Const Generics and the Compile Time Spread Operator

We're happy to announce that the Vale 0.2 beta now has const generics! Generic parameters can contain not only types (like the T in List<T>) but also integers, booleans, and other interesting substances.

This might be familiar from other languages, like C++ and Rust. However, Vale's taking them in a different direction, to enable something called the compile-time spread operator, which will serve as a useful metaprogramming tool for Vale.

This article gives a quick overview of const generics, and then where Vale plans to take them.

Const Generics

Before const generics, we often had a lot of similar classes:

struct Vec2<T> {
  elements [#2]T; // 2-element array of T
}
struct Vec3<T> {
  elements [#3]T; // 3-element array of T
}
struct Vec4<T> {
  elements [#4]T; // 4-element array of T
}

It would be nice if we could pass that 2, 3, 4 in as a generic parameter like we did T!

Enter const generics, which enables just that.

struct Vec<N Int, T> {
  elements [#N]T;
}

They're used like Vec<2, int>. Much better!

Besides types and integers, we can also have generic parameters that are:

• Booleans
• Strings
• Functions
• Mutability (imm vs mut)
• Ownership (own, borrow, share, weak)
• Location (inl, heap)
• Variability (vary, final)
• Type Lists

Compile-time Spread Operator

That last one is particularly interesting, as it allows us to implement tuples in Vale:

struct Tup<T RefList> {
  _ ..T;
}

_ means the field is unnamed, and ..T means "use each of these types".

That .. is known as the spread operator. It can basically be thought of as the "compile-time for-each loop".

(int, bool, str) is syntactic sugar for Tup<RefList[int, bool, str]> which expands to this:

struct Tup {
  0 int; // an int named 0
  1 bool; // a boolean named 1
  2 str; // a string named 2
}

The Spread Operator's Future

The above works today, in version 0.2. Now we'll show you a sneak peek of where we're heading with this delightful little operator.

We want to be able to use it for function arguments, and in expressions. For example, we could use it to implement a zero-cost variadic println function:

func println<T RefList>(args T..) {
  ..print(args..);
  print("\n");
}

If we called this with println(4, " hello ", true), it's as if the println function contains:

func println(args0 int, args1 str, args2 bool) {
  print(args0);
  print(args1);
  print(args2);
  print("\n");
}

4 hello true

In the previous snippet, the prefix .. (before print) marks the beginning of the "loop", and the postfix .. (after args) specifies what should change in each iteration.

Spread Method Call

When combined with UFCS, another interesting capability emerges, which we'll call the spread method call. The above snippet can be rewritten as:

func println<T RefList>(args T) {
  args..print();
  print("\n");
}

It's fascinating how one little symbol can enable such a powerful capability!

Thanks for visiting, hope you enjoyed it!

In the coming weeks, I'll be writing more about our "Fearless FFI" plans which will help us more safely use external C code, so subscribe to our RSS feed twitter, or the r/vale subreddit, and come hang out in the Vale discord!

If you found this interesting, please consider sponsoring us:

With your help, we can write this kind of nonsense more often!

- Evan Ovadia