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Comment by Rusky | Hacker News Reader

C++ is not a dependently typed language, for the same reason that templates do not emit errors until after they are instantiated. All non-type template parameters get fully evaluated at instantiation time so they can be checked concretely.

A truly dependently typed language performs these checks before instantiation time, by evaluating those expressions abstractly. Code that is polymorphic over values is checked for all possible instantiations, and thus its types can actually depend on values that will not be known until runtime.

The classic example is a dynamic array whose type includes its size- you can write something like `concat(vector<int, N>, vector<int, M>) -> vector<int, N + M>` and call this on e.g. arrays you have read from a file or over the network. The compiler doesn't care what N and M are, exactly- it only cares that `concat` always produces a result with the length `N + M`.

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I'm not sure what "dependently typed" means but in C++20 and beyond, concepts allow templates to constrain their parameters and issue errors for the templates when they're specialized, before the actual instantiation happens. E.g., a function template with constraints can issue errors if the template arguments (either explicit or deduced from the call-site) don't satisfy the constraints, before the template body is compiled. This was not the case before C++20, where some errors could be issued only upon instantiation. With C++20, in theory, no template needs to be instantiated to validate the template arguments if constraints are provided to check them at specialization-time.

This is the wrong side of the API to make C++20 dependently typed. Concepts let the compiler report errors at the instantiation site of a template, but they don't do anything to let the compiler report errors with the template definition itself (again before instantiation time).

To be clear this distinction is not unique to dependent types, either. Most languages with some form of generics or polymorphism check the definition of the generic function/type/etc against the constraints, so the compiler can report errors before it ever sees any instantiations. This just also happens to be a prerequisite to consider something "dependently typed."

> performs these checks before instantiation time

Notably Rust type-based generics do this, a key difference wrt. C++ templates. (You can use macros if you want checks after instantiation, of course.)

In c++ it does care what N and M are at compile time, at least the optimizer does for autovectorization and unrolling. Would that not be the case with const generic expressions?

The question of whether a language is dependently typed only has to do with how type checking is done. The optimizer doesn't come into play until later, so whether it uses the information is unrelated to whether the language is dependently typed.

Ok, I think I understand now, but is it really dependently typed just because it symbolically verified it can work with any N and M? Because it will only generate code for the instantiations that get used at compile time.

Is what really dependently typed? I'm saying C++ is not dependently typed, because it doesn't do any symbolic verification of N and M.

If rust did add const generic expressions I mean. It still would only generate code for the used instantiations.

Ah, I wasn't really talking about Rust.

Rust already does have some level of const generic expressions, but they are indeed only possible to instantiate with values known at compile time, like C++.

The difficulty of type checking them symbolically still applies regardless of how they're instantiated, but OTOH it doesn't look like Rust is really trying to go that direction.

the only thing needed here is to be able to lift N & M from run-time to the type system (which in C++ as it stands exists only at compile-time). For "small" values of N&M that's doable with switches and instantiations for instance.

The point of dependent types is to check these uses of N and M at compile time symbolically, for all possible values, without having to "lift" their actual concrete values to compile time.

Typical implementations of dependent types do not generate a separate copy of a function for every instantiation, the way C++ does, so they simply do not need the concrete values in the same way.

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