Literals and the const keyword

We want to be able to support a few things:

• writing numbers that will act as field elements directly in the code
• writing numbers that will act as relatively small numbers (for indexing into arrays for example) in the code
• writing functions that will accept constant arguments. For example to index into an array.

The first two points allow us to write things like that:

#![allow(unused)]
fn main() {
let x = 3;
assert_eq(y[5], 4);
}

The third point allows us to write things like that:

#![allow(unused)]
fn main() {
fn House.get(self, room: Field) -> Field {
    return House.rooms[room];
}
}

Since House takes a Field here, it can in theory be used with anything during type checking.

This is bad. There are two solutions:

1. During type checking, when we realize that room is used to index into an array, we enforce that it must be a constant value during compilation.
2. We create a distinct type for constants and literals.

Approach 1. is not elegant, because it means that it is not clear from the signature of the function alone that the room argument must be a constant. The user of the function will only get warned when trying to compile the program.

Approach 2. is interesting, because we already have such a type internally to track literals: a BigInt. The name is a bit misleading in the case of array accesses, because we additionally enforce that it is NOT a big integer, but rather a 32-bit integer (u32 in Rust).

Implementing a literal type

Approach 2 can be implemented in two ways:

a. Use a brand new type, like BigInt for literals. b. Use a const attribute to indicate that it is a constant.

Approach a. is a bit clumsy in my opinion because the developer need to remember about a new type name, and understand the distinction with that and Field.

On the other hand, approach b. uses the const keyword which is already well-known in many compiled programming languages.

What about this:

#![allow(unused)]
fn main() {
fn House.get(self, const room: Field) -> Field {
// versus
fn House.get(self, room: const Field) -> Field {
}

To contrast, the two other existing attributes (pub and mut) are placed in front of the variable names, not the type names.

One could argue that the type is the same, but the variable being passed is a constant, and so it makes more sense to implement the first version. This is what we do in noname.

At the time of this writing, the const keyword only seem to make sense in a function argument, and so is implemented in the same way as the pub attribute:

#![allow(unused)]
fn main() {
pub enum AttributeKind {
    Pub,
    Const,
}

pub struct Attribute {
    pub kind: AttributeKind,
    pub span: Span,
}

pub struct FnArg {
    pub name: Ident,
    pub typ: Ty,
    pub attribute: Option<Attribute>,
    pub span: Span,
}
}

When a function is parsed by the type checker, a const Field is transformed into a BigInt. And as such, the type checker will be happy with that variable being used to index into an array, or being used by other functions expecting a constant.

What about other types being const? I don’t think it makes sense for now, as I can only think of array access requiring this. So we don’t implement it.