Huh? Could you give a specific example? Because e.g. C++ and Rust definitely have dynamic dispatch through their vtable mechanisms.
Neither C++ nor Rust give you static type safety AND dynamic dispatch because all of the safety checks for C++ and Rust happen at compile time. Not runtime.
You appear to be using some other definition of dynamic dispatch than the rest of the software industry...
Dynamic dispatch happens at runtime.
C++ and Rust are compile-time tools, not runtimes.
Please read about it on Wikipedia
https://en.m.wikipedia.org/wiki/Dynamic_dispatch
And for the future to not litter HN with comments like these, next time 10 different people in thread are all explaining to you why you're mistaken, take a moment to try to listen and think through what they're saying instead of just digging deeper.
Having an open mind to learning something new, not just arguing a point is a great approach to life.
Because it can’t.
C++ and Rust let you have compile-time safety, until you choose to give it up and have runtime checks instead. Dynamic languages only allow the latter. Static languages let you choose, dynamic languages chose the latter for you in all cases. Both can have dynamic dispatch.
Besides, static languages can have compile-time type safe dynamic dispatch, if you constrain the dispatch to compile-time-known types (eg std::variant). You only lose that if you want fully unconstrained dynamism, in which case you defer type checking to runtime. Which is what dynamic languages always have.
So both C++ and Rust DO have dynamic dispatch and the programmer gets to choose what level of the dynamism/safety trade off they want. And yes, these features ARE first class features of the languages.
PRECISELY
You have to give up the safety to get the feature.
So you "want type-safety". Until you don't.
>static languages can have compile-time type safe dynamic dispatch
"Compile-time dynamic dispatch" is an oxymoron. Dynamic dispatch happens at runtime.
Trivially, because you don’t have this knowledge (and therefore you can’t encode it into your type system) at compile time.
Apparently you do know what dynamic dispatch is, you're just wrong that it can't be type checked.
In Java, say you have an interface called `Foo` with a method `String foo()`, and two classes A and B that implement that method. Then you can write this code (apologies if the syntax isn't quite right, it's been a while since I've written Java):
Foo foo = null;
if (random_boolean()) {
foo = new A();
} else {
foo = new B();
}
// This uses dynamic dispatch
System.out.println(foo.foo())
Because the implementation details of Foo are actually know at compile time. Which is why you are able to type-check it.
You have literally declared all allowed (but not all possible) implementations of Foo.
What happens when Foo() is a remote/network call?
If you can type-check your dispatcher at compile time then there is nothing dynamic about it.
Decidable (ahead of time) means your function is fully determined. Something that is fully determined is not dynamic.
It is the conception computer scientists use.
I thought you were just mistaken about something, but no, instead you've redefined a well understood term without telling anyone, then aggressively refused to clarify what you meant by it and argued for hours with people, while saying they were all wrong when they used the well established term to mean its well established meaning.
The thing you're talking about is an interesting concept, but it's not called dynamic dispatch, and you will confuse everyone you talk to if you call it that. I don't know if there's a term for it.
According to who?
Computer scientists talk about “well formed” not “well established”.
Those are categorical definitions.
Wikipedia, every textbook you can find, the top dozen search results for "dynamic dispatch", me who has a PhD in computer science plus all the other CS PhD people I know, everyone in my office who knows the term (who are industry people, not academia people), every blog post I have ever read that uses the term, and all the other HN commenters except you. I'm really not exaggerating; a lot of CS terms have disputed meanings but not this one.
EDIT: Sorry all for engaging the troll. I thought there might have been some legitimate confusion. My bad.
Oh, that's recursive! Which is Computer Science's domain of expertise, not the public domain.
We are talking about formal semantics here. What do programs (and computer languages are themselves programs) mean?
Point 0 of Wadler's law.
https://en.wikipedia.org/wiki/Semantics_(computer_science)
If you can type-check it at compile time then it is NOT dynamic dispatch. It's a contextual confusion.
What I am talking about when I say "dynamic dispatch" is the sort of dispatching done by R, LISP and Julia (and not by C++ or Java). Now, we can bicker about labels and you can insist that it's actually called "multiple dispatch" and not "dynamic dispatch", but you can't bicker about the semantic fact that "multiple dispatch" is actually more dynamic than "dynamic dispatch".
And this semantic point would've been trivial to unpack if you weren't try to win an argument.
Indeed, sorry for engaging the trolls. 20; or 30 of you. Lost count.
0. Dynamic dispatch (as you are using it) 1. Double dispatch 2. Multiple dispatch 3. Predicate dispatch 4. All of the above collectively.
What you thought was a confusing re-definition on my part might have been an ignorance of alternative uses on yours ;)
Java doesn't do 1, 2 or 3.
Focusing exclusively on what happens at compile-time is to miss the whole reason static type systems are useful in the first place: they allow compile-time reasoning about runtime behavior. Just as we can use a static type system to make predictions about programs that pass around first-class functions via static dispatch, we can also use them to make predictions about programs that use vtables or other constructions to perform dynamic dispatch. (Note that the difference between those two things isn’t even particularly well-defined; a call to a first-class function passed as an argument is a form of unknown call, and it is arguably a form of dynamic dispatch.)
Lots of statically typed languages provide dynamic dispatch. In fact, essentially all mainstream ones do: C++, Java, C#, Rust, TypeScript, even modern Fortran. None of these implementations require sacrificing static type safety in any way; rather, type systems are designed to ensure such dispatch sites are well-formed in other ways, without restricting their dynamic nature. And this is entirely in line with the OP, as there is no tension whatsoever between the techniques it describes and dynamic dispatch.
Obviously static type systems are useful. I don't even think my point is contrary to anything you are saying. This is not being said as way of undermining any particular paradigm because computation is universal - the models of computation on the other hand (programming languages) are not all “the same”. There are qualitative differences.
Every single programming paradigm is a self-imposed restriction of some sort. It is precisely this restriction that we deem useful because they prevent us from shooting off our feet with shotguns. And we also prevent ourselves from being able to express certain patterns (of course we can deliberately/explicitly turn off the self-imposed restriction! ).
Like the restriction you are posing on your self is explicit in "type systems are carefully designed so that the compile-time reasoning says something useful about what actually occurs at runtime"
If you could completely determine everything that happens at runtime you wouldn't need exception/error handling!
All software would be 100% deterministic.
And it isn't.
I can say nothing of the structure of random bitstreams from unknown sources. I only know what I EXPECT them to be. Not what they actually are.
In this context parsing untrusted data IS runtime type-checking.