> The idea is that optionals allow you to pass optional data by value.
Yes, and kj::Maybe was doing the same before std::optional was standardized.
It's disappointing that the committee chose only to solve this problem while not also solving the problem of forgetting to check for null -- often called "the billion-dollar mistake".
> Dereferencing null optionals is UB for consistency with dereferencing pointers. All uses of operator* should have the same semantics
My argument is that `std::optional` should not have an operator* at all. `kj::Maybe` does not have operator* nor `operator->`.
> If you want to dereference an optional that may be null, use the .value_or() method. For the times when you absolutely know the optional has a value use operator*.
This is putting a lot of cognitive load on the developer. They must remember which of their variables are optionals, in order to remember whether they need to check for nullness. The fact that they use the same syntax to dereference makes it very easy to get wrong. This is especilaly true in modern IDEs where developers may be relying on auto-complete. If I don't remember the type of `foo`, I'm likely to write `foo->` and look at the list of auto-complete options, then choose one, without ever realizing that `foo` is an optional that needs to be checked for null.
Or if you're really sure the maybe is non-null, you can write:
use(KJ_ASSERT_NONNULL(maybeValue));
This does a runtime check and throws an exception if the value is null. But more importantly, it makes it really clear to both the writer and the reader that as assumption is being made.
> It's disappointing that the committee chose only to solve this problem while not also solving the problem of forgetting to check for null -- often called "the billion-dollar mistake".
it's likely ~30 loc to wrap std::optional in your own type that checks for null. if std::optional checked for null and these `if` branch showed up as taking the couple nanoseconds that make you go past your time budget in your real-time system (especially if people were doing things like if(b) { foo(b); bar(b); baz(*b); }) then you have to reimplement the whole of it instead.
Don't forget that you can still use C++ on 8mhz microcontrollers.
Again, I'm not arguing that operator* should check for nullness, I'm arguing that it shouldn't exist.
With `kj::Maybe` and `KJ_IF_MAYBE`, using the syntax I demonstrated above, you check for nullness once and as a result of the check you get a direct pointer to the underlying value (if it is non-null), which you then use for subsequent access, so you don't end up repeating the check. So, you get the best of both worlds.
> it's likely ~30 loc to wrap std::optional
It's even easier to replace std::optional rather than wrap it. The value of std::optional is that it's a standard that one would hope would be used across libraries. But because it's flawed, people like me end up writing their own instead.
The ideal situation is then to not use std::optional for those cases rather than to make std::optional next to useless for it's stated case.
If it gets in the way of your goal on an 8Mhz controller, take the optionality check out of your tight loop and convert to a null pointer safely where it doesn't matter.
Deeply embedded is already used to picking and choosing features, or explicitly running with the bumper rails off in the subset of cases where it matters. We like the normal primitives not being neutered for us because we still use a lot of them outside of our tight loops.
> than to make std::optional next to useless for it's stated case.
This is such a ridiculous and obviously false assertion that it’s indistinguishable from satire. Optional is widely used and was modeled from a pre-existing boost class which was itself widely used. Do you actually write C++ professionally?
I know they're different (and the construct unfortunately named optional has some occasional uses); it's just that the semantics of optional don't help it be used as a classic optional type.
> it's just that the semantics of optional don't help it be used as a classic optional type.
what do you mean "classic optional type"? boost.optional has worked like that for something like 20 years - it's been in C++ for longer than Maybe has been in Haskell.
> My argument is that `std::optional` should not have an operator* at all. `kj::Maybe` does not have operator* nor `operator->`.
That’s fair.
>> If you want to dereference an optional that may be null, use the .value_or() method. For the times when you absolutely know the optional has a value use operator
> This is putting a lot of cognitive load on the developer. They must remember which of their variables are optionals,
Not really. Teams with programmers that are bad at keeping track of the state of their variables can simply have a policy to always use .value_or()/.value()
C++ doesn’t impose this on its users because it generally assumes its users are responsible enough to make their own policy.
> The fact that they use the same syntax to dereference makes it very easy to get wrong.
I disagree, the operator* has the same semantics as pointers did, making it no more semantically hazardous. There exists other methods on optional that have the behavior you want.
But neither of these solve the problem either. Neither one forces the programmer to really confront the possibility of nullness and write the appropriate if/else block. Throwing an exception rather than crashing is only a slight improvement IMO.
> I disagree, the operator* has the same semantics as pointers did, making it no more semantically hazardous.
It was already severely hazardous with pointers, that's the problem.
Both problem could have been solve looooooong ago by introducing a type modifier akin to const that carries if a value is verified (or safe or non-null or other. Pick your synonym).
int * p; // maybe null!
int * verified p; // guaranteed non-null!
A looooong time ago (circa... 1994-1995) I designed a hierarchy of smart pointers and had a variety for non-null so that you could declare a function like:
void foo(non_null_ptr<T>& p);
And know that you don't have to verify for null. All enforced at compile-time. (via the a function on ptr<T> returning a non_null_ptr<T>).
With language support around if() and others, C++ could have mde it even more convenient. Even C could have introduced such a tyupe modifier. Whenever I read about pointers being unsafe and how optionals and maybes are the solution, I roll my eyes, because non-null-ptr do the exact same thing.
The funny thing is C++ has a non-null ptr (with no language support guarantee though): references. Unfortunately, the language made them not resettable, which makes them unusable in many scenario when you'd want them to change value over time, like in most classes members.
But the idea of a verified type can be extended by using the verified modifier on your own type. For example, you could have a verified matrix type, where the matrix is guaranteed to be valid, non-degenerate. You can apply it to:
- matrix
- vector
- input data of any sort
And if teh compiler allowed the programmer to declare their own type modifier, the world is your oyster: you could for example tag that a matrix is a world matrix while another a local matrix and provide a function that converts from one to the other...
> But neither of these solve the problem either. Neither one forces the programmer to really confront the possibility of nullness and write the appropriate if/else block.
.value_or() actually does and you can certainly add a lint check against dereferencing optional or using .value() if you’d like. C++ does not Yet provide Case-style syntax for handling variants like rust, outside of macros and the standard library will certainly not define macros.
I think what you have done for your codebase makes sense based on your preferences but I think the standard optional works pretty well given the variety of code based and styles it’s intended to support.
> It was already severely hazardous with pointers, that's the problem.
kj::Maybe has an `orDefault()` method that is like `.value_or()` but I find that it is almost never useful. You almost always want to execute different logic in the null case, rather than treat it as some default value.
Yes, and kj::Maybe was doing the same before std::optional was standardized.
It's disappointing that the committee chose only to solve this problem while not also solving the problem of forgetting to check for null -- often called "the billion-dollar mistake".
> Dereferencing null optionals is UB for consistency with dereferencing pointers. All uses of operator* should have the same semantics
My argument is that `std::optional` should not have an operator* at all. `kj::Maybe` does not have operator* nor `operator->`.
> If you want to dereference an optional that may be null, use the .value_or() method. For the times when you absolutely know the optional has a value use operator*.
This is putting a lot of cognitive load on the developer. They must remember which of their variables are optionals, in order to remember whether they need to check for nullness. The fact that they use the same syntax to dereference makes it very easy to get wrong. This is especilaly true in modern IDEs where developers may be relying on auto-complete. If I don't remember the type of `foo`, I'm likely to write `foo->` and look at the list of auto-complete options, then choose one, without ever realizing that `foo` is an optional that needs to be checked for null.
In KJ, you MUST write:
Or if you're really sure the maybe is non-null, you can write: This does a runtime check and throws an exception if the value is null. But more importantly, it makes it really clear to both the writer and the reader that as assumption is being made.