I'm real sick of NPEs and cascading null checks in all-or-nothing flows. I would have preferred Java starting with something like Kotlin's approach where T is T not T|nil. You and the sibling might be missing the point of the post I linked, I think. It can be convenient to have formal assistance via e.g. the type checker that a function taking a non-null String returns a non-null Person with a non-null FirstName and LastName. But in the zeal to be rid of null to make programmers' lives a bit easier, when faced with a name that doesn't compose into 2 parts, someone has to decide what to do about that and who needs to care down the line. You can make up data ("FNU" as in the blog), set a convention (empty string), throw an exception, or declare either the whole Person structure Optional or at least certain fields. If you use a dynamic late-binding language you may have other options. Whatever you do, it ought to be consistent or robustly handled where the references interact with your DB, your processing programs, and your data displays. Finally when these references escape your system, as lots of real world data does, they necessarily escape any static criteria you once had on them, thus it's important to consider those third party systems have to live with your choice. Null is a convenient choice, not something to be villified so casually.

> the compiler will statically guarantee the impossibility of null dereference exceptions,

almost every language that gets rid of nulls with something like the Option type will let you still bypass it and get a null reference exception. Rust lets you unwrap, F# lets you bypass it. You could at least enforce a lint that doesn't allow the bypasses in projects where that is desired though.

Yes, but there’s a big difference between the default member access operator crashing conditionally based on null-ness — vs — the same operator guaranteeing deterministic success (thanks to static type checks), with the option to circumvent those safe defaults if the programmer really wants to (in which case they usually must be very explicit about using this discouraged, unsafe behavior).

It may seem to be just semantics, but it’s really quite important that the default (and most concise) way in these languages to read optional values is to check if they’re null/None first in an if statement, after which you can call “object.method()” all you like. It’s important that you can’t just forget this check; it’s essential to using the content of the optional, unless you explicitly type something like “.unwrap()” — in which case there’s almost no chance the programmer won’t know and think about the possibility a crash. Take this in contrast to the chance of crash literally every time you type “->” or “.” in C++, for example.

Perfect is the enemy of good. By reducing the possibility of null dereference exceptions from 100% to 10% you have reduced the cognitive burden by 90%. Removing the bypass would result in a 100% reduction in cognitive burden, only 10% more than the second best solution. However handling null cases correctly isn't free either. Especially when you know that a value cannot be "null" under certain conditions which those 10% fall under. In those cases handling the error "correctly" is actually an additional cognitive burden that can ruin the meager 10% gain you have obtained by choosing the perfect solution.

> However handling null cases correctly isn't free either. Especially when you know that a value cannot be "null" under certain conditions which those 10% fall under.

While I agree there are rare cases where .unwrap() is the right thing to do, I actually disagree here that it’s anywhere close to 10%: If you want to write a function that accepts only non-null values in Rust, you simply write it as such! In fact, this is the default, and no cognitive burden is necessary: non-nullable T is written simply as “T”. If you have an Option<T> and want to convert it into a T in Rust, you simply use “if let” or “match” control flow statements.

I actually think using .unwrap() in Rust anywhere but in test code or top-level error handling is almost always a mistake, with perhaps 0.001% of exceptions to this rule. I write code that never uses it, except those cases mentioned; while I’ve run into situations where I felt at first .unwrap() was appropriate, I took a step back to think of the bigger picture and so far always find safer solutions to yield a better overall design.

The cognitive burden from Rust comes not from this, but almost entirely from the borrow checker (a completely different toptic), and in some cases, arguably inferior “ergonomics” vs how Zig or Kotlin handle optionals.

For example, in some null-safe languages, you can write:

  if (myObject) { myObject.mehod(); }

And the compiler will understand this is safe. Whereas, in Rust, you must write:

  if let Some(x) = myObject { x.method(); }

This is not even to mention that Rust has no built-in shorthand for Option<T> (some languages write “T?” for example), but I understand why they chose not to build this into the language; rather, Option<T> in Rust is actually a component of the stranded library! In a way, that’s actually quite cool and certainly is by-design; however, it doesn’t change the fact that it’s slightly more verbose.

IMO it’s not a huge deal, but certainly Rust could benefit from some syntax sugar here at least. Either way, both examples here are safe and statically checked by the compiler.

Yeah I think unwrap is best used when experimenting/prototyping, but it can be very very useful there. Imagine trying to get started using Vulkan or Opengl without it. Big mess. But in production code you might want to lint it as a strong warning or error.

> but certainly Rust could benefit from some syntax sugar here at least

It's a tough balance. Rust could benefit from more sugaring, but on the other hand, Rust already has quite a lot of syntax at this point.

> Many of those are ruled out as modern successors (in my mind, at least), when they continue to make “the billion dollar mistake” (to use its inventor’s own words[1]) of null references.

Well, you're in luck then! You don't even need a 'modern' successor, C++ (even the ancient versions) disallow null references.

> Well, you're in luck then! You don't even need a 'modern' successor, C++ (even the ancient versions) disallow null references.

That's useful, until you realise that all its smart pointers are semantically nullable (they can all be empty with the same result as a null raw pointer) and then nothing's actually fixed.

What you mean is that C++ doesn't have a way to (easily) let you check whether a given reference is null or not. int* a = NULL; int& b = *a; compiles and runs just fine.

> compiles and runs just fine.

For fairly low values of those. Creating a null reference is UB, your program is not legal at all.

If the compiler still accepts it, then that it belongs to the "UB" class of code is not much comfort.

The whole point is to NOT have it be accepted.

Sure, we're not supposed to do that. Sometimes it happens anyway, and the C++ compiler isn't much help in that case.

No the gp is correct, references in c++ can't be null. Your code invoked undefined behavior before you did anything with a reference, namely *a which is a null pointer dereference.

> namely *a which is a null pointer dereference.

Which is a textbook example of the null reference problem.

Edit: There may be some terminological confusion here: when programming language folks talk about "references", they include in that definition what C/C++ call "pointers". See for example the Wikipedia article, which gives as the C++ example not C++ references, but C++ pointers.

https://en.wikipedia.org/wiki/Reference_(computer_science)

The "null problem" is that a static language does a run-time check instead of a compile-time check. By the time the undefined behavior is invoked, compilation ended.

>Your code invoked undefined behavior before you did anything with a reference

Since nobody stopped you, the problem is still there.