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Is there a good post out there explaining "const generics"? I'm very curious about this phrase and not sure what it means.

steveklabnik 6y ago
https://rust-lang.github.io/rfcs/2000-const-generics.html https://rust-lang.github.io/rfcs/2000-const-generics.html is the design document, it may or may not be a good resource depending on your knowledge of Rust. A classic example of them being used in Rust is to be generic over arrays, like what the blog post talks about. Arrays encode their length as part of the type; [T; N] is the type of an array of Ts that’s N elements long. In other words: fn foo<T>(array: [T; ?]) { How do you define ? to work on any length? When const generics is implemented, you’d write fn foo<T, const N>(array: [T; N]) { and now you can call this function on arrays of varying lengths. There's a bit more to it than that, but that's the core of it. Does that help?
- maxfurman 6y ago
  So in Rust you must specify the length of an array? How do you implement map, fold, etc., that work on an entire collection regardless of length? Is that a different API in Rust?
  - steveklabnik 6y ago
    > So in Rust you must specify the length of an array? Correct. > How do you implement map, fold, etc., that work on an entire collection regardless of length? So, while arrays exist, there are also things like slices and vectors, which have a runtime length, rather than an array's compile-time length. So that's one bit of it. But the other part is that these are implemented on iterators, rather than on the type directly, so usually what happens is "turn that into an iterator, then do map/fold/filter/whatever on the iterator. It is easy to convert the compile-time length array to a run-time length iterator, so it ends up being a uniform interface.
    - maxfurman 6y ago
      That makes sense. Thanks!
    - jadbox 6y ago
      So would const generics for arrays make map/fold somewhat faster because there would be no need to create an iterator?
      - steveklabnik 6y ago
        You would still need to create an iterator, because there are three different ways to iterate, and you have to pick which one you want. However, often times it entirely compiles away to nothing. What this may help with is bounds checks; it is possible for this to make it easier to remove/hoist bounds checks in some circumstances. In my understanding.
  - justadudeama 6y ago
    Yes, there is `Vec` that allows arbitrary length arrays, and iterators implement those other functions
  - deleted 6y ago
    [deleted]
  - KuiN 6y ago
    Map, fold etc. are implemented on the `Iterator` trait. [1,2,3].iter().map(|x| x + 1) https://doc.rust-lang.org/std/iter/trait.Iterator.html https://doc.rust-lang.org/std/iter/trait.Iterator.html
rhodysurf 6y ago
It allows you to be generic over things that arent types is my very dumbed down description of what it means for most people. One of the biggest missing features for me from C++ doing scientific computing.

steveklabnik 6y ago

https://rust-lang.github.io/rfcs/2000-const-generics.html https://rust-lang.github.io/rfcs/2000-const-generics.html is the design document, it may or may not be a good resource depending on your knowledge of Rust. A classic example of them being used in Rust is to be generic over arrays, like what the blog post talks about. Arrays encode their length as part of the type; [T; N] is the type of an array of Ts that’s N elements long. In other words: fn foo<T>(array: [T; ?]) { How do you define ? to work on any length? When const generics is implemented, you’d write fn foo<T, const N>(array: [T; N]) { and now you can call this function on arrays of varying lengths. There's a bit more to it than that, but that's the core of it. Does that help?

maxfurman 6y ago
So in Rust you must specify the length of an array? How do you implement map, fold, etc., that work on an entire collection regardless of length? Is that a different API in Rust?
- steveklabnik 6y ago
  > So in Rust you must specify the length of an array? Correct. > How do you implement map, fold, etc., that work on an entire collection regardless of length? So, while arrays exist, there are also things like slices and vectors, which have a runtime length, rather than an array's compile-time length. So that's one bit of it. But the other part is that these are implemented on iterators, rather than on the type directly, so usually what happens is "turn that into an iterator, then do map/fold/filter/whatever on the iterator. It is easy to convert the compile-time length array to a run-time length iterator, so it ends up being a uniform interface.
  - maxfurman 6y ago
    That makes sense. Thanks!
  - jadbox 6y ago
    So would const generics for arrays make map/fold somewhat faster because there would be no need to create an iterator?
    - steveklabnik 6y ago
      You would still need to create an iterator, because there are three different ways to iterate, and you have to pick which one you want. However, often times it entirely compiles away to nothing. What this may help with is bounds checks; it is possible for this to make it easier to remove/hoist bounds checks in some circumstances. In my understanding.
- justadudeama 6y ago
  Yes, there is `Vec` that allows arbitrary length arrays, and iterators implement those other functions
- deleted 6y ago
  [deleted]
- KuiN 6y ago
  Map, fold etc. are implemented on the `Iterator` trait. [1,2,3].iter().map(|x| x + 1) https://doc.rust-lang.org/std/iter/trait.Iterator.html https://doc.rust-lang.org/std/iter/trait.Iterator.html

rhodysurf 6y ago

It allows you to be generic over things that arent types is my very dumbed down description of what it means for most people. One of the biggest missing features for me from C++ doing scientific computing.