compile-time lookup tables via syntax extensions

[thestinger]

It would be awesome to support building constant lookup tables at compile-time similar to gperf. This is a really common pattern and could potentially be a big performance win in many places (I'm sure rustc has a lot of constant tables).

A really simple proof of concept would sort an array and provide an object implementing the core::container::Map interface (once MutableMap is split out) via binary search.

The naming would be something along the lines of map!, set!, ordered_map! and ordered_set! along with the corresponding types ConstantMap, ConstantSet, ConstantOrderedMap, ConstantOrderedSet.

[graydon]

Agreed, but I think this would make a better 3rd party library (once syntax extensions are independently loadable) rather than necessarily part of the main compiler. I'll leave it open here since we don't have a syntax-extensions-wanted page yet.

[mstewartgallus]

A functional manner of doing this would be to use a match expression such as:

fn lookup_table(key: &str) -> Option<uint> {
    match key {
        "foo" => Some(0),
        "bar" => Some(1),
        "car" => Some(2),
        _ => None
    }
}

A macro could easily be created that would provide a nicer syntax for this. Is there any reason that a hash table could not be used to implement big enough to benefit match expressions?

[sfackler]

I'm working on this: https://github.com/sfackler/rust-mphf

[sfackler]

I'm going to close this out as there's a decently mature implementation at https://github.com/sfackler/rust-phf