mathematical interval notation for slice syntax

[oli-obk]

https://github.com/rust-lang/rfcs/blob/master/text/0198-slice-notation.md introduced the [start..end] syntax for slices. There was some serious discussion about the readability of the notation (#198). I suggest using interval notation (https://en.wikipedia.org/wiki/Interval_%28mathematics%29).
Using [), (] or () like in the docs (http://doc.rust-lang.org/std/slice/trait.ImmutableSlice.html#tymethod.slice) is not a good idea, since it'll be confused with function syntax. But the alternative (US) notation [[, ]] or ][ is also part of the ISO 80000-2 standard (behind paywall, or google "international iso standard 80000-2 interval notation").

• foo][ for foo.as_slice()
• foo[n,m[ for foo.slice(n, m) or foo[n..m]
• foo[n,m] for foo.slice(n, m+1)
• foo]n,m] for foo.slice(n+1, m+1)
• foo]n,m[ for foo.slice(n+1, m)
• mut variants of all the above

I'm not sure about the unbounded versions though. Following my above suggestions it would be:

• foo[n,] for foo.slice_from(n) or foo[n..]
• foo[,m] for foo.slice_to(m) or foo[..m]

But right there the #198 notations appeals to me more. Another alternative, that also helps prevent typos would be to require an underscore for open ended intervals.

• foo[n,_] for foo.slice_from(n) or foo[n..]
• foo[_,m] for foo.slice_to(m) or foo[..m]

Intervals are not ranges. So technically this should be a way to declare intervals which then could be passed to the slice function. But i do not think the following looks readable (which is the whole point of this feature request).

• foo[[n,m[] for foo.slice(n, m) or foo[n..m]

[nrc]

Anything which uses unbalanced brackets of any kind is a non-starter since it will break the tokeniser and confuse every editor in existence. I guess the other suggestions could be considered, but most people seem pretty happy with the existing syntax (with some minor disagreement with [] vs [..]).

[oli-obk]

ok, I can see how this is not a good idea from the parser point of view.