Prototype type system design for composing structs and sum types

[DylanSp]

Issues include - can a struct be one case of a sum type?

Look what Rust does for inspiration.

[DylanSp]

Prior art from Rust

Sources:

• https://doc.rust-lang.org/book/ch06-01-defining-an-enum.html
• https://doc.rust-lang.org/book/ch06-02-match.html
• https://doc.rust-lang.org/book/ch18-03-pattern-syntax.html (for destructuring structs)

Example enum:

enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(i32, i32, i32),
}

• Structs can be variants of an enum, i.e. Move in the example.
• Variants in Rust can also have any number of anonymous fields - Write is effectively a 1-tuple, ChangeColor is effectively a 3-tuple

Pattern matching can destructure using field names. Example code:

match msg {
        Message::Move { x, y } => {
            println!("Move in the x direction {x} and in the y direction {y}");
        }
        // omitted - Quit, Write, ChangeColor cases
    }

[DylanSp]

Decisions for Faust

Type definitions:

• Allow structs as variants in enums.
• Allow variants with anonymous fields, with a max of 4 fields - treat as tuples.

Pattern matching:

• When matching on structs, match using field names.
• When matching on tuples, can use arbitrary identifiers.