Less painful range construction

[jeff-davis]

https://gist.github.com/sfackler/4c4e93f896e881fae811

I like that solution there. It is reasonably readable, does compile-time checking, and matches the postgres constructors.

However, it was also suggested on IRC to use something more like:

range!('[' a, b ')')

which is also acceptable to me.

[sfackler]

I kind of like the range!('[' a, b ')') syntax. Would e.g. range!('(' , b ')') be the best syntax for a half-open range?

[jeff-davis]

That's a good point: it does seem to extend a little better to unbounded ranges.

Another thought I had is to use textual tokens like II, EI, IE, EE. That is a little awkward to read though and doesn't seem to work naturally for unbounded ranges.

[jeff-davis]

I tried implementing it that way, and I ran into a problem. The ranges with no upper bound work fine, but the ranges with no lower bound won't compile and I don't understand why.

macro_rules! range(
   ('(' $low:expr, $high:expr ')') => ("(A,B)");
   ('(' $low:expr, $high:expr ']') => ("(A,B]");
   ('[' $low:expr, $high:expr ')') => ("[A,B)");
   ('[' $low:expr, $high:expr ']') => ("[A,B]");
   ('('          , $high:expr ')') => ("(-inf,B)");
   ('('          , $high:expr ']') => ("(-inf,B]");
   ('(' $low:expr,            ')') => ("(A,inf)");
   ('[' $low:expr,            ')') => ("[A,inf)");
   ($low: expr, $high:expr) => (
      range!('[' $low, $high ')')
   );
)

println!("{}",    range!('('  , 2 ')')   ); // error
println!("{}",    range!('(' 1,   ')')   ); // works fine

[sfackler]

It tries to parse an expression first, fails and doesn't try the next option. Moving the no-lower-bound cases to the top appear to fix it.

[jeff-davis]

Yes, thank you, I had just realized that and was about to update my comment.

+1 to this approach

We'll also need a basic constructor for an empty range and a range unbounded on both sides.

[sfackler]

There's already Range::empty() to construct the empty range, but we might as well add range!(empty) and range!('(',')') for completeness.

[sfackler]

I've added a macro to types/range.rs. You'll have to copy/paste it into your source file for now until rust-lang/rust#11151 is finished up.