table-enum

A convenient rust macro to create enums with associated constant data. It lets you associate constant values to enum variants, similar to how enums work in Java, or how X macros are often used in C.

Only the enum tag is ever passed around, the data is accessed through generated const fn functions that match the enum tag to the relevant data.

This is different from how enums are typically used in Rust, which are actually tagged unions. (also known as variant types or sum types in computer science theory).

When would you use this?

An example where non-tagged-union[^1] enums are very useful is compiler or interpreter development. For example:

use table_enum::table_enum;

table_enum! {
    enum BinaryOp(text: &'static str, precedence: i32, right_assoc: bool) {
        Add("+", 10, false),
        Sub("-", 10, false),
        Mul("*", 20, false),
        Div("/", 20, false),
        Pow("**", 30, true),
        ...
    }
}

[^1]: I really wish Rust and Swift hadn't called their tagged unions "enums". To me enums are meant to be used as in this macro. A tagged union should be a kind of union.

How does it work?

The example above expands into the following code:

enum BinaryOp {
    Add,
    Sub,
    Mul,
    Div,
    Pow,
    ...
}
impl BinaryOp {
    const fn text(&self) -> &'static str {
        match self {
            BinaryOp::Add => "+",
            BinaryOp::Sub => "-",
            BinaryOp::Mul => "*",
            BinaryOp::Div => "/",
            BinaryOp::Pow => "**",
            ...
        }
    }
    const fn precedence(&self) -> i32 {
        match self {
            BinaryOp::Add => 10,
            BinaryOp::Sub => 10,
            BinaryOp::Mul => 20,
            BinaryOp::Div => 20,
            BinaryOp::Pow => 30,
            ...
        }
    }
    const fn right_assoc(&self) -> bool {
        match self {
            BinaryOp::Add => false,
            BinaryOp::Sub => false,
            BinaryOp::Mul => false,
            BinaryOp::Div => false,
            BinaryOp::Pow => true,
            ...
        }
    }
}

Alternative Crates

• enum_assoc: more powerful but less convenient.