RFC: Enum trait

Issue by ghost
Sunday Mar 17, 2013 at 02:56 GMT

For earlier discussion, see https://github.com/rust-lang/rust/issues/5417

This issue was labelled with: A-libs, A-traits, B-RFC in the Rust repository

Since I've been doing some work with the deriving code in libsyntax, I thought I'd solicit opinions on adding a new trait to libcore:

trait Enum {
    static fn enumerate(blk: &fn(Self) -> bool);
}

Obviously, this would be most useful for enums with only nullary variants, but other enumerable types could take advantage of it as well (bool::all_values currently serves the same purpose).

This would make it possible to write

// enum Dir { North, East, South, West }
for Enum::enumerate |dir: Dir| { ... }

instead of

for [North, East, South, West].each |dir| { ... }

A standard implementation for enums with only nullary variants would be made available through #[deriving(Enum)], and a default method (or utility trait/impl until those are working) would be provided for obtaining all the values in a vector. It might be beneficial to have a static fn cardinality() -> Option<uint> method on the trait as well.

If the trait name is too easily confused with the keyword, another option is Enumerable.

Start by reading the earlier discussion in rust-lang/rust issue 5417, then inspect the deriving code in libsyntax and the proposed libcore trait. Determine whether an Enum or Enumerable trait, enumeration behavior, and cardinality API have consensus; done means an accepted design and RFC decision, not just an implementation.