Type macros

gistfile1.md

• Start Date: 2014-07-22
• RFC PR #: (leave this empty)
• Rust Issue #: (leave this empty)

Summary

This adds the ability to have macros in type signatures.

Motivation

Use procedural macros to generate values at the type level.

For natural number we could define something like this:

struct Zero;
struct Succ<T>;

Now we can make numbers like 0 (Zero), 1 (Succ<Zero>), 2 (Succ<Succ<Zero>>), etc. on the type level. However these are inconvenient to write out, so we can make an procedural macro which generates these for us. PackInt!(3) could expand to Succ<Succ<Succ<Zero>>>. To turn this back into a value, we can use a trait.

trait IntVal {
    fn get(&self) -> uint;
}

impl IntVal for Zero {
    fn get(&self) -> uint {
        0
    }
}

impl<T: IntVal> IntVal for Succ<T> {
    fn get(&self) -> uint {
        unsafe {
            1 + unpack_int<T>()
        }
    }
}

fn unpack_int<T: IntVal>() -> uint {
    unsafe {
        mem::uninitialized<T>().get()
    }
}

An more advanced example would be PackStr!("hello") which could use cons cells to encode a string literal as a type.

Completeness

It's currently surprising that macros won't work in type signatures.

Detailed design

Anywhere a type is allowed, an type macro is also allowed using the existing macro infrastructure.

Drawbacks

None.

Alternatives

None.

Unresolved questions

None.