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> In computer science and logic, a dependent type is a type whose definition depends on a value. (source Wikipedia) So rusts const generics are (very limited)

> In computer science and logic, a dependent type is a type whose definition depends on a value. (source Wikipedia) So rusts const generics are (very limited) dependent types. C++ templates on the other hand you could argue are not types (but templates), but that is nit-picking. Weather or not dependent types are can be runtime evaluated often is a separate question then weather or not a language has dependent types. EDIT: The following part is partial nonsense, very POV dependent. I wanted to delete it but that wouldn't be right, so I just added this note. Through not allowing run-time creation of dependent types makes them much more limited in usability. But then allowing it requires either reflections (still likely very limited/slow/etc.), or part of the type being stored as a "magic" field of the type instance (fast but is that still a dependent type?), or a interpreted language).

yakubin 6y ago
Are you saying you consider std::array<int, 5> in C++ to be a dependent type? I'm pretty sure it's not right. I'm pretty sure that the "value" in that quote is, implicitly, a "runtime value", or "value of a variable", similar to how we often say "type" without saying "type of a variable". This "5" isn't the value of a variable, it's just an expression. If you continue on to the next paragraph on this Wikipedia page, you will read, first: > The return type of a dependent function may depend on the value (not just type) of one of its arguments. For instance, a function that takes a positive integer n may return an array of length n, where the array length is part of the type of the array. second: > A dependent pair may have a second value of which the type depends on the first value. Sticking with the array example, a dependent pair may be used to pair an array with its length in a type-safe way. Generally, it's the same nomenclature as with the explanation of static vs dynamic type systems: in static type systems, types are attributes of variables, functions and expressions, whereas in dynamic type systems types are attributes of values. And in the static type system of C, 5 has type int, because it's an expression. It representing a value is secondary. A side effect of it is: you can write a compile-time constant in C that will overflow its type, because the type is assigned based on the lexical category, not on the value. As far as I understand, in a dependent type system it should be possible to implement a resizable array "std::vector<int, n>", where n is the length of the array (a variable), then write a "concat" function which concatenates two such arrays, and whose return (static) type depends on the lengths of the arrays passed as arguments. This is different than parametric polymorphism, such as the one that C++ has, since we can't know at compile-time all the possible lengths of arrays that we're going to get at runtime. Idris example here: https://idris2.readthedocs.io/en/latest/tutorial/typesfuns.html#vectors https://idris2.readthedocs.io/en/latest/tutorial/typesfuns.h... If someone has a better understanding of the topic, I'd be delighted to be shown wrong.
- jcelerier 6y ago
  > As far as I understand, in a dependent type system it should be possible to implement a resizable array "std::vector<int, n>", where n is the length of the array (a variable), then write a "concat" function which concatenates two such arrays, and whose return (static) type depends on the lengths of the arrays passed as arguments. worksforme https://gcc.godbolt.org/z/x9GjE7KGM https://gcc.godbolt.org/z/x9GjE7KGM (ok, except the "resizeable" bit - but since we're doing functional programming we want immutable data structures, no ? :p)
  - yakubin 6y ago
    "n" is not a variable in your example. It's still just syntactic sugar for templates, where the n needs to be known statically. Even without the "resizable" bit, you still limit this function to operating on arrays of sizes known statically, which is a severe limitation, making the whole thing impractical.
mhh__ 6y ago
> C++ templates on the other hand you could argue are not types (but templates), but that is nit-picking. Not only is it nitpicking, it seems wrong even in an academic sense of the term. Whether the template itself is a type or not probably depends on what theoretical language you use to discuss it. The template is monomorphized into a new type per instantiation, yes, but the moment it actually has a number in it it's a type. In D the template can quite happily accept pretty much anything as a parameter (particularly structs i.e. better code quality), and I'm surprised other languages don't allow this. Why have a bunch of template parameters on a line when you can just program the way you normally would.
- steveklabnik 6y ago
  > I'm surprised other languages don't allow this. Rust doesn't (arbitrary values, anyway, const generics is the start of this being possible) because we don't have templates, we have generics. I agree if you're going with a template-based system, it makes a lot of sense to accept whatever, but if you aren't, then things are more complicated.
  - mhh__ 6y ago
    I think the typechecking here should be possible for rust in the sense that it's just a value of a known type (this isn't like C++ hacks). https://d.godbolt.org/z/Tb5jKv4ns https://d.godbolt.org/z/Tb5jKv4ns
    - steveklabnik 6y ago
      It's possible, (and I think might even work on nightly? Don't have the time to check right this second) but is just a lot more work to support than in a template-based system, that's all. We'll get there!

yakubin 6y ago

Are you saying you consider std::array<int, 5> in C++ to be a dependent type? I'm pretty sure it's not right. I'm pretty sure that the "value" in that quote is, implicitly, a "runtime value", or "value of a variable", similar to how we often say "type" without saying "type of a variable". This "5" isn't the value of a variable, it's just an expression. If you continue on to the next paragraph on this Wikipedia page, you will read, first: > The return type of a dependent function may depend on the value (not just type) of one of its arguments. For instance, a function that takes a positive integer n may return an array of length n, where the array length is part of the type of the array. second: > A dependent pair may have a second value of which the type depends on the first value. Sticking with the array example, a dependent pair may be used to pair an array with its length in a type-safe way. Generally, it's the same nomenclature as with the explanation of static vs dynamic type systems: in static type systems, types are attributes of variables, functions and expressions, whereas in dynamic type systems types are attributes of values. And in the static type system of C, 5 has type int, because it's an expression. It representing a value is secondary. A side effect of it is: you can write a compile-time constant in C that will overflow its type, because the type is assigned based on the lexical category, not on the value. As far as I understand, in a dependent type system it should be possible to implement a resizable array "std::vector<int, n>", where n is the length of the array (a variable), then write a "concat" function which concatenates two such arrays, and whose return (static) type depends on the lengths of the arrays passed as arguments. This is different than parametric polymorphism, such as the one that C++ has, since we can't know at compile-time all the possible lengths of arrays that we're going to get at runtime. Idris example here: https://idris2.readthedocs.io/en/latest/tutorial/typesfuns.html#vectors https://idris2.readthedocs.io/en/latest/tutorial/typesfuns.h... If someone has a better understanding of the topic, I'd be delighted to be shown wrong.

jcelerier 6y ago
> As far as I understand, in a dependent type system it should be possible to implement a resizable array "std::vector<int, n>", where n is the length of the array (a variable), then write a "concat" function which concatenates two such arrays, and whose return (static) type depends on the lengths of the arrays passed as arguments. worksforme https://gcc.godbolt.org/z/x9GjE7KGM https://gcc.godbolt.org/z/x9GjE7KGM (ok, except the "resizeable" bit - but since we're doing functional programming we want immutable data structures, no ? :p)
- yakubin 6y ago
  "n" is not a variable in your example. It's still just syntactic sugar for templates, where the n needs to be known statically. Even without the "resizable" bit, you still limit this function to operating on arrays of sizes known statically, which is a severe limitation, making the whole thing impractical.

mhh__ 6y ago

> C++ templates on the other hand you could argue are not types (but templates), but that is nit-picking. Not only is it nitpicking, it seems wrong even in an academic sense of the term. Whether the template itself is a type or not probably depends on what theoretical language you use to discuss it. The template is monomorphized into a new type per instantiation, yes, but the moment it actually has a number in it it's a type. In D the template can quite happily accept pretty much anything as a parameter (particularly structs i.e. better code quality), and I'm surprised other languages don't allow this. Why have a bunch of template parameters on a line when you can just program the way you normally would.

steveklabnik 6y ago
> I'm surprised other languages don't allow this. Rust doesn't (arbitrary values, anyway, const generics is the start of this being possible) because we don't have templates, we have generics. I agree if you're going with a template-based system, it makes a lot of sense to accept whatever, but if you aren't, then things are more complicated.
- mhh__ 6y ago
  I think the typechecking here should be possible for rust in the sense that it's just a value of a known type (this isn't like C++ hacks). https://d.godbolt.org/z/Tb5jKv4ns https://d.godbolt.org/z/Tb5jKv4ns
  - steveklabnik 6y ago
    It's possible, (and I think might even work on nightly? Don't have the time to check right this second) but is just a lot more work to support than in a template-based system, that's all. We'll get there!